Insect nicotinic receptor interactions in vivo with neonicotinoid,organophosphorus, and methylcarbamate insecticides and a synergist

The nicotinic acetylcholine (ACh) receptor (nAChR) is the principal insecticide target. Nearly half of the insecticides by number and world market value are neonicotinoids acting as nAChR agonists or organophosphorus (OP) and methylcarbamate (MC) acetylcholinesterase (AChE) inhibitors. There was no previous evidence for in vivo interactions of the nAChR agonists and AChE inhibitors. The nitromethyleneimidazole (NMI) analog of imidacloprid, a highly potent neonicotinoid, was used here as a radioligand, uniquely allowing for direct measurements of house fly (Musca domestica) head nAChR in vivo interactions with various nicotinic agents. Nine neonicotinoids inhibited house fly brain nAChR [³H]NMI binding in vivo, corresponding to their in vitro potency and the poisoning signs or toxicity they produced in intrathoracically treated house flies. Interestingly, nine topically applied OP or MC insecticides or analogs also gave similar results relative to in vivo nAChR binding inhibition and toxicity, but now also correlating with in vivo brain AChE inhibition, indicating that ACh is the ultimate OP- or MC-induced nAChR active agent. These findings on [³H]NMI binding in house fly brain membranes validate the nAChR in vivo target for the neonicotinoids, OPs and MCs. As an exception, the remarkably potent OP neonicotinoid synergist, O-propyl O-(2-propynyl) phenylphosphonate, inhibited nAChR in vivo without the corresponding AChE inhibition, possibly via a reactive ketene metabolite reacting with a critical nucleophile in the cytochrome P450 active site and the nAChR NMI binding site.The nicotinic nervous system has two principal sites of insecticide action, the nicotinic receptor (nAChR) activated by acetylcholine (ACh) and neonicotinoid agonists (1–6), and acetylcholinesterase (AChE) inhibited by organophosphorus (OP) and methylcarbamate (MC) compounds to generate and maintain localized toxic ACh levels (Fig. 1) (7). The nAChR and AChE targets have been identified in insects by multiple techniques but not by direct assays of the ACh binding site in the brain of poisoned insects. Here we use the outstanding insecticidal potency of the nitromethyleneimidazole (NMI) analog of imidacloprid (IMI) (8) as a radioligand (9), designated [³H]NMI, to directly measure the house fly (Musca domestica) nAChR not only in vitro but also in vivo, allowing us to validate by a previously undescribed method the neonicotinoid direct and OP/MC indirect nAChR targets (Fig. 2). This approach also helped solve the intriguing mechanism by which an O-(2-propynyl) phosphorus compound strongly synergizes neonicotinoid insecticidal activity (10) by dual inhibition of cytochrome P450 (CYP) (11–13) and the nAChR agonist site (described herein). Insecticide disruption at the insect nAChR can now be readily studied in vitro and in vivo with a single radioligand allowing better understanding of the action of several principal insecticide chemotypes (Fig. 3).Open in a separate windowFig. 1.The insect nicotinic receptor is the direct or indirect target for neonicotinoids, organophosphorus compounds and methylcarbamates, which make up about 45% of the insecticides by number and world market value (2, 7).Open in a separate windowFig. 2.In this study, Musca nicotinic receptor in vivo interactions with major insecticide chemotypes are revealed by a [³H]NMI radioligand reporter assay. *Position of tritium label.Open in a separate windowFig. 3.Two neonicotinoid nicotinic agonists and two anticholinesterase insecticides.     

Emerging single-phase state in small manganite nanodisks

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La_0.325Pr_0.3Ca_0.375MnO₃ (LPCMO) single crystalline disks to study the effect of spatial confinement on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.Owing to strong coupling between spin, charge, orbital, and lattice (1, 2), different electronic phases often coexist spatially in strongly correlated materials known as electronic phase separation (EPS) (3, 4). For colossal magnetoresistance (CMR) manganites, EPS has been observed to have strong influence on the global magnetic and transport properties (5, 6). Regarding the physical origin of EPS, it has been shown theoretically that quenched disorder can lead to inhomogeneous states in manganites (1, 3, 7). Once long-range effects such as coulombic forces (8), cooperative oxygen octahedral distortions (9), or strain effects (10) are included, calculations show infinitesimal disorder (8, 11) or even no explicit disorder (10) may lead to EPS. Within a phenomenological Ginzburg–Landau theory, it has been shown that EPS is intrinsic in complex systems as a thermodynamic equilibrium state (12).Although the details of the origin of the EPS remain as a matter of dispute, its very existence as a new form of electronic state has been well accepted. The length scale of the EPS has been observed to vary widely from nanometers to micrometers depending on many parameters that can affect the competition between different electronic phases (13–20). It is thus of great interest to examine whether the EPS state still exists as the system is scaled down, especially when the spatial dimension of the system is smaller than the length scale of the EPS domains.In this work, we use La_0.325Pr_0.3Ca_0.375MnO₃ (LPCMO) as a prototype system to show a spatial confinement-induced transition from the EPS state to a single ferromagnetic phase state. The LPCMO system is chosen because of its well-known large length scale of EPS domains (approximately a micrometer) (21), which allows us to conveniently fabricate LPCMO epitaxial thin films into disks with diameters that are smaller than the EPS domain size. In LPCMO bulk (21) and thin films (6, 22), the EPS state was observed to be the low-temperature ground state. Using magnetic force microscope, we observe that the EPS state remains to be the ground state in disks with the size of 800 nm in diameter or larger but vanishes in the 500-nm-diameter disks whose size is distinctly smaller than the characteristic length scale of the EPS domains. In the 500-nm disks, only the ferromagnetic phase can be observed at all temperatures below Curie temperature T_c, indicating that the system is in a single-phase state rather than a EPS state. Our results further indicate that the large length scale EPS in the LPCMO system does not cost extra Coulomb energy, which otherwise should lead to a scaling down of EPS with decreasing size of the LPCMO disks (23, 24).LPCMO films with 60-nm thickness were epitaxially grown on SrTiO₃(001) substrates by pulsed-laser deposition. The substrates were kept at 780 °C in oxygen atmosphere of 5 × 10⁻³ millibars during growth. Unit cell by unit cell growth was achieved as indicated by oscillations of intensity of reflection high-energy electron diffraction (RHEED). The films were postannealed to 950 °C for 3 h in flowing oxygen to reduce oxygen vacancy and make sure that the films have the same magnetic properties as the bulk. The LPCMO disks with diameters from 500 nm to 20 μm were fabricated from the epitaxial thin films by electron beam lithography with a negative tone resist (for details, see the sample fabrication method and Fig. S1 in the Supporting Information). Magnetic properties of the LPCMO disk arrays were carried out using superconducting quantum interference device (SQUID) and magnetic force microscope (MFM) measurements.Open in a separate windowFig. S1.(A and B) Schematics of LPCMO disks samples for magnetic property measurement (A) and MFM mapping (B). (C) Optical microscopic image of LPCMO disk array with specific diameter. (D) SEM image of LPCMO disk sample for MFM imaging.A distinct signature of the EPS state in the LPCMO system is the thermal hysteresis for temperature-dependent magnetic and transport properties. Fig. 1 A–D shows temperature dependent magnetic properties of LPCMO disks with different diameters. To enhance the measuring signal for SQUID, we fabricate disk arrays for each selected diameter (the optical microscopic image shown in Fig. 1B, Inset for the 1-μm disk array). Fig. 1 A–D shows temperature-dependent magnetization measured under 1,000 Oe in-plane field for 7-μm, 1-μm, 800-nm, and 500-nm disk arrays, respectively. Thermal hysteresis can be observed for disk arrays with size down to 800 nm, reflecting the fact that ferromagnetic metallic (FMM) and charge order insulating (COI) phases coexist during the first-order phase transition (7, 25). For the 500-nm disk array, however, no thermal hysteresis can be observed. This observation implies that the EPS state may no longer exist in the system (7, 26).Open in a separate windowFig. 1.Temperature dependence of magnetization (black lines for cooling and red lines for warming) under 1,000 Oe (A–D) and initial magnetization (red lines) and hysteresis loop (black lines) (E–H) at 5 K of arrays of LPCMO disks with sizes of 7 μm (A and E), 1 μm (B and F), 800 nm (C and G), and 500 nm (D and H) in diameter and an area of 3 mm × 3 mm. (B, Inset) The optical microscopic image of d = 1 μm array. (C and D, Insets) Zoomed-in M vs. T loop around the thermal hysteresis region.The lack of EPS state in the 500-nm disk array is supported by the field-dependent magnetization measurements. Fig. 1 E–H shows in-plane initial magnetization curves and magnetic hysteresis loops (M-H loops) for the disk arrays measured at 5 K after zero-field cooling. For 800-nm or larger disk arrays, there is a clear difference between the initial magnetization curves and the corresponding M-H loops due to the coexistence of FMM and COI phases. When the magnetic field is applied from the initial state, the magnetization of the FMM phase first quickly aligns along the field direction, leading to the low field fast rise of the initial magnetization curve. With increasing field, the COI phase is melted and transits into the FMM phase. Once transited, the FMM phase will mostly stay even if the field is reduced, giving rise to the difference between initial magnetization curve and the M-H loop. The difference, however, becomes smaller with decreasing size. For the 500-nm disk array, the initial magnetization curve and the M-H loop virtually superimpose each other, indicating no melting of COI phase occurs. Both the temperature- and field-dependent magnetization measurements show a transition from the EPS state to a single FMM state with decreasing size of the disk, and the critical size should be between 500 nm and 800 nm.The transition from the EPS state to a single FMM state can be seen in MFM images shown in Fig. 2 (for MFM imaging details, see micromagnetic mapping method in Supporting Information). Fig. 2A shows morphological appearance of LPCMO disks with different sizes acquired by atomic force microscope (AFM). Fig. 2 B–D shows the corresponding MFM images of the LPCMO disks acquired at different temperatures under a perpendicular magnetic field of 1T. Here, the perpendicular magnetic field is applied to yield some perpendicular magnetization components for MFM imaging because the easy magnetization axis is in the plane. In the present color scale, the contrast below zero (red or black) represents FMM phase, whereas the contrast above zero (green or blue) represents nonferromagnetic phase [i.e., COI phase based on previous knowledge of the LPCMO system (21, 22)]. Apparently, except the 500-nm disk, all other disks show distinct features of the EPS state (i.e., the coexistence of the FMM and COI phases). Although the portion of FMM phase increases noticeably with decreasing temperature, the system stays in the EPS state even at 10 K. The typical length scale of the EPS domains is around a micrometer, which is consistent with previous reports (21, 27).Open in a separate windowFig. 2.(A) AFM images of LPCMO disks with sizes of 500 nm, 1 μm, 2 μm, 3.8 μm, 5 μm, and 7 μm in diameter. (B–D) The MFM images of LPCMO disks under 1T field (external magnetic field direction is pointing perpendicularly to the sample surface plane) taken at 10 K (B), 100 K (C), and 180 K (D). The sizes of disks in MFM images are adjusted and corrected to have same scales for each size with the help of scanning electron microscope (SEM) images (shown in Fig. S2) and dash lines show the approximate physical boundary of disks. The negative value in MFM image indicates attractive force and positive value indicates repulsive force.In stark contrast to the larger disks, the 500-nm disk does not exhibit any features of EPS in Fig. 2. Instead, the whole disk is in a ferromagnetic phase with a magnetization profile peaking in the center. To ensure that the EPS state is not diminished by the magnetic field applied during MFM imaging, we took MFM images of the 500-nm disk at 10 K under different perpendicular magnetic fields from 0T to 1T, as shown in Fig. 3A. At 0T, signals with opposite sign can only be seen on two sides of the disk along the marked line (MFM images of 4 disks shown in Fig. S3). This pattern is a typical MFM image for an in-plane ferromagnetic single domain, because only the two ends of an in-plane magnetic dipole yield perpendicular field gradient (with opposite signs) for the MFM tip to detect. Once a perpendicular field of 0.15T is applied, the in-plane magnetization is driven out of plane, leading to a center peaked MFM contour. The MFM signal increases with increasing field, as shown in Fig. 3B by the marked line profiles extracted from Fig. 3A.Open in a separate windowFig. 3.(A) MFM images of 500-nm disks at 10 K under different magnetic field. The lines show the path the line profile extracted. (B) Line profiles extracted from MFM images partly shown in Fig. 3A. (C) Simulated results corresponding to line-profiles in B. (D and E) The simulated Z-component of stray field 100 nm above sample disks and magnetic structure of 500-nm disks under a different magnetic field. In E, the direction and size of arrows show the direction and relative value of in-plane component of magnetization, and the color of disk indicates the value of Z-component of magnetization presented by ratio of Z-component to total magnetization, as shown in the color bar.Open in a separate windowFig. S3.MFM images of 500-nm disks taken at 10 K under zero field after zero-field cooling.The field-dependent behavior of the MFM contrast of the 500-nm disk is in qualitative agreement with micromagnetic simulations. Based on the MFM observation, the 500-nm disk is in an in-plane, single-domain state. Using this model as input, we performed micromagnetic simulation and obtained the Z-component of magnetic stray field distribution at 100 nm above sample surface (Fig. 3D; for details, see the micromagnetic simulation method in the Supporting Information), which is virtually the signal detected by MFM tip. The corresponding magnetic structures under different magnetic fields are shown in Fig. 3E. The marked line profiles extracted from simulation (Fig. 3D) are shown in Fig. 3C alongside with the experimental MFM line profiles (Fig. 3B). The subtle differences between Fig. 3B and Fig. 3C are likely caused by the fact that experimental MFM images are convoluted from signals of both the LPCMO disks and the MFM tips (∼100 nm in size). The consistency of MFM images and simulation confirms that the 500-nm disk is in a ferromagnetic single-domain state with an in-plane easy magnetization axis.Finally, we show that the 500-nm disk is in a single FMM state at all temperatures. Fig. 4 shows MFM images of the 500-nm disk acquired every 20 K, from 20 K to 200 K under 1,000 Oe. Other than the center-peaked FMM phase, no traces of COI phase can be observed. The MFM signal decreases with increasing temperature, which is consistent with the behavior of the temperature-dependent magnetization shown in Fig. 1. Considering the fact that we have never observed pure COI phase in the 500-nm disks, we believe this phenomenon may be caused by the existence of the ferromagnetic metallic edge state in the LPCMO system (22), which assists the 500-nm disk to be in pure ferromagnetic state when a single state is energetically preferred in the 500-nm disk.Open in a separate windowFig. 4.MFM images of 500-nm disks taken every 20 K, from 20 K to 200 K.In summary, we discovered a spatial confinement-induced transition from a EPS state featuring coexistence of FMM and COI phases to a single FMM state in the LPCMO system. The critical size for the transition is between 500 nm and 800 nm, which is similar to the characteristic length scale of the EPS state in the LPCMO system. Combining the MFM data and the micromagnetic simulation, we conclude that the 500-nm LPCMO disk is in a single-domain ferromagnetic state at all temperatures below T_c. A similar conclusion can be reached for 300-nm LPCMO disks (shown in Fig. S4), although it needs to be studied further whether a new state would emerge if the disk size becomes a few tens of nanometers or smaller. Our work opens a way to control EPS without external field or introducing strain and disorder, which is potentially useful to design electronic and spintronic devices in complex oxides systems.Open in a separate windowFig. S4.MFM images of 300-nm disks taken at different temperatures and magnetic fields.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Pelvic girdle and fin of Tiktaalik roseae

A major challenge in understanding the origin of terrestrial vertebrates has been knowledge of the pelvis and hind appendage of their closest fish relatives. The pelvic girdle and appendage of tetrapods is dramatically larger and more robust than that of fish and contains a number of structures that provide greater musculoskeletal support for posture and locomotion. The discovery of pelvic material of the finned elpistostegalian, Tiktaalik roseae, bridges some of these differences. Multiple isolated pelves have been recovered, each of which has been prepared in three dimensions. Likewise, a complete pelvis and partial pelvic fin have been recovered in association with the type specimen. The pelves of Tiktaalik are paired and have broad iliac processes, flat and elongate pubes, and acetabulae that form a deep socket rimmed by a robust lip of bone. The pelvis is greatly enlarged relative to other finned tetrapodomorphs. Despite the enlargement and robusticity of the pelvis of Tiktaalik, it retains primitive features such as the lack of both an attachment for the sacral rib and an ischium. The pelvic fin of Tiktaalik (NUFV 108) is represented by fin rays and three endochondral elements: other elements are not preserved. The mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives.At first glance, the origin of tetrapods (limbed vertebrates) from finned precursors seems an almost insurmountable transition between life in water and life on land. If the basis of comparison were living taxa alone, then the anatomical and behavioral differences among finned and limbed vertebrates could appear vast: for example, fin structure and function differ dramatically from those of limbs. Fossil evidence, in particular vertebrates from the middle and late part of the Devonian period (393–359 Mya), offers intermediate conditions that bridge this gap (1). The fossils that provide the most informative anatomical intermediates are from the tetrapodomorph lineage (also known as stem tetrapods) and have been recovered from a variety of nonmarine and marginal marine deposits from around the globe (2–4). The creatures closest to the node containing the most basal limbed vertebrates—elpistostegalids, such as Panderichthys, Tiktaalik, and Elpistostege—are most enlightening in understanding the primitive conditions from which tetrapods arose. Although most work has focused on revealing homologies and function of the pectoral appendage of these forms (4–7), relatively little is known of the pelvic appendage beyond limited material of Panderichthys (8). Consequently, analyses of the pelvic fin have been given only sporadic attention over the past decades (4, 8–11) largely because they are often poorly preserved or not preserved at all. In most cases, it is thought that this poor preservation of the pelvic appendage is due to its putative small size and fragility (10).Pelves, and in some cases pelvic appendages, of taxa that span the fin-to-limb transition are known from Gooloogongia (Rhizodontida) (12), Eusthenopteron (Osteolepida) (10, 13), Panderichthys (Elpistostegalia) (7, 8), and Acanthostega and Ichthyostega (Tetrapoda) (14–17). Comparisons of these forms reveal large differences between the pelvic appendages of finned tetrapodomorphs and tetrapods (Fig. 1). Most noticeable is that, in finned taxa, the entire pelvic appendage is significantly smaller than the pectoral. In particular, the pelvic girdle of finned tetrapodomorphs is diminutive relative to the pectoral: the pelvis represents a small fraction of the length of the body (the maximum length of pelvis-to-body length is 1:20 in Eusthenopteron per ref. 10). In addition, there are major differences in the morphology of the pelvic girdles of finned and limbed taxa. The girdles of Eusthenopteron and Gooloogongia have posteriorly facing acetabulae and lack sacral ribs and ischial bones, among other features (10, 12). Unfortunately, the pelvic girdle of Panderichthys is not preserved in sufficient detail to understand the distribution of these morphological features in elpistostegalids (8). However, the best comparisons available from these data strongly supported the hypothesis that the closest finned relatives of tetrapods were “front wheel drive animals,” possessing enlarged pectoral fins, robust pectoral girdles, and relatively small pelvic appendages that were incapable of providing extensive degrees of body support and propulsion.Open in a separate windowFig. 1.Right pelves of Gooloogongia (A), Eusthenopteron (B), and Acanthostega (C) in lateral view. Gooloogongia is preserved as a natural cast in one orientation. Figures were modified from refs. 10, 12, and 14. Cranial is to the right.Previously undescribed material of the stem tetrapod, Tiktaalik roseae, can inform these issues. The type specimen (NUFV108), recovered in 2004 and described in 2006 (5, 6), has since been revealed to contain a partial pelvic appendage, including the right side of the pelvic girdle and an incomplete pelvic fin consisting of endochondral bones and lepidotrichia (Figs. 2 and ​and3).3). This specimen allows direct comparison of the relative size of the pelvic girdle and appendage with the rest of the body because the type consists of a relatively articulated skeleton from head to pelvis. In addition, work at the same site in Nunavut Territory during 2006, 2008, and 2013 has revealed additional isolated pelves of four other individuals. Together, these specimens offer the possibility to test the front wheel drive hypothesis and provide insights into the sequence in the acquisition of tetrapod pelvic appendage structure and locomotor function.Open in a separate windowFig. 2.Type specimen (NUFV108): ventral surface of cranial block (figured in ref. 6) aligned in preserved position with ventral view of the block containing the pelvic fin. (Inset) Line diagram of lepidotrichia and preserved portions of endochondral bones of pelvic fin. f, fin; i, intermedium?; l, lepidotrichia; r, radials.Open in a separate windowFig. 3.Tiktaalik roseae, stereopairs of the right pelvis from NUFV108 in (A) ventral (cranial is to the Top), (B) dorsal (cranial is to the Bottom), (C) caudal (lateral is to the Right), and (D) cranial views (lateral is to the Left). A, acetabulum; i, ilium; p, pubis; r, ossified ridge; u, unfinished bone.     

Drought,agricultural adaptation,and sociopolitical collapse in the Maya Lowlands

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800–950 C.E.). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands but does not explain more pronounced and earlier societal disruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ∼200 C.E. to 500 C.E., during the Terminal Preclassic and Early Classic periods. Plant wax carbon isotope records indicate a decline in C₄ plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water-conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.The decline of the lowland Classic Maya during the Terminal Classic period (800–900/1000 C.E.) is a preeminent example of societal collapse (1), but its causes have been vigorously debated (2–5). Paleoclimate inferences from lake sediment and cave deposits (6–11) indicate that the Terminal Classic was marked by a series of major droughts, suggesting that climate change destabilized lowland Maya society. Most evidence for drought during the Terminal Classic comes from the northern Maya Lowlands (Fig. 1) (6–8, 10), where societal disruption was less severe than in the southern Maya Lowlands (12, 13). There are fewer paleoclimate records from the southern Maya Lowlands, and they are equivocal with respect to the relative magnitude of drought impacts during the Terminal Classic (9, 11, 14). Further, the supposition that hydrological impacts were a primary cause for societal change is often challenged by archaeologists, who stress spatial variability in societal disruption across the region and the complexity of human responses to environmental change (2, 3, 12). The available paleoclimate data, however, do not constrain possible spatial variability in drought impacts (6–11). Arguments for drought as a principal cause for societal collapse have also not considered the potential resilience of the ancient Maya during earlier intervals of climate change (15).Open in a separate windowFig. 1.Map of the Maya Lowlands indicating the distribution of annual precipitation (64) and the location of paleoclimate archives discussed in the text. The locations of modern lake sediment and soil samples (Fig. 2) are indicated by diamonds.For this study, we analyzed coupled proxy records of climate change and ancient land use derived from stable hydrogen and carbon isotope analyses of higher-plant leaf wax lipids (long-chain n-alkanoic acids) in sediment cores from Lakes Chichancanab and Salpeten, in the northern and southern Maya Lowlands, respectively (Fig. 1). Hydrogen isotope compositions of n-alkanoic acids (δD_wax) are primarily influenced by the isotopic composition of precipitation and isotopic fractionation associated with evapotranspiration (16). In the modern Maya Lowlands, δD_wax is well correlated with precipitation amount and varies by 60‰ across an annual precipitation gradient of 2,500 mm (Fig. 2). This modern variability in δD_wax is strongly influenced by soil water evaporation (17), and it is possible that changes in potential evapotranspiration could also impact paleo records. Accordingly, we interpret δD_wax values as qualitative records of water availability influenced by both precipitation amount and potential evapotranspiration. These two effects are complementary, since less rainfall and increased evapotranspiration would lead to both increased δD_wax values and reduced water resources, and vice versa.Open in a separate windowFig. 2.Scatter plot showing the negative relationship between annual precipitation and δD_wax-corr measured in modern lake sediment and soil samples (Fig. 1). Results from Lake Chichancanab (CH) and Salpeten (SP) are indicated. The black line indicates a linear regression fit to these data, with regression statistics reported at the bottom of the plot. Large squares indicate mean values for each sampling region, with error bars indicating SEM in both δD_wax-corr and annual precipitation. The black error bar indicates the 1σ error for δD_wax-corr values (SI Text). Original δD_wax data from ref. 17. VSMOW, Vienna Standard Mean Ocean Water.Plant wax carbon isotope signatures (δ¹³C_wax) in sediments from low-elevation tropical environments, including the Maya lowlands, are primarily controlled by the relative abundance of C₃ and C₄ plants (18–20). Ancient Maya land use was the dominant influence on the relative abundance of C₃ and C₄ plants during the late Holocene, because Maya farmers cleared C₃ plant-dominated forests and promoted C₄ grasses, in particular, maize (21–24). Thus, we apply δ¹³C_wax records as an indicator of the relative abundance of C₄ and C₃ plants that reflects past land use change (SI Text). Physiological differences between plant groups also result in differing δD_wax values between C₃ trees and shrubs and C₄ grasses (16), and we use δ¹³C_wax records to correct for the influence of vegetation change on δD_wax values (25) (δD_wax-corr, SI Text and Fig. S1).Plant waxes have been shown to have long residence times in soils in the Maya Lowlands (26). Therefore, age−depth models for our plant wax isotope records are based on compound-specific radiocarbon ages (Fig. 3), which align our δD_wax records temporally with nearby hydroclimate records derived from other methodologies (26) (SI Text and Fig. S2). The mean 95% confidence range for the compound-specific age−depth models is 230 y at Lake Chichancanab and 250 y at Lake Salpeten. Given these age uncertainties, we focus our interpretation on centennial-scale variability (26). The temporal resolution of our plant wax isotope records is lower than speleothem-derived climate records (8, 9), but combining plant wax records from multiple sites allows comparisons of climate change and land use in the northern and southern Maya Lowlands, which would otherwise not be possible. In addition, plant wax isotope records extend to the Early Preclassic/Late Archaic period (1500–2000 B.C.E.), providing a longer perspective on climate change in the Maya Lowlands than most other regional records (6, 8–11).Open in a separate windowFig. 3.Plant wax (green; left) and terrigenous macrofossil (red; right) age−depth models for (A) Lake Chichancanab and (B) Lake Salpeten. The age probability density of individual radiocarbon analyses is shown. The black lines indicate the best age model based on the weighted mean of 1,000 age model iterations (62). Colored envelopes indicate 95% confidence intervals. Cal, calendar.     

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet–visible (UV–Vis) absorption bands. Bulky ligands on the metal suppress π–π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV–Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Conjugated polymers are macromolecules usually featuring a backbone chain with alternating double and single bonds (1–3). These characteristics allow the overlapping p-orbitals to form a system with highly delocalized π-electrons, thereby giving rise to intriguing chemical and physical properties (4–6). They have exhibited many applications in organic light-emitting diodes, organic thin film transistors, organic photovoltaic cells, chemical sensors, bioimaging and therapies, photocatalysis, and other technologies (7–10). To facilitate the use of solar energy, tremendous efforts have been devoted in recent decades to developing previously unidentified conjugated polymers exhibiting broad and strong absorption bands (11–13). The common strategies for increasing absorption involve extending π-conjugation by incorporating conjugated cyclic moieties, especially fused rings; modulating the strength of intramolecular charge transfer between donor and acceptor units (D–A effect); increasing the coplanarity of π conjugation through weak intramolecular interactions (e.g., hydrogen bonds); and introducing heteroatoms or heavy atoms into the repeat units of conjugated polymers (11–16). Additionally, appropriate solubility is a prerequisite for processing and using polymers and is usually achieved with the aid of long alkyl or alkoxy side chains (12, 17).Aromatic rings are among the most important building blocks for conjugated polymers. In addition to aromatic hydrocarbons, a variety of aromatic heterocycles composed of main-group elements have been used as fundamental components. These heteroatom-containing conjugated polymers show unique optical and electronic properties (4–10). However, while metalla-aromatic systems bearing a transition metal have been known since 1979 due to the pioneering work by Thorn and Hoffmann (18), none of them have been used as building blocks for conjugated polymers. The HOMO–LUMO gaps (E_g) of metalla-aromatics are generally narrower (Fig. 1) than those of their organic counterparts (19–22). We reasoned that this feature should broaden the absorption window if polymers stemming from metalla-aromatics are achievable.Open in a separate windowFig. 1.Comparison of traditional organic skeletons with metalla-aromatic building blocks (the computed energies are in eV). (A) HOMO–LUMO gaps of classic aromatic skeletons. (B) Carbolong frameworks as potential building blocks for novel conjugated polymers with broad absorption bands and improved solubility.In recent years, we have reported a series of readily accessible metal-bridged bicyclic/polycyclic aromatics, namely carbolong complexes, which are stable in air and moisture (23–25). The addition of osmium carbynes (in carbolong complexes) and alkynes gave rise to an intriguing family of d_π–p_π conjugated systems, which function as excellent electron transport layer materials in organic solar cells (26, 27). These observations raised the following question: Can this efficient addition reaction be used to access metalla-aromatic conjugated polymers? It is noteworthy that incorporation of metalla-aromatic units into conjugated polymers is hitherto unknown. In this contribution, we disclose a polymerization reaction involving M≡C analogs of C≡C bonds, which involves a unique carbyne shuttling strategy (Fig. 2A). This led to examples of metalla-aromatic conjugated polymers (polycarbolongs) featuring metal carbyne units in the main chain. On the other hand, the development of polymerization reactions plays a crucial role in involving certain building blocks in conjugated polymers (28–32). These efficient, specific, and feasible polymerizations could open an avenue for the synthesis of conjugated polymers.Open in a separate windowFig. 2.Design of polymers and synthesis of monomers. (A) Schematic illustration of the polymerization strategy. (B) Preparation of carbolong monomers. Insert: X-ray molecular structure for the cations of complex 3. Ellipsoids are shown at the 50% probability level; phenyl groups in PPh₃ are omitted for clarity.     

Late Miocene episodic lakes in the arid Tarim Basin,western China

The Tibetan Plateau uplift and Cenozoic global cooling are thought to induce enhanced aridification in the Asian interior. Although the onset of Asian desertification is proposed to have started in the earliest Miocene, prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources, is only a geologically recent phenomenon. Here we report episodic occurrences of lacustrine environments during the Late Miocene and investigate how the episodic lakes vanished in the basin. Our oxygen isotopic (δ¹⁸O) record demonstrates that before the prevailing desert environment, episodic changes frequently alternating between lacustrine and fluvial-eolian environments can be linked to orbital variations. Wetter lacustrine phases generally corresponded to periods of high eccentricity and possibly high obliquity, and vice versa, suggesting a temperature control on the regional moisture level on orbital timescales. Boron isotopic (δ¹¹B) and δ¹⁸O records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at ∼4.9 million years ago (Ma), permanently and irreversibly. Although the episodic occurrences of lakes appear to be linked to orbitally induced global climatic changes, the plateau (Tibetan, Pamir, and Tianshan) uplift was primarily responsible for the final vanishing of the episodic lakes in the Tarim Basin, occurring at a relatively warm, stable climate period.Once part of the Neo-Tethys Sea, indicated by the Paleogene littoral-neritic deposits with intercalated marine strata (1), the Tarim Basin in western China (Fig. 1) has experienced dramatic environmental and depositional changes during the Cenozoic. The eventual separation of the basin from the remnant sea probably occurred during the middle to late Eocene (1, 2), and since then, terrestrial sedimentation and environment have prevailed. Today, the basin has relatively flat topography with elevation ranging between 800 and 1,300 m above sea level (asl), surrounded by high mountain ranges with average elevation exceeding 4,000 m. Hyperarid climate prevails with mean annual precipitation <50 mm and evaporation ∼3,000 mm. Active sand dunes occupy 80% of the basin, forming the Taklimakan Desert, the largest desert in China and the second largest in the world (3). Thick desert deposits in the basin also provide a major source for dust storms occurring in East Asia (4).Open in a separate windowFig. 1.Digital elevation model of the Tarim Basin and surrounding mountain ranges. The studied 1,050-m sediment core is retrieved from Lop Nor (1), at a relatively low elevation (∼800 m asl) in the eastern basin. Also indicated are locations of previous studied sections, near Sanju (2), Kuqa (3), and Korla (4) in the basin.The Tibetan Plateau uplift, long-term global cooling, and the associated retreat of the remnant sea during the Cenozoic, through their complex interplay, may have all contributed to the enhanced aridification in the Asian interior and eventually the desert formation (2, 5–11). Although the onset of Asian desertification is proposed to have started in the earliest Miocene (12, 13), prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources (4), is only a geologically recent phenomenon (9). Previous lithological and pollen studies (9, 14, 15) suggest that the currently prevailing desert environment started probably at ∼5 Ma. However, what kind of environmental conditions prevailed before that and how the dramatic changes occurred largely remain elusive.To better decipher the aridification history, we used a 1,050-m-long, continuous sediment core retrieved from Lop Nor (39°46''0''''N, 88°23''19''''E) in the eastern basin (Fig. S1). The core site has a relatively low elevation (∼800 m asl) (Fig. 1) and thus effectively records basin environment. Previous study sites came from elevated basin margins (9, 14). The core mainly consists of lacustrine sediments with associated fluvial-eolian sands (Fig. 2). The core chronology was established previously based on paleomagnetic polarity (15). The total 706 remanence measurements on the continuous sediment profile allow straightforward correlation with the CK95 geomagnetic polarity timescale (16) and identification of 14 normal (N1–N14) and 13 reversal (R1–R13) polarity zones over the last 7.1 Ma (Fig. S1). The CK95 timescale is largely consistent (within a few ky) with that inferred from marine archives (17) over the last 5.23 Ma, and before 5.23 Ma a practical measure of chronological uncertainty was estimated to be within ∼100 ky (18), assuring that the terrestrial records can be directly compared with marine records and orbital changes at the timescale of >100 ky. The derived chronology yields an average sedimentation rate of ∼200 m/Ma at lower sections (>1.77 Ma), allowing high-resolution studies of the desertification history at the critical interval.Open in a separate windowFig. 2.Records of δ¹¹B, δ¹⁸O, TOC, and CaCO₃ changes in the Lop Nor profile. Representative photos show lithological changes mainly from lacustrine bluish gray argillaceous limestone to fluvial-eolian brown/red clayey silt, as shown in the lithological column with visual colors indicated (15). High levels and large fluctuations of proxy values occurred only before ∼4.9 Ma, and since then, those remain low and within a small range, largely similar to modern conditions. The transition from episodic occurrences of lacustrine phases to prevailing desert environments thus appears to be permanent and irreversible.Boron and oxygen isotopes from carbonates are important environmental indicators (19–25) and used here to infer the environmental evolution in the Tarim Basin. We also present total organic carbon (TOC), calcium carbonate (CaCO₃), ostracod, and grain size records to substantiate the isotopic evidence (SI Materials and Methods). Our δ¹¹B profile shows substantial, stepwise changes over the last ∼7.1 Ma (Fig. 2). Carbonate δ¹¹B values were −5.0 ± 1.6‰ (n = 3) before 6.0 Ma, increased rapidly to ∼11‰ at 4.9–6.0 Ma, and then stayed at roughly the same level (10.7 ± 2.2‰, n = 25) for the remaining 4.9 Ma. Higher-resolution δ¹⁸O, TOC, and CaCO₃ profiles generally confirm the pattern observed in the low-resolution δ¹¹B one (Fig. 2). The δ¹⁸O values remained low, ranging from −10‰ to −4‰ over the last 4.9 Ma. However, δ¹⁸O values frequently oscillated between −10‰ and 5‰ before that. Similarly, the TOC profile shows consistently low organic carbon content (0–0.2%) after 4.9 Ma and large fluctuations (0–1.0%) before then. The CaCO₃ profile also indicates consistently low values (0–25%) after 4.9 Ma and large fluctuations (0–50%) earlier (Fig. 2).The multiple proxy records strongly suggest that critical environmental changes must have occurred at ∼4.9 Ma. δ¹¹B values of carbonates from marine sources differ substantially from those of nonmarine carbonates (19–21). δ¹¹B values after 4.9 Ma are close to those from marine carbonates, but values before 6 Ma fall into the range of lacustrine carbonates (22). Positive δ¹⁸O values before 4.9 Ma also indicate lacustrine environments at that time. Carbonates from modern lakes in arid and semiarid regions of northwestern China show similar positive δ¹⁸O values (23), due to strong evaporation processes. High TOC and CaCO₃ contents (Fig. 2) further support that lacustrine environments existed in the basin before ∼4.9 Ma. δ¹⁸O values after 4.9 Ma are comparable to those in Cenozoic soil carbonates (24) and ancient marine carbonates in the Tarim Basin (25). However, the accompanying carbonate δ¹³C values throughout the record, ranging from −4‰ to 1‰ (Dataset S1), are significantly higher than those from Cenozoic soil carbonates reported (26), essentially ruling out the possibility of soil carbonate source. Using modern prevailing desert environment in the basin as an analog, the combined δ¹¹B and δ¹⁸O evidence thus suggests that the sediment deposits in the basin after 4.9 Ma must be eolian-fluvial in origin and their sources, at least carbonate grains, came from weathered ancient marine carbonates in nearby regions.Sedimentological and stratigraphic patterns in other exposed sections from different parts of the basin (9, 14) share great similarity with the Lop Nor core profile (Fig. S1). Episodic lacustrine mudstones and/or siltstones during the Late Miocene were present in all sections and were replaced by fluvial-eolian deposits later. Studies of ostracod assemblages (27) also suggest a shallow paleolake with brackish water environments in the northern basin during the Late Miocene. Changes in the depositional environment from our Lop Nor profile alone could be plausibly explained by a shift in basin center due to tectonic compressions, as evidenced from the slightly uplifted central basin (Fig. 1). However, similar temporal changes occurring basin-wide at ∼4.9 Ma argue against it. Instead, our results, together with previous studies (5, 14, 15, 27), suggest that paleolakes were widely present in the low lands of the basin during the Late Miocene, much different from currently prevailing desert environments with a few scattered small lakes. The existing evidence, although still limited (Fig. 1), would point to the occurrence of a possible megalake in the Tarim Basin during the Late Miocene.Three high-resolution records, δ¹⁸O, TOC, and CaCO₃, further suggest that lacustrine environments before ∼4.9 Ma were not permanent (Fig. 2). These large fluctuations indicate frequent switches between lacustrine and fluvial-eolian environments in the basin. High proxy values, δ¹⁸O in particular, appear to indicate lacustrine environments, whereas low values, similar to ones after 4.9 Ma, correspond to fluvial-eolian deposits. This is consistent with lithological features at this interval, showing argillaceous limestone intercalated with clayey layers (15), the occurrence of ostracod assemblages (Fig. 3) from lacustrine sediments, grain size changes (Fig. S2), and detrital carbonate grains identified in photomicrographs of fluvial-eolian deposits (Fig. S3).Open in a separate windowFig. 3.δ¹⁸O fluctuations linked to eccentricity and obliquity orbital variations at 4.5–7.1 Ma. Lacustrine phases (high δ¹⁸O) generally correspond to periods of high eccentricity and obliquity. Fluvial-eolian environment (low δ¹⁸O, highlighted with gray bars), developed more around 6.5, 6.1, 5.7, and 5.2 Ma, at a ∼400-ky eccentricity beat. The red bar indicates the last occurrence of lacustrine environment at ∼4.9 Ma. Occurrences of ostracod assemblages with total number >100 are also indicated.To further investigate such episodic changes, we performed spectral analysis on the δ¹⁸O record over the interval 4.5–7.1 Ma. Strong spectral power at orbital frequencies were identified, with periods of ∼400 ky throughout the interval, ∼41 ky particularly at 6–6.5 Ma, and ∼100 ky at 5–6 Ma and 6.5–7.1 Ma (Fig. S4). Precessional ∼20-ky power might also have existed but was relatively weak and discontinuous. Lacustrine phase as indicated by high δ¹⁸O values and ostracod assemblages generally occurred at periods of high eccentricity and obliquity (28) (Fig. 3). At 6–6.5 Ma, δ¹⁸O shows clear correspondence to orbital obliquity variation (Fig. 3A). Additionally, the number of low δ¹⁸O values (fluvial-eolian environment) occurred more around 6.5, 6.1, 5.6, and 5.2 Ma, at a ∼400-ky beat following orbital eccentricity variation (Fig. 3B). The cluster of high δ¹⁸O values (>0‰) at 4.9–5.0 Ma signals the last occurrence of lacustrine environments in the basin. Our orbital association thus allows us to precisely determine the timing of desert formation at ∼4.9 Ma, ∼400 ky (an eccentricity cycle) later than the age inferred from basin margins (9, 14) and yet all occurring at eccentricity minima (Fig. 3B). As high eccentricity and obliquity generally correspond to warm conditions at orbital timescales, lacustrine (wet) phase could be associated with warm periods, consistent with the notion that cooler conditions would reduce moisture in the atmosphere and enhance continental drying (10, 11). We recognize that the chronological uncertainty from the geomagnetic polarity timescale before 5.23 Ma, within ∼100 ky (18), could confound our association of wet phase with high obliquity, although it is unlikely affected at the 400-ky eccentricity beat. However, the opposite association, wet phase with low obliquity, and the combination with high eccentricity, would require a different, yet unknown mechanism that is inconsistent with the orbital theory of Pleistocene ice ages.Superimposed on the orbitally episodic changes, the δ¹⁸O record also shows a long-term trend of deteriorating lacustrine conditions at 4.9–7.1 Ma. As δ¹⁸O values indicate two depositional environments, lacustrine and fluvial-eolian, the range of δ¹⁸O changes (between −10‰ and 5‰) does not vary much over this period (Fig. 3). Rather, the duration of high δ¹⁸O vs. low values would reflect the long-term trend. The mean δ¹⁸O values over 40-ky and 400-ky intervals both show a decreasing trend, with dominant lacustrine phase before 6.1 Ma, more developed fluvial-eolian environment at 5.7–6.1 Ma, a return to slightly better lacustrine environment at 5.3–5.7 Ma, and lacustrine phase permanently vanished around 4.9 Ma (Fig. 4).Open in a separate windowFig. 4.Long-term δ¹⁸O changes compared with global climatic conditions and regional tectonic activities. Global benthic δ¹⁸O (29) and northwestern Pacific SST (30) records show minimal long-term changes at 4–7 Ma, whereas occurrences of detrital apatite fission track ages (35) in northern western Kunlun peaked, and the Tarim episodic lakes gradually vanished. The dark thick lines are their 400-ky running means, and the light green line on δ¹⁸O represents the 40-ky running mean.The gradual disappearance of the Tarim episodic lakes could be potentially explained by the two driving forces, plateau uplift (the Tibetan Plateau, Pamir Plateau, and Tianshan) and long-term global cooling. However, the long-term global climate was relatively warm and stable during this period (Fig. 4). The global benthic δ¹⁸O record (29) shows that much of the Miocene cooling occurred between 15 and 11 Ma, and the cooling between 8 and 5 Ma was minimal. Supporting this view, sea surface temperature records from the northwestern Pacific (30) show that almost no cooling occurred between 6 and 4 Ma (perhaps further to 3 Ma) (Fig. 4). Particularly, the mean global climate (29, 30) was even warmer at 4.1–4.5 Ma than at 5.7–6.1 Ma, whereas lacustrine phase permanently disappeared after ∼4.9 Ma (Fig. 4), indicating decoupling of the lake evolution from global climate. Therefore, long-term global cooling might have played a subordinate role in the lake disappearance.Instead, the growth of surrounding mountain ranges (Tibetan, Pamir, and Tianshan) may have blocked moisture from the west and south, changed air circulations, and eventually led to the permanent lake disappearance within the basin. Today, the Tarim Basin receives limited moisture from westerlies (31) through the Pamir and Tianshan Ranges (and perhaps from the Indian Ocean in summertime as well). Although the Indo–Eurasian convergence since the Late Eocene resulted in high elevations of the Tibetan Plateau and, to a lesser degree, surrounding mountains including Pamir and Tianshan by the mid-Miocene time (8), tectonic activities in broad areas around the Tarim Basin appear to be rejuvenated since the Late Miocene. Tectonic deformations during the Late Miocene–Early Pliocene inferred from growth strata, sedimentary facies changes, and low-temperature thermochronologic studies occurred in Tianshan to the north of the Tarim Basin, in the Kunlun Mountains to its south and the Pamir to its west (32–35). Syntectonic growth strata from the foreland basins of the Kunlun and Tianshan Ranges (32) show that strong crust shortening and potential mountain uplift initiated ∼6.5–5 Ma and lasted to the Early Pleistocene (Fig. S5), similarly reported in northern Pamir (33). Cenozoic sequences in the Pamir–Tianshan convergence zone, changing from an arid continental plain to an intermountain basin by ∼5 Ma, support surface uplift of the west margin of the Tarim Basin (34). Occurrence of detrital apatite fission track ages from West Kunlun Ranges (35) also peaked at ∼4.5 Ma (Fig. 4). Thus, the reactivated uplift of Pamir and West Kunlun Ranges and northward movement of Pamir during the Late Miocene–Pliocene would progressively block moisture into the basin and enhance regional aridity to a certain threshold to terminate lacustrine environments even during warm periods with favorable orbital configuration. Although inconsistencies indeed exist in linking regional climatic and environmental changes to tectonic events (8), the final vanishing of the Tarim episodic lakes is better explained by tectonic factors.Therefore, our multiple-proxy results consistently show that the Taklimakan sand sea began to form at ∼4.9 Ma and that the transition into prevailing desert environment was permanent and irreversible. The episodic occurrences of lacustrine environments at favorable climatic conditions (warm periods) during the Late Miocene suggest that uplifted mountain ranges then were not high enough to effectively block moisture from being transported into the basin. The transition from episodic lacustrine environments to prevailing desert deposits was gradual, from 7.1 Ma (or earlier beyond our record) to 4.9 Ma, for which we suggest that although long-term global cooling enhanced the overall aridification in the Asian interior during the Late Cenozoic (10, 11), plateau uplift played a more important role in finally drying up the episodic lakes within a relatively warm, stable climate period, thus decoupling regional climate temporally from a global trend. Our high-resolution records thus demonstrate that regional climate in the Tarim Basin reached a critical state in the Late Miocene, with the dual effects from global climate conditions and regional tectonic settings then. With global climate remaining relatively stable and warm entering the Pliocene, by ∼3–4 Ma (Fig. 4), drying up of episodic lakes at ∼4.9 Ma could thus be largely attributed to rejuvenated tectonic activities.     

Groundwater sapping as the cause of irreversible desertification of Hunshandake Sandy Lands,Inner Mongolia,northern China

In the middle-to-late Holocene, Earth’s monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation–atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China’s Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.Earth’s climate is subject to abrupt, severe, and widespread change, with nonlinear vegetation–atmosphere feedbacks that produced extensive and catastrophic ecosystem shifts and subsequent cultural disruption and dispersion during the Holocene (1–7). In the early and middle Holocene, northern China’s eastern deserts, including much of the currently sparsely vegetated and semistabilized Hunshandake (Figs. 1 and ​and2),2), were covered by forests (8), reflecting significantly wetter climate associated with intensification of monsoon precipitation by up to 50% (6).Open in a separate windowFig. 1.Geographical location of the Hunshandake Sandy Lands (A) and its area (encircled by red line in B). The black rectangle in B marks the location of the enlarged maps C and D on the Right, and the green rectangle shows the location of Fig. 2. Map C shows the localities of water samples, and map D shows the localities of sections with stratigraphy presented in Fig. 3. The sand–paleosol section P (Fig. 3) is on the southern margin, and the site Bayanchagan marks the coring site of ref. 8. Rivers with headwaters in the Hunshandake likely formed by groundwater sapping are marked in blue. Drainages to the southwest and west are currently undergoing groundwater sapping, with substantial spring-driven flow found at the current river base level.Open in a separate windowFig. 2.(Left) Holocene lakes and channels in the Hunshandake and lake extent at selected epochs. Upper, middle, and lower lakes are indicated by points A, B, and C, respectively. Xilamulun River (point D) drains to the east. Groundwater-sapping headcuts at the upper reaches of incised canyons (point E) suggest a mid-Holocene interval of easterly surface flow, followed by groundwater drainage beginning at the ca. 4.2 ka event. Northern and central channels at point E are currently abandoned, and groundwater sapping has migrated to the southerly of the three channels shown. (Right) Cross-sections of the predrainage shift, northerly drainage into Dali Lake (Localities shown on the Left), showing the increase in widths of channels downstream (Vertical exaggeration ∼30:1).Monsoonal weakening, in response to middle-to-late Holocene insolation decrease, reduced precipitation, leading to a green/sandy shift and desertification across Inner Mongolia between ca. 5,000 and 3,000 y (years) ago (6). However, variations in the timing of this transition (9, 10) suggest local/regional thresholds or possibly environmental tipping by stochastic fluctuations. The impacts of this wet-to-dry shift in the Hunshandake, expressed as variations in surface and subsurface hydrology coincident with the termination of the formation of thick and spatially extensive paleosols, and the impacts of a ca. 4.2 ka (1 ka = 1,000 years) mid-Holocene desiccation of the Hunshandake on the development of early Chinese culture remain poorly understood and controversial (6, 11). Here we report for the first time to our knowledge on variability in a large early-to-middle Holocene freshwater lake system in China’s Hunshandake Sandy Lands and associated vegetation change, which demonstrates a model of abrupt green/desert switching. We document a possible hydrologic trigger event for this switching and discuss associated vegetation and hydrologic disruptions that significantly impacted human activities in the region.     

From the Cover: Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna

Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.Three-dimensional photonic crystals (1–5), materials with two-dimensional micro- or nanosized periodic morphologies (6–8), and one-dimensional multilayer configurations (9) have been identified as the primary cause of structural coloration in a wide variety of nonrelated biological organisms. In contrast, surface-confined diffraction elements for the separation of incident light into specific colors are less abundant in nature and have only been discovered in a handful of organisms (10), including a fossil polychaete (7), sea mouse Aphrodita sp. (6), and some flowering plants (11). Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the scales of the butterfly Pierella luna (12).Inspired by this biological light manipulation strategy, we devised an artificial material morphology mimicking the butterfly’s diffraction effect by creating periodic arrays of vertically oriented individual microdiffraction gratings. In addition to the butterfly-inspired reverse color-order diffraction arising from each individual micrograting, the periodicity between the individual gratings causes diffraction on a different length scale, leading to complex intensity distributions in experimentally measured angularly resolved reflection spectra. An in-depth analysis of the observed diffraction phenomenon complemented by optical modeling revealed a strong dependence of the optical signature on the orientation of the gratings. Such an effect can only be seen because of the hierarchical nature of the superposed, orthogonal grating features. To further elucidate the role of the different structural components for the emerging reflection spectra, the initially vertically oriented individual microgratings were subjected to a tilt, resulting in a predictable change of the surface''s optical signature.The dorsal side of the fore- and hind wings of P. luna males are dull brown in diffuse ambient illumination (Fig. 1A, Left). When exposed to directional illumination at grazing incidence, a coin-sized spot on each fore wing displays an angle-dependent color variation across the whole visible spectrum (Fig. 1A, Right). The color changes from red to blue with increasing observation angle, unlike the variation from blue to red normally observed in conventional diffraction gratings (13). This reverse color diffraction effect results from the local morphology of individual scales within the colored spot on the fore wings (12). The top parts of the scales are curled upward, orienting lines of periodically arranged cross-ribs perpendicular to the wing surface. (Fig. 1 C and D). Light incident at an angle onto the curled parts of the scales is diffracted by the cross-rib structure acting as a diffraction grating, with a periodicity of ∼400 nm. The alignment of the grating perpendicular to the surface results in the reverse color sequence that can be observed in angularly resolved reflection spectra and in the diffraction pattern (Fig. 1 E and F).Open in a separate windowFig. 1.Optical properties of the curled scales in butterfly P. luna. (A) Optical image of P. luna under diffuse lighting (Left) and directional lighting at grazing incidence (Right). Scale bar, 10 mm. (B) Optical micrograph of P. luna scales under oblique illumination. Scale bar, 50 µm. (C) SEM of scales in the colored wing region. Scale bar, 50 µm. White dashed boxes in B and C mark the curled tops of the scales from which the color originates. (D) Close-up image of the curled region of the scale from which the color originates. Scale bar, 20 µm (E) Gray-scale encoded reflection intensity as a function of wavelength and propagation direction showing the inverse color-order diffraction pattern for 65° light incidence. The red dashed line indicates the predicted location of the diffraction due to the cross-rib structures for an orientation of the curled scale sections of −25° relative to the surface normal and a cross-rib periodicity of 390 nm. The blue shaded region signifies the angle range for which the diffraction microscopy image of curled P. luna scales in F was obtained. The color bar under the graph shows the human-eye-perceived color for the spectra observed at the corresponding angles calculated by the CIE 1931 standards (14). (F) Diffraction microscopy image of the colored spot of a P. luna wing showing multiple diffracted orders in similar angular locations due to the variation in the position and angle of the diffracting scales.The identification of this unusual diffraction effect on the wings of P. luna (12) provided inspiration for the development of a bioinspired photonic system that incorporates vertically oriented microdiffraction gratings with submicrometer periodicity analogous to the key features observed in the natural structure. In addition, the artificial system displays a periodic arrangement of the individual vertical gratings in large arrays with 2D microscale periodicity. This structural feature, which is not found in the natural organism, enriches the optical signature of the artificial system via coupling of the diffractive modes of the two present hierarchical morphologies. In the following, we discuss the optical properties of the artificial system and demonstrate that the modification of either one of the grating morphologies changes the diffraction signature in predictable ways.The artificial system consists of an array of individual 10-µm-long, 2-µm-wide, and 18-µm-high plates arranged in rows with an interplate spacing of 10 µm and a separation of 5 µm between individual rows of plates (Fig. 2A). These parameters result in an overall periodicity of 12 µm in the direction perpendicular to the plates and 15 µm in the direction collinear with the plane of the plates and the sample surface. A periodic wave pattern—termed “scallops”—of ∼500-nm pitch runs along the sides of each individual plate (Fig. 2B).Open in a separate windowFig. 2.Geometry and optical properties of the artificial photonic structure mimicking P. luna with vertically oriented (Top) and tilted (Bottom) diffraction gratings. (A) SEM of the array of scalloped microplates. Scale bar, 5 µm. (B) SEM of an individual plate with regular scallops. Scale bar, 2 µm. (C) Variable-angle spectroscopic data for 45° light incidence showing the arc-shaped diffraction pattern caused by the diffraction from the scallops coupled with the diffraction due to the plates. (Inset, Top Right) Measurement geometry. The red overlay displays where the first diffraction order of the scallops based on the grating equation (Eq. 2) is expected for tilt angles of ‒3° to 5°. The color bar under the graph shows the human-eye-perceived color for the spectra observed at the corresponding angles calculated by the CIE 1931 standards (14). (D) Diffraction pattern caused by the periodic ensemble of microplates for 45° light incidence. A choice of propagation angles is visualized by the white dashed lines. The diffraction orders within the blue frame correspond to the diffraction observed in the angular range marked in blue in C. (E) Diffraction pattern resulting from the scallops on individual plates. The diffraction orders within the yellow frame correspond to the diffraction observed in the angular range marked in yellow in C. (F–K) Same as A–E, for tilted gratings. Red overlay in H marks the range where higher intensities are predicted by the grating equation for tilt angles between 19° and 27°.The bioinspired diffraction elements are fabricated in a double-molding procedure. Starting from a silicon master, a periodic array of scalloped microplates is first cast into polydimethylsiloxane (PDMS) to form a negative mold (15), which is then replicated with a UV-curable epoxy to produce a positive replica of the master silicon structure with the original scallops on the individual plates well preserved. The silicon master is formed using the Bosch process (16), in which multiple etching and passivation steps give rise to the periodic undulations on the microplate surface. The pitch and height of these grating structures can be controlled by adjusting the etching parameters (17, 18). Here, they are chosen to be comparable to the spacings and dimensions of the diffraction-inducing microribs on the P. luna scales (Fig. 2B), and hence are expected to cause a similar diffraction effect.Notice that in the biological system, the periodicity of the diffraction grating-supporting scales is of the order of 80 ± 10 µm along their length and 60 ± 10 µm perpendicular to the scale axis (Fig. 1B). Due to these large distances between diffraction elements and a nonnegligible amount of irregularity in the location of individual scales, no coherence is observed for light diffracted from adjacent scales. The overall color splitting only results from the diffraction caused by the cross-rib gratings on the individual scales, which is confirmed by variable-angle spectroscopy and by diffraction microscopy measurements (Fig. 1 D–F). Unlike in the biological system, the individual microdiffraction gratings in the artificial system are intentionally arranged in a highly periodic manner, which is expected to result in a richer diffraction signature and provide additional possibilities of tailoring the interaction of light beyond the diffraction induced by the scallops on the plates.Variable observation-angle spectroscopy performed on the artificial system serves to spectrally and angularly resolve parts of the complex diffraction pattern (Fig. 2C). For each measurement the plane of light incidence is chosen to be perpendicular to the surface of the individual microplates. The light incidence angle θ_I is fixed and the observation angle θ_D is varied in the plane of light incidence to capture light reflected in an angular range of ±75° around the sample surface normal. Two main features are observed in these measurements:

• i)Straight lines of higher intensity resulting from diffraction caused by the interplate periodicity represent the individual diffraction orders; the experimentally observed locations of these diffraction orders (shown in Fig. 2 C and H) can be directly calculated using the grating equation (13). An example of such a calculation can be seen in Fig. S1. Due to the large interplate pitch of 12 µm, the angular separation between adjacent diffraction orders and the free spectral range of each individual order are small. For light incident at an angle θ_I = 45°, eight positive propagating diffraction orders [left of the zeroth order at sin(θ) = ‒0.71 in Fig. 2C] and 53 negative propagating orders are captured with the highest intensity in the direct reflection (zeroth order) and in the adjacent orders.
• ii)There is an arc-shaped distribution of intensity maxima across different diffraction orders (emphasized by the red shaded region in Fig. 2C).

This anomalous redistribution of light in the diffraction orders is caused by the scallops on each individual plate. To properly describe the diffraction resulting from the microdiffraction gratings oriented normally to the substrate, the grating equation (13) has to be reformulated taking into account their vertical orientation for diffraction in the plane of the scallops:λ=dm(cos⁡θI+cos⁡θD),[1]where d is the grating periodicity, m is the diffraction order, θ_I is the light incidence angle, and θ_D is the diffraction angle. This equation describes the arc-shaped pattern observed in the experiment under the assumption that the plates are vertical (Fig. 2C, red shaded region).Imaging of the diffraction patterns by diffraction microscopy provides a direct visualization of the effects observed in the variable-angle spectroscopy measurements. The angle range that can be visualized in these measurements is determined by the numerical aperture of the microscope objective. For a given light incidence angle θ_I the diffraction caused by the periodic ensemble of plates is most clearly observed when collecting light with the objective''s axis aligned with the specular reflection direction θ_D = ‒θ_I (Fig. 2D, with the signal in the blue box corresponding to the blue shaded spectral range in Fig. 2C). Imaging of the sample''s diffraction in the Littrow mounting (13), where light is incident on the sample through the microscope objective, allows for the capturing of the diffraction component induced by the scallops on the individual plates (Fig. 2E, with the signal in the yellow box corresponding to the yellow shaded spectral range in Fig. 2C). This feature is easily distinguished from the diffraction of the plate ensemble by the wider color spread.Overall, the measurements provide clear evidence of coupling between the first-order mode of the scallop diffraction and the interplate periodicity-based diffraction modes. The diffraction coupling can be controlled by adjusting the pitch of the scallops on individual plates and the interplate geometry in the manufacturing procedure. Likewise, a change in inclination of the microdiffraction gratings with respect to the substrate is expected to be reflected in a variation of the diffraction patterns. This change can indeed be observed by imposing a controlled tilt of ∼20° on the microplates by shearing the sample (Fig. 2 F and G).The bending leaves the locations of the diffraction modes resulting from the interplate periodicity largely unaffected. Only minor wavelength-dependent intensity variations in each diffraction mode are apparent (Fig. 2 H and I). At large angles θ_D, measured from the sample surface normal, a region of higher reflection intensity appears, which is related to the inclination of the individual plates, i.e., the blaze of the grating array. By contrast, the diffraction caused by the scallops on the individual plates is strongly influenced by the variation in plate tilt angle shifting the observed arc-shaped pattern in wavelength and angular position (Fig. 2 H and K). For micrometer-sized diffraction gratings, which have a tilt angle β relative to the surface normal, the diffraction grating equation can be reformulated asλ=dm(cos(θI−β)+cos(θD−β)).[2]By calculating a fit for the arc-shaped intensity distribution across the different diffraction orders in the variable-angle spectroscopic data (Fig. 2H, red shaded region), the tilt angle of the plates relative to the surface normal is found to be β ∼ 23°, in agreement with the SEM image analysis.These findings are further supported by finite-difference time-domain simulations (FDTD) (19). By comparing the plates with and without scalloping, the arc-shaped intensity distribution across diffraction orders is clearly identified as the diffraction resulting from the scallops (Fig. 3). Simulations of 23° tilted plates with 45° incident illumination show that this arc-shaped diffraction pattern gets skewed and spectrally shifted as predicted by Eq. 2 and observed in the experiments; the influence of the blaze of the regular array of individual gratings, separate from the effect of the scallops, can also be seen by closely examining Fig. 3 and noting that the two tilted plate simulations have regions of high-intensity reflection at sin(θ) ∼ 0 whereas the upright plates do not. Furthermore, the simulations serve to predict and optimize the influence of variations in interplate geometry, scallop grating pitch and shape, and plate tilt angle before the manufacturing of the system.Open in a separate windowFig. 3.Modeling of the expected diffraction patterns originating from the ordered array of gratings with 45° illumination. (A) Calculated diffraction pattern for upright plates with scallops. (B) Calculated diffraction pattern for upright plates without scallops. (C) Calculated diffraction pattern for 23° tilted plates with scallops. (D) Calculated diffraction pattern for 23° tilted plates without scallops.Whereas the angular positions of the diffraction modes resulting from the interplate periodicity only vary with the angle of light incidence θ_I, for a given θ_I the diffraction pattern caused by the regular scallops on individual plates is strongly affected by a change in tilt of the plates relative to the sample normal. Consequently, a means of reversibly varying the plate tilt angle can provide the possibility to dynamically tune the diffraction pattern independent of light incidence. Possible means of achieving such reversible actuation include embedding the plates in a stimuli-responsive hydrogel (20–23), using a soft material negative of the structure and applying shear force to bend the microgratings, modifying the tips of the plates with ferromagnetic particles to allow for dynamic reconfiguration of the plate geometry using magnetic fields (24), or implementing tuning mechanisms shown for simple planar diffraction gratings that rely on electric fields or mechanical deformation (25–28).Biological strategies for light manipulation have already been successfully implemented in nanophotonic devices for applications in chemically selective vapor sensing, pH determination, infrared imaging, surface-enhanced Raman spectroscopy-based chemical analysis, and localized heating from infrared absorption (29–34). The rich and tunable optical signature of our hierarchical bioinspired diffraction-based photonic material platform could provide a basis for novel developments in biosensing (35–37), efficient light management in photovoltaic systems (38–40), enhanced light extraction and radiation profile shaping in light-emitting diodes (41–43), and optically variable devices in consumer product design and anticounterfeiting (44–46).Our results demonstrate the versatility of a bioinspired approach toward the creation of novel photonic systems. The unique diffraction-inducing nano- and microscale architecture previously discovered in the scales of the male butterfly P. luna served as inspiration for artificial microdiffraction grating arrays. Our photonic system mimics the reverse diffraction color sequence found in the butterfly''s scales and also provides additional complexity in the diffraction patterns due to a periodic arrangement of the diffraction elements not found in the natural structure. Such arrays provide a platform for hierarchical photonic systems displaying unique diffraction coupling. A detailed optical analysis and modeling of the diffraction patterns allowed us to observe, understand, and decouple diffraction effects induced by the plate ensemble and by the regular scalloping of individual plates. A variation of the light incidence angle results in the expected shifting of the plate ensemble diffraction modes but does not affect the diffraction resulting from individual plates. On the other hand, a variation in plate inclination leaves the interplate geometry diffraction modes untouched but has a strong influence on the scallop-induced diffraction.The intensity distribution of the diffraction induced by the scallops is modulated by the diffraction induced by the plate ensemble. Where the scallop diffraction arc coincides with a plate ensemble diffraction order a peak in intensity is observed. A decrease in the plate ensemble period would result in fewer propagating plate ensemble diffraction orders with a larger free angular range in between. This would lead to a discretization of the scallop diffraction pattern (seen in the FDTD simulation in Fig. 4) inducing a discrete and easily perceivable color variation with potential applications in the development of novel optically variable devices in security printing and consumer product labeling. Currently, efforts are underway to fabricate the diffraction structures in different material combinations that provide a higher refractive index contrast, thereby strengthening the diffractive signal.Open in a separate windowFig. 4.Diffraction discretization. (A) One-μm-wide unit cell for diffraction grating simulated via FDTD. (B) Reflected spectra calculated from FDTD simulation of A showing bright spots in each diffracted order with discrete jumps in the diffracted wavelength.     

Porcupine inhibitor suppresses paracrine Wnt-driven growth of Rnf43;Znrf3-mutant neoplasia

Rnf43 (RING finger protein 43) and Znrf3 (zinc/RING finger protein 3) (RZ) are two closely related transmembrane E3 ligases, encoded by Wnt target genes, that remove surface Wnt (wingless-int) receptors. The two genes are mutated in various human cancers. Such tumors are predicted to be hypersensitive to, yet still depend on, secreted Wnts. We previously showed that mutation of RZ in the intestine yields rapidly growing adenomas containing LGR5⁺ (leucine-rich repeat-containing G-protein coupled receptor 5) stem cells and Wnt3-producing Paneth cells. We now show that removal of Paneth cells by Math1 mutation inhibits RZ^−/− tumor formation. Similarly, deletion of Wnt3 inhibits tumorigenesis. Treatment of mice carrying RZ^−/− intestinal neoplasia with a small molecule Wnt secretion inhibitor (porcupine inhibitor C59) strongly inhibited growth, whereas adjacent normal crypts remained intact. These results establish that paracrine Wnt secretion is an essential driver of RZ^−/− tumor growth and imply that a therapeutic window exists for the use of porcupine inhibitors for RZ-mutant cancers.RNF43 (RING finger protein 43) and ZNRF3 (zinc/RING finger protein 3) target Wnt (wingless-int) receptors (the Frizzleds) to the lysosomal degradation pathway by promoting their endocytosis (1, 2). Encoded by stem cell-specific Wnt target genes, they constitute a crucial negative feedback loop in the Wnt signaling pathway (2). RNF43 is mutated in subsets of human cancers of the colon (2, 3), pancreas (4–6), stomach (7), ovary (8), and liver (9), whereas ZNRF3 is mutated in adrenocortical carcinoma (10) and osteoblastoma (11). Indeed, when both genes are mutated simultaneously in small intestinal stem cells in mice, tumors arise that consist almost entirely of LGR5⁺ (leucine-rich repeat-containing G-protein coupled receptor 5) stem cells and their Wnt3-producing niche cells, the Paneth cells (2). The activity of these E3 ligases is negatively controlled by R-spondins (RSPO) and LGR4/5/6 (LGR) receptors that sequestrate RZ from Frizzled by forming a triple complex (1). The molecular detail of the RSPO/RZ/LGR interaction has been resolved by multiple independent X-ray structures (12–14). In our previous report, we have shown that epithelial organoids (15) derived from RZ-mutant tumors can grow in the absence of the Wnt-amplifier RSPO (2), indicative of their hypersensitivity to Wnt. However, RZ-null mutant organoids die in the presence of porcupine inhibitor IWP1, implying that they require paracrine Wnt (2), which, in organoids, is provided in the form of Wnt3 by Paneth cells (16). Of note, additional Wnts are secreted by nonepithelial cells surrounding crypts, making Wnt3 nonessential in vivo (16–19). This result clearly indicated the Wnt dependency of tumorigenic RZ-null epithelial cells.We asked whether Wnt3 produced by Paneth cells is essential for the unrestricted growth of RZ-mutant neoplasia in vivo. We first addressed this Paneth cell dependency by genetically blocking Paneth cell formation by ablating mouse atonal homolog 1 (Math1) (Atoh1, essential for Paneth cell formation; refs. 20 and 21). A floxed Math1 allele was crossed into Rnf43^f/f: Znrf3^f/f: Villin-Cre^ERT2 conditional knockout mice. Cre-mediated deletion was induced by tamoxifen administration. Rnf43^f/f: Znrf3^f/f: Villin-Cre^ERT2 mutant intestine showed extensive cellular proliferation with Paneth cell metaplasia, as reported previously (Fig. 1 C and D). Rnf43^f/f: Znrf3^f/f: Math1^f/f: Villin-Cre^ERT2 mutants rapidly lost their Lysozyme⁺ Paneth cells, whereas the aberrant proliferation significantly attenuated (Fig. 1 E and F). A similar experiment using a conditional Wnt3 allele yielded similar results (Fig. 1 G and H). These observations implied that RZ-null neoplasia depends on Wnt3-secreting Paneth cells.Open in a separate windowFig. 1.Genetic depletion of Math1 and Wnt3 prevent the formation of tumorigenic neoplasia in the intestine of the Rnf43^f/f: Znrf3^f/f: Villin-Cre^ERT2 conditional knockout mice. Intestinal sections were analyzed for Ki67 (proliferative cells) (A, C, E, and G) and Lysozyme (Paneth cells) (B, D, F, and H). Rnf43 and Znrf3 (RZ) compound mutant intestine shows dramatic increase in the number of Ki67⁺ proliferating cells (C) and Lysozyme⁺ Paneth cells (D) compared with control (A and B). Depletion of Math1 (E) or Wnt3 (G) in the RZ mutant background significantly attenuates the strong proliferative response. Note that Math1 ablation causes the loss of Paneth cells (F).We cultured organoids from Rnf43^f/f: Znrf3^f/f: Math1^f/f: Villin-Cre^ERT2 and from Rnf43^f/f: Znrf3^f/f: Wnt3^f/f: Villin-Cre^ERT2 mutants. Both types of mutant organoids (tumoroids) grow normally when provided with exogenous Wnt3 (Fig. 2 H and K). The mutant organoids were hypersensitive to Wnt, because they did not require RSPO when provided with Wnt3A (Fig. 2 I, L, and N), unlike control organoids (Fig. 2 C and N). This survivability under RSPO withdrawal implied that the triple mutant tumoroids retained the characteristics of the original RZ double mutants. However, triple mutant tumoroids could not be cultured without Wnt ligands (Fig. 2 G, J, and M), unlike control and RZ double mutants (Fig. 2 A, D, and M), which confirms the dependency of RZ mutant tumor cells on Wnt3 secreted by Paneth cells.Open in a separate windowFig. 2.(A–L) Tumorigenic Rnf43 and Znrf3 mutant intestinal organoids depend on the paracrine Wnt source. Various Intestinal organoids (control, RZ, RZ:Math1, and RZ:Wnt3) were isolated and induced by tamoxifen before being cultured under indicated conditions (ENR, WENR, and WEN medium; E, EGF; N, Noggin; R, R-spondin; W, Wnt3a). All RZ mutant organoid lines (RZ, RZ:Math1, and RZ:Wnt3) show unaffected growth in the absence of R-spondin (F, I, and L), in contrast to control organoids (C, asterisks), whereas the removal of exogenous Wnt3a suppresses the growth of RZ:Math1 (G, asterisks) and RZ:Wnt3 (J, asterisks) organoids. (M, N) The percentage of surviving organoids were scored at three time points [day 1 (d1), day 3 (d3), and day 5 (d5)] after seeding in indicated conditions [ENR (M) and WEN (N)].These observations suggested that blocking the interaction between the tumorigenic intestinal stem and niche cell mediated by Wnt ligand can be an effective therapeutic intervention of tumors with mutations in Rnf43 and/or Znrf3. Small molecule porcupine inhibitors block palmitoylation of Wnt proteins, an essential step for secretion of Wnts (22). It has been tested in the Wnt1-driven mouse mammary tumor model (MMTV-Wnt1; refs. 23 and 24).We treated 8-wk-old Rnf43^f/f: Znrf3^f/f: Villin-Cre^ERT2 conditional knockout mice with C59 porcupine inhibitor (50 mg/kg) directly after inducing RZ-null neoplasia by tamoxifen administration (Fig. 3A). At this dosing, we noted no effects on functioning of normal crypts as shown by the presence of olfactomedin-4 (Olfm4⁺) intestinal stem cells (Fig. 3B) and Cryptdin1⁺ Paneth cells (Fig. 3C), although the presence of intestinal stem cells and Paneth cells crucially depend on Wnt (16–19). However, the C59 porcupine inhibitor strongly suppressed the formation of RZ-null tumorigenic epithelium when provided from the neoplasia induction date onward (Fig. 3 F and G versus Fig. 3 D and E, respectively). Additional analysis for stem cell (Lgr5) and Paneth cell (Lysozyme) markers showed similar results (Fig. S1).Open in a separate windowFig. 3.Porcupine inhibitor C59 prevents neoplastic outgrowth of Rnf43 and Znrf3 mutant Olfm4⁺ intestinal stem cells. Schematic drawing of drug treatments (A). Mice were treated with C59 porcupine inhibitor for seven consecutive days upon tamoxifen (Tam) administration to control (B and C) and RZ (F and G) mutant mice. C59 untreated, but tamoxifen treated RZ mutant mouse was used as tumorigenic-positive control (D and E). Sections were analyzed by Olfm4, a robust marker for intestinal stem cells (B, D, and F), and Cryptin1, a marker for Paneth cells (C, E, and G) via in situ hybridization.Next, we tested whether C59 porcupine inhibitor affected established RZ-null intestinal neoplasia. RZ mutations were induced first by tamoxifen administration to Rnf43^f/f: Znrf3^f/f: Villin-Cre^ERT2 conditional knockout mice. C59 porcupine inhibitor treatment was initiated 5 d after tamoxifen induction. At this time point, the mutant intestine is filled with Olfm4⁺ stem cells (Fig. 4B) and Cryptdin1⁺ Paneth cells (Fig. 4C) throughout the crypt epithelium. Again, C59 porcupine treatment for 5 d suppressed the outgrowth of RZ-mutant intestinal epithelium (Fig. 4 F and G versus Fig. 4 D and E, respectively). Additional stainings for stem cell (Lgr5) and Paneth cell (Lysozyme) markers showed similar results (Fig. S2).Open in a separate windowFig. 4.Porcupine inhibitor C59 eradicates preestablished neoplasia induced by Rnf43 and Znrf3 loss. Schematic drawing of drug treatments (A). Mice were treated with C59 porcupine inhibitor 5 d after initial tamoxifen (Tam) administration to RZ (F and G) mutant mice. C59 untreated, but tamoxifen-treated RZ mutant mice were used as control (D and E). RZ mutant mice were analyzed on day 5 to examine the status of hyperplasia at the time point when C59 treatment was started (B and C). Sections were analyzed for Olfm4 (intestinal stem cells) (B, D, and F) and Cryptdin 1 (Paneth cells) expression (C, E, and G) via in situ hybridization.Our data imply that cancer cells, carrying mutations in RNF43 and/or ZNRF3 as drivers of tumor growth, require a Wnt-secreting niche. We demonstrate that blocking this Wnt niche function by porcupine inhibition attenuates aberrant RZ-null hyperplasia at doses that do not affect normal crypts. The existence of this therapeutic dose of porcupine inhibitor is particularly interesting with a high incidence of RNF43 mutations in various types of human cancer (2–9), including colorectal cancer (18%) (3). These observations indicate that the growing numbers of small molecule Wnt inhibitors may be tested in clinical trials of patients carrying malignant tumors with demonstrated mutations in RNF43 or ZNRF3.     

Atomic-number (Z)-correlated atomic sizes for deciphering electron microscopic molecular images

With the advent of atomic resolution transmission electron microscopy (AR-TEM) achieving sub-Ångstrom image resolution and submillisecond time resolution, an era of cinematic molecular science where chemists can visually study the time evolution of molecular motions and reactions at atomistic precision has arrived. However, the appearance of experimental TEM images often differs greatly from that of conventional molecular models, and the images are difficult to decipher unless we know in advance the structure of the specimen molecules. The difference arises from the fundamental design of the molecular models that represent atomic connectivity and/or the electronic properties of molecules rather than the nuclear charge of atoms and electrostatic potentials that are felt by the e-beam in TEM imaging. We found a good correlation between the atomic number (Z) and the atomic size seen in TEM images when we consider shot noise in digital images. We propose Z-correlated (ZC) atomic radii for modeling AR-TEM images of single molecules and ultrathin crystals with which we can develop a good estimate of the molecular structure from the TEM image much more easily than with conventional molecular models. Two parameter sets were developed for TEM images recorded under high-noise (ZC_HN) and low-noise (ZC_LN) conditions. The molecular models will stimulate the imaginations of chemists planning to use AR-TEM for their research.

The world of atoms and molecules invisible to human eyes has been illuminated by the development of three-dimensional models in which atoms represented by spheres are connected to each other with appropriate spatial disposition. Here, the choice of the radii of the spheres, in addition to coloring, is the most crucial visual identifier of atoms and atomic ions, and a variety of useful constructions of radii, either empirical or quantum mechanical (1, 2), have been proposed in accord with chemical intuition. Whereas wire and ball-and-stick models are primitive, showing only bond lengths, bond angles, and torsional angles, chemists have implemented further information into molecular models. Representing the van der Waals isosurface, a Corey–Pauling–Koltun (CPK) model was developed to illustrate the molecular surface for analysis of intermolecular interactions (3). Shannon’s ionic model (4) is useful for inorganic chemistry. The atomic radii chosen in these models reflect the electronic properties of atoms and molecules.With the advent of atomic-resolution transmission electron microscopy (AR-TEM) achieving sub-Ångstrom image resolution and submillisecond time resolution (5–8), an era of “cinematic molecular science” has arrived, where chemists can visually study the time evolution of molecular motions and reactions at atomistic precision (9–11). However, experimental TEM images often differ greatly from what chemists expect using their favorite molecular models (Fig. 1), and this discrepancy contributed to reducing the popularity of AR-TEM in molecular science. The difficulty is particularly important when we deal with unknown structures that are mobile and lack structural periodicity. For example, structure assignment of the AR-TEM images of a rotating zinc cluster (Fig. 1A) that forms in the synthesis of a crystal of a metal–organic framework (MOF; MOF-5) (12) was impossible without reducing the number of initial guess structures down to several hundred from a total of 4,096 isomers and their rotamers as to the 2-iodo-1,4-phenylene bridges (Fig. 1 D and E). Here, we found the CPK atomic radii (Fig. 1B) more confusing than revealing, and we became convinced of a need for new atomic radii that better represent the experimental images (compare Fig. 1 B and C). In doing so, we found that the atom size seen in TEM images shows a strong correlation to the atomic number (Z), particularly if we consider the shot noise in digital images. In a noise-free image, we find little correlation, as theory predicts (13). Our idea may be symbolically described as comparing the sizes of mountains by measuring the width of the lowest portion of each mountain that is visible above a sea of clouds. We report here a set of Z-correlated (ZC) atomic radii for deciphering AR-TEM images, with which we can develop a good estimate of the molecular structure from the TEM image much more easily than with conventional molecular models. Two parameter sets are described: one for TEM images under high-noise conditions (ZC_HN, Fig. 2C) and another under low-noise conditions (ZC_LN, Fig. 2D). The former is useful for single-molecule imaging and the latter for an ultrathin film such as UiO-66 MOF (Fig. 1F) (14, 15). The ZC radii reflect the relative sizes of various elements and help chemists develop an initial estimate of the molecular structures. Note, however, that we do not intend the ZC model to reproduce the TEM images exactly. No chemists expect the CPK model to represent the van der Waals properties of the molecule exactly, and molecular models are there to stimulate chemists’ imaginations.Open in a separate windowFig. 1.Experimental AR-TEM image and atomic-number-correlated molecular model (ZC model). (A) Time evolution of TEM image of a MOF-5 precursor, a cubic cluster made of Zn²⁺ and 2-iodoterephthalic acid captured on a carbon nanotube. The molecular images are blurred by rotation and vibration. Time is shown in seconds. Taken from ref. 12 (Copyright 2019, Nature Publishing Group). (B) A CPK model corresponding to the image at 10.0 s. (C) A ZC_HN model also for the image at 10.0 s. (D) Chemical structure of the cubic cluster. (E) Schematic illustration of isomers of the cubic MOF-5 precursor. Four structures out of 4096 possible isomers are shown. The structure seen in Fig. 1A is highlighted by a red square. (F) TEM image of an ultrathin crystal of UiO-66 (adapted from ref. 15). The benzene ring in a benzene dicarboxylate linker is indicated by an arrow (Copyright 2018, The American Association for the Advancement of Science). (G) A ZC_LN model corresponding to (F). (Scale bars: 1 nm.)Open in a separate windowFig. 2.Periodic table of atomic sizes used in molecular models. (A) CPK model. (B) Shannon’s ionic parameters. (C) ZC_HN model. (D) ZC_LN model. (Scale bar: 2 Å.) (E) List of ZC_HN radii for single-molecule imaging and ZC_LN radii for ultrathin crystals (in Å).In Fig. 2, we compare the atomic radii used in the CPK model, Shannon’s ionic model, and the ZC_LN and ZC_HN models. CPK radii are smaller for transition metals and larger for main group elements, and Shannon’s radii decrease with the increasing atomic number within the same period. These atomic radii are unsuitable for deciphering AR-TEM images, where the behavior of the e-beam in the electrostatic potential created by atomic nuclei plays a decisive role (16, 17). In the ZC model proposed here, the atomic radius shows element dependence, changing systematically as Z increases. The ZC_HN atomic radii (Fig. 2C) are smaller in signal size than the ZC_LN radii (Fig. 2D), show large Z-dependence, and are suitable for fast molecular imaging where the level of shot noise is large (Fig. 1 A–C) (18). The ZC_LN radii are suitable for images of ultrathin crystalline films where the level of shot noise is comparatively low. The atomic sizes for the graphical construction of the ZC models are listed in Fig. 2E. We anticipate that the molecular model will stimulate the imaginations of chemists planning to use AR-TEM for their research.     

Long-term reliability of the Athabasca River (Alberta,Canada) as the water source for oil sands mining

Exploitation of the Alberta oil sands, the world’s third-largest crude oil reserve, requires fresh water from the Athabasca River, an allocation of 4.4% of the mean annual flow. This allocation takes into account seasonal fluctuations but not long-term climatic variability and change. This paper examines the decadal-scale variability in river discharge in the Athabasca River Basin (ARB) with (i) a generalized least-squares (GLS) regression analysis of the trend and variability in gauged flow and (ii) a 900-y tree-ring reconstruction of the water-year flow of the Athabasca River at Athabasca, Alberta. The GLS analysis removes confounding transient trends related to the Pacific Decadal Oscillation (PDO) and Pacific North American mode (PNA). It shows long-term declining flows throughout the ARB. The tree-ring record reveals a larger range of flows and severity of hydrologic deficits than those captured by the instrumental records that are the basis for surface water allocation. It includes periods of sustained low flow of multiple decades in duration, suggesting the influence of the PDO and PNA teleconnections. These results together demonstrate that low-frequency variability must be considered in ARB water allocation, which has not been the case. We show that the current and projected surface water allocations from the Athabasca River for the exploitation of the Alberta oil sands are based on an untenable assumption of the representativeness of the short instrumental record.Over the past several decades, the province of Alberta has had Canada’s fastest growing economy, driven largely by the production of fossil fuels. Climatic change, periodic drought, and expanding human activities impact the province’s water resources, creating the potential for an impending water crisis (1). The Athabasca River (Fig. 1) is the only major river in Alberta with completely unregulated flows. It is the source of surface water for the exploitation of the Alberta oil sands, the world’s third-largest proven crude oil reserve at roughly 168 billion barrels. The oil and gas industry accounted for 74.5% of total surface water allocations in the Athabasca River Basin (ARB) in 2010 (Fig. 2) (2). An almost doubling of ARB water allocations since 2000, or 13 times the provincial average, is attributable to expanding oil sands production, which began in 1967 (Fig. 2).Open in a separate windowFig. 1.Alberta’s seven major drainage basins. The numbers refer to hydrometric gauges identified in 6). Reproduced with permission from the Government of Alberta.Open in a separate windowFig. 2.Licensed water allocations by sector and decades to 2000 and selected subsequent years in the Athabasca River Basin, 1900–2010. Reproduced with permission from the Government of Alberta.According to the Canadian Association of Petroleum Producers, in 2012, surface mining of the oil sands and in situ extraction (drilling) required 3.1 and 0.4 barrels of fresh water, respectively, to produce a barrel of crude oil (3). This amounted to 187 million cubic meters of fresh water use in 2012, or the equivalent of the residential water use of 1.7 million Canadians (4). Within the next decade, cumulative water use for oil sands production is projected to peak at about 505 million cubic meters per year or a rate of 16 m³⋅s⁻¹ (5). The current (2010 data) total water allocation represents only 4.4% of the mean annual Athabasca River flow (2) (Fig. 3); however, allocation and use as a proportion of average water levels does not account for the large variability in flow between seasons and years (interannual coefficient of variation = 22%). During 1952–2013, the Athabasca River gauge at Athabasca recorded mean seasonal flows of 100 m³⋅s⁻¹ in winter (December−February) and 911 m³⋅s⁻¹ in summer (June−August), with extreme monthly flows of 48 m³⋅s⁻¹ in December 2000 and 2,280 m³⋅s⁻¹ in June 1954. The mean, maximum, and minimum annual flows were 422 m³⋅s⁻¹, 702 m³⋅s⁻¹, and 245 m³⋅s⁻¹, respectively (Water Survey of Canada, wateroffice.ec.gc.ca/search/search_e.html?sType=h2oArc).Open in a separate windowFig. 3.Mean annual flow of the Athabasca River at Athabasca (gauge 07BE001, Water Survey of Canada) for the water year (October 1 to September 30, 1913–2013).Assessing the sustainability of ARB surface water availability is difficult because most regional streamflow gauges have been operational for a few years to a few decades, with long intervals of missing values, including during the 1930s and 1940s when drought was prevalent throughout the North American western interior. Various critics (e.g., ref. 4) emphasize the intraannual variability and express concern about water withdrawals in the low-flow winter season. Less attention has been given to flow variability at interannual to decadal scales associated with climate oscillations such as the Pacific Decadal Oscillation (PDO) and Pacific North American mode (PNA), which are known to have significant impacts on runoff from the Rocky Mountains, including in the ARB (6–9), and the potential consequences of a long period of predominantly low flow. We investigate the response of ARB flows to the low-frequency components of the PDO and PNA. This paper is the first, to our knowledge, to explicitly model the variability linked to the PDO and PNA, when testing for trends in discharge, thereby removing the tendencies associated with these climatic oscillations, which can confound trend detection in the relatively short instrumental records (6, 7). However, the available instrumental hydrologic records provide a limited sample of these low-frequency fluctuations; exploring longer records of this variability is critical. Therefore, we also reconstructed the Athabasca River annual flow from the growth rings in moisture-sensitive conifers at a network of sites in the upper reaches of the ARB and in an adjacent watershed (Fig. S1). We compare 900 y of inferred proxy flows to the streamflow variability recorded in recent decades, identifying extended periods of paleo low flow that exceeded the worst-case scenario in the instrumental records. We consider (i) the extent to which gauged flows (e.g., Fig. 3), which are the basis for surface water allocation, fail to capture the full range of hydroclimatic variability and extremes evident in a longer proxy record and (ii) the temporal evolution of low-frequency Athabasca River variability, related to the PDO and PNA, with its consequences for ARB surface water availability.Open in a separate windowFig. S1.The tree-ring sampling sites within and near the Athabasca River Basin in north-central Alberta. The colored triangles represent four coniferous tree species.     

Little evidence for enhanced phenotypic evolution in early teleosts relative to their living fossil sister group

Since Darwin, biologists have been struck by the extraordinary diversity of teleost fishes, particularly in contrast to their closest “living fossil” holostean relatives. Hypothesized drivers of teleost success include innovations in jaw mechanics, reproductive biology and, particularly at present, genomic architecture, yet all scenarios presuppose enhanced phenotypic diversification in teleosts. We test this key assumption by quantifying evolutionary rate and capacity for innovation in size and shape for the first 160 million y (Permian–Early Cretaceous) of evolution in neopterygian fishes (the more extensive clade containing teleosts and holosteans). We find that early teleosts do not show enhanced phenotypic evolution relative to holosteans. Instead, holostean rates and innovation often match or can even exceed those of stem-, crown-, and total-group teleosts, belying the living fossil reputation of their extant representatives. In addition, we find some evidence for heterogeneity within the teleost lineage. Although stem teleosts excel at discovering new body shapes, early crown-group taxa commonly display higher rates of shape evolution. However, the latter reflects low rates of shape evolution in stem teleosts relative to all other neopterygian taxa, rather than an exceptional feature of early crown teleosts. These results complement those emerging from studies of both extant teleosts as a whole and their sublineages, which generally fail to detect an association between genome duplication and significant shifts in rates of lineage diversification.Numbering ∼29,000 species, teleost fishes account for half of modern vertebrate richness. In contrast, their holostean sister group, consisting of gars and the bowfin, represents a mere eight species restricted to the freshwaters of eastern North America (1). This stark contrast between teleosts and Darwin''s original “living fossils” (2) provides the basis for assertions of teleost evolutionary superiority that are central to textbook scenarios (3, 4). Classic explanations for teleost success include key innovations in feeding (3, 5) (e.g., protrusible jaws and pharyngeal jaws) and reproduction (6, 7). More recent work implicates the duplicate genomes of teleosts (8–10) as the driver of their prolific phenotypic diversification (8, 11–13), concordant with the more general hypothesis that increased morphological complexity and innovation is an expected consequence of genome duplication (14, 15).Most arguments for enhanced phenotypic evolution in teleosts have been asserted rather than demonstrated (8, 11, 12, 15, 16; but see ref. 17), and draw heavily on the snapshot of taxonomic and phenotypic imbalance apparent between living holosteans and teleosts. The fossil record challenges this neontological narrative by revealing the remarkable taxonomic richness and morphological diversity of extinct holosteans (Fig. 1) (18, 19) and highlights geological intervals when holostean taxonomic richness exceeded that of teleosts (20). This paleontological view has an extensive pedigree. Darwin (2) invoked a long interval of cryptic teleost evolution preceding the late Mesozoic diversification of the modern radiation, a view subsequently supported by the implicit (18) or explicit (19) association of Triassic–Jurassic species previously recognized as “holostean ganoids” with the base of teleost phylogeny. This perspective became enshrined in mid-20th century treatments of actinopterygian evolution, which recognized an early-mid Mesozoic phase dominated by holosteans sensu lato and a later interval, extending to the modern day, dominated by teleosts (4, 20, 21). Contemporary paleontological accounts echo the classic interpretation of modest teleost origins (22–24), despite a systematic framework that substantially revises the classifications upon which older scenarios were based (22–25). Identification of explosive lineage diversification in nested teleost subclades like otophysans and percomorphs, rather than across the group as a whole, provides some circumstantial neontological support for this narrative (26).Open in a separate windowFig. 1.Phenotypic variation in early crown neopterygians. (A) Total-group holosteans. (B) Stem-group teleosts. (C) Crown-group teleosts. Taxa illustrated to scale.In contrast to quantified taxonomic patterns (20, 23, 24, 27), phenotypic evolution in early neopterygians has only been discussed in qualitative terms. The implicit paleontological model of morphological conservatism among early teleosts contrasts with the observation that clades aligned with the teleost stem lineage include some of the most divergent early neopterygians in terms of both size and shape (Fig. 1) (see, for example, refs. 28 and 29). These discrepancies point to considerable ambiguity in initial patterns of phenotypic diversification that lead to a striking contrast in the vertebrate tree of life, and underpins one of the most successful radiations of backboned animals.Here we tackle this uncertainty by quantifying rates of phenotypic evolution and capacity for evolutionary innovation for the first 160 million y of the crown neopterygian radiation. This late Permian (Wuchiapingian, ca. 260 Ma) to Cretaceous (Albian, ca. 100 Ma) sampling interval permits incorporation of diverse fossil holosteans and stem teleosts alongside early diverging crown teleost taxa (Figs. 1 and ​and2A2A and Figs. S1 and ​andS2),S2), resulting in a dataset of 483 nominal species-level lineages roughly divided between the holostean and teleost total groups (Fig. 2B and Fig. S2). Although genera are widely used as the currency in paleobiological studies of fossil fishes (30; but see ref. 31), we sampled at the species level to circumvent problems associated with representing geological age and morphology for multiple congeneric lineages. We gathered size [both log-transformed standard length (SL) and centroid size (CS); results from both are highly comparable (Figs. S3 and ​andS4);S4); SL results are reported in the main text] and shape data (the first three morphospace axes arising from a geometric morphometric analysis) (Fig. 2A and Figs. S1) from species where possible. To place these data within a phylogenetic context, we assembled a supertree based on published hypotheses of relationships. We assigned branch durations to a collection of trees under two scenarios for the timescale of neopterygian diversification based on molecular clock and paleontological estimates. Together, these scenarios bracket a range of plausible evolutionary timelines for this radiation (Fig. 2B). We used the samples of trees in conjunction with our morphological datasets to test for contrasts in rates of, and capacity for, phenotypic change between different partitions of the neopterygian Tree of Life (crown-, total-, and stem-group teleosts, total-group holosteans, and neopterygians minus crown-group teleosts), and the sensitivity of these conclusions to uncertainty in both relationships and evolutionary timescale. Critically, these include comparisons of phenotypic evolution in early crown-group teleosts—those species that are known with certainty to possess duplicate genomes—with rates in taxa characterized largely (neopterygians minus crown teleosts) or exclusively (holosteans) by unduplicated genomes. By restricting our scope to early diverging crown teleost lineages, we avoid potentially confounding signals from highly nested radiations that substantially postdate both genome duplication and the origin of crown teleosts (26, 32). This approach provides a test of widely held assumptions about the nature of morphological evolution in teleosts and their holostean sister lineage.Open in a separate windowFig. 2.(A) Morphospace of Permian–Early Cretaceous crown Neopterygii. (B) One supertree subjected to our paleontological (Upper) and molecular (Lower) timescaling procedures to illustrate contrasts in the range of evolutionary timescales considered. Colors of points (A) and branches (B) indicate membership in major partitions of neopterygian phylogeny. Topologies are given in Datasets S4 and S5. See Dataset S6 for source trees.Open in a separate windowFig. S1.Morphospace of 398 Permian–Early Cretaceous Neopterygii. Three major axes of shape variation are presented. Silhouettes and accompanying arrows illustrate the main anatomical correlates of these principal axes, as described in Open in a separate windowFig. S2.Morphospace of 398 Permian–Early Cretaceous Neopterygii, illustrating the major clades of (A) teleosts and (B) holosteans.Open in a separate windowFig. S3.Comparisons of size rates between (A) holosteans and teleosts, (B) crown teleosts and all other neopterygians, (C) crown teleosts and stem teleosts, (D) crown teleosts and holosteans, and (E) stem teleosts and holosteans. Comparisons were made using the full-size SL dataset, a CS dataset, and a smaller SL dataset pruned to exactly match the taxon sampling of the CS dataset. Identical taxon sampling leads the CS and pruned SL datasets to yield near identical results. Although the larger SL dataset results often differ slightly, the overall conclusion from each pairwise comparison (i.e., which outcome is the most likely in an overall majority of trees) is identical in all but one comparison (E, under molecular timescales).Open in a separate windowFig. S4.Comparisons of size innovation between (A) holosteans and teleosts, (B) crown teleosts and all other neopterygians, (C) crown teleosts and stem teleosts, (D) crown teleosts and holosteans, and (E) stem teleosts and holosteans. Comparisons were made using the full-size SL dataset, a CS dataset, and a smaller SL dataset pruned to exactly match the taxon sampling of the CS dataset. Comparisons of size innovation are presented for K value distributions of the three datasets resemble each other closely.     

A neural surveyor to map touch on the body

Perhaps the most recognizable sensory map in all of neuroscience is the somatosensory homunculus. Although it seems straightforward, this simple representation belies the complex link between an activation in a somatotopic map and the associated touch location on the body. Any isolated activation is spatially ambiguous without a neural decoder that can read its position within the entire map, but how this is computed by neural networks is unknown. We propose that the somatosensory system implements multilateration, a common computation used by surveying and global positioning systems to localize objects. Specifically, to decode touch location on the body, multilateration estimates the relative distance between the afferent input and the boundaries of a body part (e.g., the joints of a limb). We show that a simple feedforward neural network, which captures several fundamental receptive field properties of cortical somatosensory neurons, can implement a Bayes-optimal multilateral computation. Simulations demonstrated that this decoder produced a pattern of localization variability between two boundaries that was unique to multilateration. Finally, we identify this computational signature of multilateration in actual psychophysical experiments, suggesting that it is a candidate computational mechanism underlying tactile localization.

In the 18th century, surveyors in France completed the world’s first topographically accurate map of an entire country. To do so, they relied on the computation of triangulation; given a precisely known distance between two baseline landmarks, the location of a third landmark could be computed from its angles of intersection with the baseline landmarks. Countries could utilize this simple geometric computation to accurately map the location of all landmarks in their borders (Fig. 1A). This is also possible using multilateration (or trilateration, more specifically), where the known distance between multiple baseline landmarks is used to compute the location of another landmark. These computations are simple yet robust ways to localize objects and therefore still used in modern surveying and global position systems.Open in a separate windowFig. 1.Examples of geometric computations. (A) Idealized example of how multi/trilateration can be used to localize an object within a country. Given the distance between two baseline landmarks whose locations are known (d₁), an object (e.g., the Eiffel tower) can be localized by calculating its distance from each landmark individually (d₂ and d₃). (B) Path integration: By computing over distances and angles traveled (d₁ and d₂), a rat can calculate how much it needs to travel (d₃) to return to its starting position. (C) Visuomotor reaching: The distance between the hand and object (d₃) can be computed by evaluating over the eye-centered hand distance (d₁) and object distance (d₂). (D) Egocentric tactile localization: The vector (black arrow) for reaching to a touch (black dot) on the arm may be computed using trilateration. First, an arm-centered touch location is trilaterated (see next section; see Fig. 2A) by computing distance between the touch and the elbow (d₁) and wrist (d₂). An egocentric representation of touch could be derived by further taking into account the distance between the reaching hand and the elbow (d₃) and wrist (d₄).Geometric computations involving manipulating distances and angles are also employed by the nervous system of animals to localize and interact with objects in the environment. During spatial navigation (Fig. 1B), mammals can readily return to their starting location by taking into account all computed distances and heading directions traveled (1), a phenomenon known as path integration. Reaching to grasp a visible target is another behavior involving geometric computations (Fig. 1C). To do so, the brain must compute a reach vector from distances derived from hand and target position signals (2, 3), involving transformations that take place in the frontal and parietal cortices (4, 5).Equally crucial to localizing objects in the environment is localizing objects on the personal space of the body. In the 180 y since Weber’s seminal investigations on the sense of touch (6), researchers have extensively characterized tactile localization at both the behavioral (7–18) and neural (19–25) level. However, despite this progress, the computations underlying tactile localization remain largely unknown. Recent accounts have suggested that tactile localization requires two computational steps (26, 27). First, afferent input must be localized within a topographic map in somatosensory cortex (19). However, an activation within this map is not sufficient for localization since it alone is meaningless without a reference frame that is centered on the body. Localizing touch on the body therefore requires a neural decoder (28) that can “read” the topographic landscape of the population response within the map (22) and reference this information to stored spatial representations of the body (29). However, given that the nature of these computations—and how they might be implemented by neural circuits—remains largely unknown, it is unclear whether the brain uses geometric computations to localize objects touching the body.We propose that, like a surveyor, the human brain employs multi/trilateration to localize an object in body-centered coordinates (Fig. 1D). To do so, this “neural surveyor” uses simple arithmetic to calculate the relative distance between the location of the afferent input and the boundaries of the body part (the baseline landmarks, e.g., elbow and wrist of the forearm in Fig. 2A). This body-centered coordinate system is tied to a neural architecture that represents the body’s geometry (29) and is influenced by proprioceptive feedback about limb size (30). In the present study, we provide multiple lines of evidence that the brain may use multi/trilateration to localize touch on a limb. We first develop a Bayesian formulation of this computation in the nervous system. We then develop a population-based neural network model that implements trilateration, thus allowing us to identify its computational signatures. Simulations revealed that trilateration produces a unique pattern of localization variability across a limb. Finally, we identify this pattern in actual psychophysical experiments and provide further predictions of the model to be tested in future work.Open in a separate windowFig. 2.Neural network implementing trilateration. (A) Trilateral computation for tactile localization: The location of touch on the arm is computed by integrating two estimates (d₁ and d₂) of the distance between each joint (x₁ and x₂; elbow and wrist) and the sensory input x₃. (B) Neural network implementation of trilateration: (Lower) the encoding layer is composed of homogenous tuning curves across the space of the sensory surface (in percent); (Upper) the decoding layer is composed of two subpopulations of neurons with distance-dependent gradients in tuning properties (shown: firing rate and tuning width). The distance of a tuning curve from its “anchor” is coded by the luminance, with darker colors corresponding to neurons that are closer to the limb boundary. (C) Activations for each layer of the network averaged over 5,000 simulations. Each circle corresponds to a unit of the neural network. (Lower) Encoding layer; (Middle) decoding layer; (Upper) posterior probabilities of localization for each decoding subpopulation (blue and red) and their integration by the Bayesian decoder (purple).     

Universal features in the photoemission spectroscopy of high-temperature superconductors

The energy gap for electronic excitations is one of the most important characteristics of the superconducting state, as it directly reflects the pairing of electrons. In the copper–oxide high-temperature superconductors (HTSCs), a strongly anisotropic energy gap, which vanishes along high-symmetry directions, is a clear manifestation of the d-wave symmetry of the pairing. There is, however, a dramatic change in the form of the gap anisotropy with reduced carrier concentration (underdoping). Although the vanishing of the gap along the diagonal to the square Cu–O bond directions is robust, the doping dependence of the large gap along the Cu–O directions suggests that its origin might be different from pairing. It is thus tempting to associate the large gap with a second-order parameter distinct from superconductivity. We use angle-resolved photoemission spectroscopy to show that the two-gap behavior and the destruction of well-defined electronic excitations are not universal features of HTSCs, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped samples either show two-gap behavior or not. In contrast, many other characteristics of HTSCs, such as the dome-like dependence of on doping, long-lived excitations along the diagonals to the Cu–O bonds, and an energy gap at the Brillouin zone boundary that decreases monotonically with doping while persisting above (the pseudogap), are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high- superconductivity are relatively insensitive to differences in the electronic states along the Cu–O bond directions.Elucidating the mechanism of high-temperature superconductivity in the copper–oxide materials remains one of the most challenging open problems in physics. It has attracted the attention of scientists working in fields as diverse as materials science, condensed matter physics, cold atoms, and string theory. To clearly define the problem of high-temperature superconductors (HTSCs), it is essential to establish which of the plethora of observed features are universal, namely, qualitatively unaffected by material-specific details.An important early result concerns the universality of the symmetry of the order parameter for superconductivity. The order parameter was found to change sign under a 90° rotation (1, 2), which implies that the energy gap must vanish along the diagonal to the Cu–O bonds, i.e., the Brillouin zone diagonal. This sign change is consistent with early spectroscopic studies of near-optimally-doped samples (those with the highest in a given family), where a energy gap (3, 4) was observed (ϕ being the angle from the Cu–O bond direction), the simplest functional form consistent with d-wave pairing. More recently, there is considerable evidence (5–8) that, with underdoping, the anisotropy of the energy gap deviates markedly from the simple form. Although the gap node at is observed at all dopings, the gap near the antinode (near and 90°) is significantly larger than that expected from the simplest d-wave form. Further, the large gap continues to persist in underdoped (UD) materials as the normal-state pseudogap (9–11) above . This suggests that the small (near-nodal) and large (antinodal) gaps are of completely different origin, the former related to superconductivity and the latter to some other competing order parameter.This two-gap picture has attracted much attention (8), raising the possibility that multiple energy scales are involved in the HTSC problem. There is mounting evidence for additional broken symmetries (12–14) in UD cuprates, once superconductivity is weakened upon approaching the Mott insulating state. The central issue is the role of these additional order parameters in impacting the universal properties of high- superconductivity.In this paper we use angle-resolved photoemission (ARPES) to examine the universality of the two-gap scenario in HTSCs by addressing the following questions. To what extent are the observed deviations from a simple d-wave energy gap independent of material details? How does the observed gap anisotropy correlate, as a function of doping, with other spectroscopic features such as the size of the antinodal gap, and the spectral weights of the nodal and antinodal quasiparticle excitations?We systematically examine the electronic spectra of various families of cation-substituted Bi₂Sr₂CaCu₂O_8+δ single crystals as a function of carrier concentration to elucidate which properties are universal and which are not. We present ARPES data on four families of float-zone-grown Bi₂Sr₂CaCu₂O_8+δ single crystals, where was adjusted by both oxygen content and cation doping. As-grown samples, labeled Bi2212, have an optimal of 91 K. These crystals were UD to by varying the oxygen content. Ca-rich crystals (grown from material with a starting composition Bi_2.1Sr_1.4Ca_1.5Cu₂O_8+δ) with an optimal of 82 K are labeled Ca. Two Dy-doped families grown with starting compositions Bi_2.1Sr_1.9Ca_{1 − x}Dy_xCu₂O_8+δ with x = 0.1 and 0.3 are labeled Dy1 and Dy2, respectively. A full list of the samples used and their determined from magnetization measurements are shown in SI Text, where we also show high-resolution X-ray data that give evidence for the excellent structural quality of our samples.Our main result is that the Dy1 and Dy2 samples show clear evidence of a two-gap behavior in the UD regime , with loss of coherent quasiparticles in the antinodal region of k space where the gap deviates from a simple d-wave form. In marked contrast, the UD Bi2212 samples and the Ca samples show a simple d-wave gap in the superconducting state and sharp quasiparticles over the entire Fermi surface in a similar range of the UD regime. We conclude by discussing the implications of the nonuniversality of the two-gap behavior for the phenomenon of high superconductivity.We begin our comparison of the various families of samples by focusing in Fig. 1 on the superconducting state antinodal spectra as a function of underdoping. The antinode is the Fermi momentum k_F on the Brillouin zone boundary, where the energy gap is a maximum and, as we shall see, the differences between the various samples are the most striking. We show data at optimal doping, corresponding to the highest in each family, in Fig. 1A. Increasing Dy leads to a small suppression of the optimal compared with Bi2212, together with an increase in the antinodal gap and a significant reduction of the quasiparticle weight. This trend continues down to moderate underdoping, as seen in Fig. 1B, where we show UD Bi2212 and Dy2 samples with very similar . For more severely UD samples, with , spectral changes in the Dy-substituted samples are far more dramatic. In Fig. 1C, we see that quasiparticle peaks in the Dy samples are no longer visible, even well below , consistent with earlier work on Y-doped Bi2212 and also Bi2201 and La_1.85Sr_0.15CuO₄ (5, 15–18). In contrast, Bi2212 and Ca-doped samples with comparable continue to exhibit quasiparticle peaks. In this respect the latter two are similar to epitaxially grown thin-film samples that exhibit quasiparticle peaks all of the way down to the lowest (19).Open in a separate windowFig. 1.Superconducting state antinodal ARPES spectra. We use the label “Bi2212” for samples without cation doping, “Dy1” for 10% Dy, “Dy2” for 30% Dy, and “Ca” for Ca-doped samples. The temperature is indicated along with . OP denotes optimal doped, UD underdoped, and OD overdoped samples. (A) Antinodal spectra for OP samples of three different families: Bi2212 (blue), Dy1 (green), and Dy2 (red), showing an increase in gap and a decrease in quasiparticle weight with increasing Dy content. (B) Antinodal spectra for UD samples with similar (≃66 K) for Bi2212 (blue) and Dy2 (red). As in A, there is a larger gap and smaller coherent weight in the Dy-substituted sample. (C) Same as in B, but for four UD samples with near 55 K for Bi2212 (dark blue), Ca (light blue), Dy1 (green), and Dy2 (red). The Bi2212 and Ca spectra are very similar to each other and quite different from those of the Dy1 and Dy2 materials. (D) Doping evolution of the antinodal spectra of four Dy1 samples from OP to UD . (E) Doping evolution of the antinodal spectra of four Dy2 samples from OP to UD . We see in D and E the sudden loss of quasiparticle weight for below 60 K. (F) Doping evolution of the antinodal spectra of three Bi2212 samples and three Ca samples, showing well-defined quasiparticle peaks in all cases.A significant feature of the highly UD Dy samples in Fig. 1C is that, in addition to the strong suppression of the quasiparticle peak, there is severe loss of low-energy spectral weight. To clearly highlight this, we show the doping evolution of antinodal spectra for the Dy1 (Fig. 1D) and Dy2 (Fig. 1E) samples. These observations are in striking contrast with the Bi2212 and Ca-doped data in Fig. 1F, where we do see a systematic reduction of the quasiparticle peak with underdoping, but not a complete wipeout of the low-energy spectral weight. To the extent that the superconducting state peak–dip–hump line shape (20, 21) originates from one broad normal-state spectral peak, the changes in spectra of the Dy materials are not simply due to a loss of coherence, but more likely a loss of the entire spectral weight near the chemical potential.The doping evolution of the k-dependent gap is illustrated in Figs. 2 and​and 3. 3. In Fig. 2 we contrast the optimally doped Dy1 (T_c = 86 K) sample (Fig. 2 A and B) with a severely UD Dy1 (T_c = 38 K) sample (Fig. 2 C and D ), the spectra being particle–hole-symmetrized to better illustrate the gap. The OP 86 K sample shows a well-defined quasiparticle peak over the entire Fermi surface (Fig. 2A) with a simple d-wave gap of the form (blue curve in Fig. 2B). For the UD 38 K sample, we see in Fig. 2C well-defined quasiparticles near the node (red spectra), but not near the antinode (blue spectra). The near-nodal gaps (red triangles in Fig. 2D) are obtained from the energy of quasiparticle peaks and continue to follow a d-wave gap (blue curve in Fig. 2D). However, once the quasiparticle peak is lost closer to the antinode, one has to use some other definition of the gap scale. We identify a break in the slope of the spectrum, by locating the energy scale at which it deviates from the black straight lines (Fig. 2C), which leads to the gap estimates (blue squares) in Fig. 2D.Open in a separate windowFig. 2.Superconducting state spectra and energy gap for OD and highly UD Dy1 samples. (A) Symmetrized spectra at k_F, from the antinode (Upper) to the node (Lower) for an OP 86 K Dy1 sample. (B) Gap as a function of Fermi surface angle (0° is the antinode and 45° the node). The blue curve is a d-wave fit to the data. (C) Same as A for an UD 38 K Dy1 sample. Curves, near the node, with discernible quasiparticle peaks are shown in red; those near the antinode are shown in blue. (D) Gap along the Fermi surface from data of C.Open in a separate windowFig. 3.Energy gap anisotropies of various samples. (A) OD 79 K Ca (where ); (B) UD 54 K Ca; (C) OP 81 K Dy2; and (D) UD 59 K Dy2. The two near-optimal samples in A and C both show a simple d-wave gap. This behavior persists in the UD Ca sample of B, but the UD Dy2 sample of D has a two-gap behavior despite having a similar to the UD Ca sample.Despite the larger error bar associated with gap scale extraction in the absence of quasiparticles, it is nevertheless clear (Fig. 2D) that the UD 38 K Dy1 sample has an energy gap that deviates markedly from the simple d-wave form. This observation is called two-gap in the UD regime, in contrast with a single gap near optimality (Fig. 2B). It is easy to observe from Fig. 2 that the Fermi surface angle at which the energy gap starts to deviate from the form matches the one at which the spectral peak gets washed out. This is very similar to the two-gap behavior demonstrated in refs. 5, 15–18. From this, one might conclude that two-gap behavior is directly correlated with a loss of well-defined quasiparticle excitations in the antinodal region. However, we point to recent ARPES data on Y-doped Bi2212 (6, 7), where two-gap behavior has been observed despite the presence of small antinodal quasiparticle peaks.We next show that the two-gap behavior is not a universal feature of all UD samples. To make this point, we compare in Fig. 3 the gap anisotropies of the Ca-doped samples (Fig. 3 A and B) with the Dy2 samples (Fig. 3 C and D) with essentially identical , where both families have the same optimal . The near-optimal samples, OD 79 K Ca (Fig. 3A) and OP 81 K Dy2 (Fig. 3C) samples, both have a simple d-wave anisotropy (although different maximum gap values at the antinode). However, upon underdoping to similar values, the two have markedly different gap anisotropies. The UD 59 K Dy2 sample (Fig. 3D) shows two-gap behavior, and an absence of quasiparticles near the antinode (similar to the discussion in connection with Fig. 2 above). However, the UD 54 K Ca sample (Fig. 3B) continues to exhibit sharp spectral peaks and a single-gap, despite a very similar as the UD 59 K Dy2.Having established the qualitative differences in the gap anisotropies for various samples as a function of underdoping, we next summarize in Fig. 4 the doping evolution of various spectroscopic features. Instead of estimating the carrier concentration in our samples using an empirical equation (22) (that may or may not be valid for various cation substitutions), we prefer to use the measured to label the doping. In Fig. 4A we show the doping evolution of the antinodal energy gap, which is consistent with the known increase in the gap with underdoping.Open in a separate windowFig. 4.Antinodal gaps and quasiparticle weights. (A) Antinodal energy gap as a function of doping for various samples is seen to grow monotonically with underdoping. Here, and in B and C, the doping is characterized by the measured quantity , with UD samples shown to the left of and OD samples to the right. All results are at temperatures well below . (B) Coherent spectral weight for antinodal quasiparticles as a function of doping. Dy-doped samples exhibit a rapid suppression of this weight to zero for UD , whereas the Ca-doped samples show robust antinodal peaks even for . (C) Coherent spectral weight for nodal quasiparticles as a function of doping, which is seen to be much more robust than the antinodal one.The coherent spectral weight Z for antinodal quasiparticles is plotted in Fig. 4B (for details on the procedure used to estimate this weight, from a ratio of spectral areas, see SI Text). The Dy1 and Dy2 samples both show a sudden and complete loss of Z with underdoping (23), which coincides with the appearance of two-gap behavior. In marked contrast with the Dy samples, the Bi2212 and Ca samples that exhibit a single d-wave gap show a gradual drop in the antinodal Z. On the other hand, we find that the nodal excitations are much less sensitive to how the sample is UD compared with the antinodal ones. Similar sharp nodal excitations have been observed in Dy-doped Bi2212 samples in ref. 7 as well. The nodal quasiparticle weight Z in Fig. 4C decreases smoothly with underdoping for all families of samples, as expected for a doped Mott insulator (24).The two-gap behavior and the attendant loss of quasiparticle weight near the antinode imply a nodal–antinodal dichotomy, aspects of which have been recognized in k space (25–27) and in real space (28–30). Two possible, not mutually exclusive, causes of this behavior are disorder and competing orders.It is known that antinodal states are much more susceptible to impurity scattering, whereas near-nodal excitations are protected (31). However, it is not a priori clear why certain cation substitutions (Dy) should lead to more electronic disorder than others (Ca). As shown by our X-ray studies in SI Text, there is no difference in the structural disorder in Dy and Ca samples. One possibility is that Dy has a local moment, but there is no direct experimental evidence for this.The two-gap behavior in UD materials, with a large antinodal gap that persists above , is suggestive of an order parameter, distinct from d-wave superconductivity, which sets in at the pseudogap temperature . There are several experiments (12–14) that find evidence for a broken symmetry at . However, it is not understood how the observed small, and often subtle, order parameter(s) could lead to large antinodal gaps of , with a loss of spectral weight over a much larger energy range (Fig. 1 D and E).We now discuss the pertinence of competing order parameters based on our measurements. First, in our ARPES data, we have not found any direct evidence for density wave ordering (say, from zone folding). Second, our X-ray data did not provide any signature for additional diffraction peaks expected for long-range density wave ordering. However, none of these null results provide definitive evidence for the absence of a density wave ordering, particularly if it were short range. In contrast, in previously published work (5, 15–18), two-gap behavior has been conjectured to be a direct consequence of phase competition between d-wave superconductivity and some type of density wave ordering. As we have demonstrated, two-gap behavior in and of itself is a sample-specific issue and hence, even if we assume a linkage between competing order and two-gap behavior, it cannot be central to the question of superconductivity in HTSC systems.Whatever the mechanism leading to qualitatively different gap anisotropies for the UD Dy and Ca samples, it only produces relatively small, quantitative changes in key aspects of these materials, such as the dependence of on doping, the presence of sharp nodal quasiparticles, and the pseudogap. We thus conclude that antinodal states do not make a substantial contribution to the universal features of HTSCs. Clearly, two gaps are not necessary for high-temperature superconductivity.     

Amphitheater-headed canyons formed by megaflooding at Malad Gorge,Idaho

Many bedrock canyons on Earth and Mars were eroded by upstream propagating headwalls, and a prominent goal in geomorphology and planetary science is to determine formation processes from canyon morphology. A diagnostic link between process and form remains highly controversial, however, and field investigations that isolate controls on canyon morphology are needed. Here we investigate the origin of Malad Gorge, Idaho, a canyon system cut into basalt with three remarkably distinct heads: two with amphitheater headwalls and the third housing the active Wood River and ending in a 7% grade knickzone. Scoured rims of the headwalls, relict plunge pools, sediment-transport constraints, and cosmogenic (³He) exposure ages indicate formation of the amphitheater-headed canyons by large-scale flooding ∼46 ka, coeval with formation of Box Canyon 18 km to the south as well as the eruption of McKinney Butte Basalt, suggesting widespread canyon formation following lava-flow diversion of the paleo-Wood River. Exposure ages within the knickzone-headed canyon indicate progressive upstream younging of strath terraces and a knickzone propagation rate of 2.5 cm/y over at least the past 33 ka. Results point to a potential diagnostic link between vertical amphitheater headwalls in basalt and rapid erosion during megaflooding due to the onset of block toppling, rather than previous interpretations of seepage erosion, with implications for quantifying the early hydrosphere of Mars.Landscapes adjust to perturbations in tectonics and base level through upstream propagation of steepened river reaches, or knickzones, thereby communicating environmental signals throughout a drainage basin (e.g., ref. 1). Nowhere are knickzones more important and apparent than in landscapes where canyon heads actively cut into plateaus, such as tributaries of the Grand Canyon, United States, and the basaltic plains of Mars (e.g., refs. 2–4). Here the stark topographic contrast between low-relief uplands and deeply incised canyons sharply delineates canyon rims and planform morphology. Canyon heads can have varied shapes from amphitheaters with vertical headwalls to more pointed planform shapes with lower gradients, and a prominent goal in geomorphology and planetary science is to link canyon morphology to formation processes (e.g., refs. 4–8), with implications for understanding the history of water on Mars.Amphitheater-headed canyons on Mars are most likely cut into layered basalt (9, 10), and canyon-formation interpretations have ranged widely from slow seepage erosion to catastrophic megafloods (4–6, 11, 12). Few studies have been conducted on the formation of amphitheater-headed canyons in basalt on Earth, however, and instead, terrestrial canyons in other substrates are often used as Martian analogs. For example, groundwater sapping is a key process in forming amphitheater-headed canyons in unconsolidated sand (e.g., refs. 8, 13, 14), but its importance is controversial in rock (5, 12, 15). Amphitheater-headed canyons are also common to plateaus with strong-over-weak sedimentary rocks (3, 16); however, here the tendency for undercutting is so strong that canyon-head morphology may bear little information about erosional processes, whether driven by groundwater or overland flow (e.g., refs. 3, 5, 17). Canyons in some basaltic landscapes lack strong-over-weak stratigraphy, contain large boulders that require transport, and show potential for headwall retreat by block toppling (18–21), all of which make extension of process–form relationships in sand and sedimentary rocks to basalt and Mars uncertain.To test the hypothesis of a link between canyon formation and canyon morphology in basalt, we need field measurements that can constrain formation processes for canyons with distinct morphologies, but carved into the same rock type. Here we report on the origin of Malad Gorge, a canyon complex eroded into columnar basalt with markedly different shaped canyon heads. Results point to a potential diagnostic link between canyon-head morphology and formative process by megaflood erosion in basalt.Malad Gorge is a tributary to the Snake River Canyon, Idaho, within the Snake River Plain, a broad depression filled by volcanic flows that erupted between ∼15 Ma and ∼2 ka (22, 23). The gorge sits at the northern extent of Hagerman Valley, a particularly wide (∼7 km) part of the Snake River Canyon (Fig. 1). Malad Gorge is eroded into the Gooding Butte Basalt [⁴⁰Ar/³⁹Ar eruption age: 373 ± 12 ka (25)] which is composed of stacked lava beds, each several meters thick with similar well-defined columns bounded by cooling joints and no apparent differences in strength between beds. The Wood (or Malad) River, a major drainage system from the Sawtooth Range to the north, drains through Malad Gorge before joining the Snake River. The Wood River is thought to have been diverted from an ancestral, now pillow lava-filled canyon into Malad Gorge by McKinney Butte basalt flows (24) [⁴⁰Ar/³⁹Ar eruption age: 52 ± 24 ka (25) (Fig. 1).Open in a separate windowFig. 1.Shaded relief map of the study region (50-m contour interval) showing basalt flows (23), their exposure age sample locations, and the path of the ancestral Wood River following Malde (24) (US Geological Survey).Malad Gorge contains three distinct canyon heads herein referred to as Woody’s Cove, Stubby Canyon, and Pointed Canyon (Fig. 2A). Woody’s Cove and Stubby have amphitheater heads with ∼50-m-high vertical headwalls (Fig. 2C), and talus accumulation at headwall bases indicates long-lived inactive fluvial transport (Fig. 3 A and B). Woody’s Cove, the shortest of the three canyons, lacks major spring flows and has minor, intermittent overland flow partially fed by irrigation runoff that spills over the canyon rim. Stubby has no modern-day overland flow entering the canyon, and springs emanate from a pool near its headwall (Fig. 3B). In contrast, Pointed Canyon is distinctly more acute in planform morphology, contains a 7% grade knickzone composed of multiple steps rather than a vertical headwall (Figs. 2C and ​and3C),3C), and extends the farthest upstream.Open in a separate windowFig. 2.Malad Gorge topography (10-m contour interval) and aerial orthophotography (US Geological Survey). (A) Overview map and (B) close-up for Stubby and Pointed canyons showing mapped bedrock scours (white arrows), exposure age sample locations (red circles) with age results, location of the uppermost active knickpoint (black circle), abandoned bedrock channels (blue dashed lines), and grain-size analysis sites (blue squares). The blue star shows the reconstructed location of the headwall of Pointed Canyon at 46 ka (see Discussion and Fig. 5). (C) Longitudinal profile along Stubby and Pointed canyons from their confluence (shown as white lines in B) with local slope, S, averaged over regions demarked by dashed lines (Fig. 4A shows close-up of profile in Stubby Canyon).Open in a separate windowFig. 3.Photographs of (A) headwall of Woody’s Cove (person for scale, circled), (B) ∼50-m-high headwall of Stubby Canyon, (C) downstream-most waterfall at Pointed Canyon knickzone (12-m-high waterfall with overcrossing highway for scale), (D) fluted and polished notch at the rim of Stubby Canyon (notch relief is 10 m), (E) upstream-most waterfall at Pointed Canyon knickzone (within the southern anabranch of Fig. S2), and (F) upstream-most abandoned channel in Fig. 2B and Fig. S2 (channel relief is ∼10 m). White coloring on the headwalls in A and B is likely residue from irrigation runoff.Early work attributed the amphitheater-headed canyons in this region—Malad Gorge, Box Canyon, located 18 km south of Malad Gorge (Fig. 1), and Blue Lakes Canyon located 42 km to the SE—to formation by seepage erosion because of no modern overland flow and the occurrence of some of the largest springs in the United States in this region (7). Because spring flows (e.g., ∼10 m³/s in Box Canyon; US Geological Survey gauge 13095500) are far deficient to move the boulders that line the canyon floors, Stearns (7) reasoned that the boulders must chemically erode in place. This explanation is improbable, however, given the young age of the Quaternary basalt (25), spring water saturated in dissolved solids (19), and no evidence of rapid chemical weathering (e.g., talus blocks are angular and have little to no weathering rinds). Instead of groundwater sapping, Box Canyon was likely carved by a large-scale flood event that occurred ∼45 ka based on ³He cosmogenic exposure age dating of the scoured rim of the canyon headwall (19, 26). In addition, Blue Lakes Canyon was formed during the Bonneville Flood [∼18–22 ka (27, 28)], one of the world’s largest outburst floods that occurred as a result of catastrophic draining of glacial lake Bonneville (21). In both cases, canyon formation was inferred to have occurred through upstream headwall propagation by waterfall erosion.Herein we aim to test whether the amphitheater-headed canyons at Malad Gorge also owe their origin to catastrophic flooding, whether Pointed Canyon has a different origin, and whether canyon morphology is diagnostic of formation process. To this end we present field observations, sediment-size measurements, hydraulic modeling, and cosmogenic exposure ages of water-scoured rock surfaces and basalt-flow surfaces (Methods and Tables S1 and S2).