Abrupt change of Antarctic moisture origin at the end of Termination II

The deuterium excess of polar ice cores documents past changes in evaporation conditions and moisture origin. New data obtained from the European Project for Ice Coring in Antarctica Dome C East Antarctic ice core provide new insights on the sequence of events involved in Termination II, the transition between the penultimate glacial and interglacial periods. This termination is marked by a north–south seesaw behavior, with first a slow methane concentration rise associated with a strong Antarctic temperature warming and a slow deuterium excess rise. This first step is followed by an abrupt north Atlantic warming, an abrupt resumption of the East Asian summer monsoon, a sharp methane rise, and a CO₂ overshoot, which coincide within dating uncertainties with the end of Antarctic optimum. Here, we show that this second phase is marked by a very sharp Dome C centennial deuterium excess rise, revealing abrupt reorganization of atmospheric circulation in the southern Indian Ocean sector.     

Climatically driven emissions of hydrocarbons from marine sediments during deglaciation

Marine hydrocarbon seepage emits oil and gas, including methane ( approximately 30 Tg of CH(4) per year), to the ocean and atmosphere. Sediments from the California margin contain preserved tar, primarily formed through hydrocarbon weathering at the sea surface. We present a record of variation in the abundance of tar in sediments for the past 32,000 years, providing evidence for increases in hydrocarbon emissions before and during Termination IA [16,000 years ago (16 ka) to 14 ka] and again over Termination IB (11-10 ka). Our study provides direct evidence for increased hydrocarbon seepage associated with deglacial warming through tar abundance in marine sediments, independent of previous geochemical proxies. Climate-sensitive gas hydrates may modulate thermogenic hydrocarbon seepage during deglaciation.     

Widespread collapse of the Ross Ice Shelf during the late Holocene

The stability of modern ice shelves is threatened by atmospheric and oceanic warming. The geologic record of formerly glaciated continental shelves provides a window into the past of how ice shelves responded to a warming climate. Fields of deep (−560 m), linear iceberg furrows on the outer, western Ross Sea continental shelf record an early post-Last Glacial Maximum episode of ice-shelf collapse that was followed by continuous retreat of the grounding line for ∼200 km. Runaway grounding line conditions culminated once the ice became pinned on shallow banks in the western Ross Sea. This early episode of ice-shelf collapse is not observed in the eastern Ross Sea, where more episodic grounding line retreat took place. More widespread (∼280,000 km²) retreat of the ancestral Ross Ice Shelf occurred during the late Holocene. This event is recorded in sediment cores by a shift from terrigenous glacimarine mud to diatomaceous open-marine sediment as well as an increase in radiogenic beryllium (¹⁰Be) concentrations. The timing of ice-shelf breakup is constrained by compound specific radiocarbon ages, the first application of this technique systematically applied to Antarctic marine sediments. Breakup initiated around 5 ka, with the ice shelf reaching its current configuration ∼1.5 ka. In the eastern Ross Sea, the ice shelf retreated up to 100 km in about a thousand years. Three-dimensional thermodynamic ice-shelf/ocean modeling results and comparison with ice-core records indicate that ice-shelf breakup resulted from combined atmospheric warming and warm ocean currents impinging onto the continental shelf.Ice shelves are among the most rapidly changing elements of the modern cryosphere, due to their internal weaknesses, atmospheric warming, and melting from beneath by warm ocean currents. In the northern Antarctic Peninsula, accelerated atmospheric warming is the principle cause of ongoing ice-shelf retreat (1, 2), and collapse of the Larsen Ice Shelf has resulted in rapid retreat of tidewater glaciers flowing into the ice shelf (2, 3). Farther south in Pine Island Bay, thermal erosion of the floating terminus of Pine Island Glacier by impinging Circumpolar Deep Water (CDW) results in basal melt rates of 6–12.5 m⋅y⁻¹ and is causing rapid grounding line retreat that poses a threat of ice-stream collapse in the foreseeable future (4, 5). Geological evidence for ice-shelf collapse has been reported for Marguerite Bay in the southern Antarctic Peninsula (6, 7) and in Pine Island Bay in West Antarctica (8, 9), but the timing and rate of these events are poorly constrained.The modern Ross Ice Shelf is the largest ice shelf on Earth, covering an area of ∼500,000 km². It provides a buttress to the outflow of several large outlet glaciers and ice streams that drain the West Antarctic ice sheet (WAIS) and East Antarctic Ice Sheet (EAIS) and thus plays a crucial role in ice-sheet stability (10, 11). Here we report compelling geomorphological, sedimentological, and geochemical evidence for widespread retreat of the Ross Ice Shelf at ∼5 ka to 1.5 ka. Modeling results and comparison with ice-core records indicate that ice-shelf breakup was triggered by oceanic and atmospheric warming.     

Asynchronous marine-terrestrial signals of the last deglacial warming in East Asia associated with low- and high-latitude climate changes

A high-resolution multiproxy record, including pollen, foraminifera, and alkenone paleothermometry, obtained from a single core (DG9603) from the Okinawa Trough, East China Sea (ECS), provided unambiguous evidence for asynchronous climate change between the land and ocean over the past 40 ka. On land, the deglacial stage was characterized by rapid warming, as reflected by paleovegetation, and it began ca. 15 kaBP, consistent with the timing of the last deglacial warming in Greenland. However, sea surface temperature estimates from foraminifera and alkenone paleothermometry increased around 20–19 kaBP, as in the Western Pacific Warm Pool (WPWP). Sea surface temperatures in the Okinawa Trough were influenced mainly by heat transport from the tropical western Pacific Ocean by the Kuroshio Current, but the epicontinental vegetation of the ECS was influenced by atmospheric circulation linked to the northern high-latitude climate. Asynchronous terrestrial and marine signals of the last deglacial warming in East Asia were thus clearly related to ocean currents and atmospheric circulation. We argue that (i) early warming seawater of the WPWP, driven by low-latitude insolation and trade winds, moved northward via the Kuroshio Current and triggered marine warming along the ECS around 20–19 kaBP similar to that in the WPWP, and (ii) an almost complete shutdown of the Atlantic Meridional Overturning Circulation ca. 18–15 kaBP was associated with cold Heinrich stadial-1 and delayed terrestrial warming during the last deglacial warming until ca. 15 kaBP at northern high latitudes, and hence in East Asia. Terrestrial deglacial warming therefore lagged behind marine changes by ca. 3–4 ka.     

Eastern equatorial pacific productivity and related-CO2 changes since the last glacial period

Understanding oceanic processes, both physical and biological, that control atmospheric CO(2) is vital for predicting their influence during the past and into the future. The Eastern Equatorial Pacific (EEP) is thought to have exerted a strong control over glacial/interglacial CO(2) variations through its link to circulation and nutrient-related changes in the Southern Ocean, the primary region of the world oceans where CO(2)-enriched deep water is upwelled to the surface ocean and comes into contact with the atmosphere. Here we present a multiproxy record of surface ocean productivity, dust inputs, and thermocline conditions for the EEP over the last 40,000 y. This allows us to detect changes in phytoplankton productivity and composition associated with increases in equatorial upwelling intensity and influence of Si-rich waters of sub-Antarctic origin. Our evidence indicates that diatoms outcompeted coccolithophores at times when the influence of Si-rich Southern Ocean intermediate waters was greatest. This shift from calcareous to noncalcareous phytoplankton would cause a lowering in atmospheric CO(2) through a reduced carbonate pump, as hypothesized by the Silicic Acid Leakage Hypothesis. However, this change does not seem to have been crucial in controlling atmospheric CO(2), as it took place during the deglaciation, when atmospheric CO(2) concentrations had already started to rise. Instead, the concomitant intensification of Antarctic upwelling brought large quantities of deep CO(2)-rich waters to the ocean surface. This process very likely dominated any biologically mediated CO(2) sequestration and probably accounts for most of the deglacial rise in atmospheric CO(2).     

20th-Century doubling in dust archived in an Antarctic Peninsula ice core parallels climate change and desertification in South America

Crustal dust in the atmosphere impacts Earth's radiative forcing directly by modifying the radiation budget and affecting cloud nucleation and optical properties, and indirectly through ocean fertilization, which alters carbon sequestration. Increased dust in the atmosphere has been linked to decreased global air temperature in past ice core studies of glacial to interglacial transitions. We present a continuous ice core record of aluminum deposition during recent centuries in the northern Antarctic Peninsula, the most rapidly warming region of the Southern Hemisphere; such a record has not been reported previously. This record shows that aluminosilicate dust deposition more than doubled during the 20th century, coincident with the approximately 1 degrees C Southern Hemisphere warming: a pattern in parallel with increasing air temperatures, decreasing relative humidity, and widespread desertification in Patagonia and northern Argentina. These results have far-reaching implications for understanding the forces driving dust generation and impacts of changing dust levels on climate both in the recent past and future.     

Timing and dynamics of Late Pleistocene mammal extinctions in southwestern Australia

Explaining the Late Pleistocene demise of many of the world's larger terrestrial vertebrates is arguably the most enduring and debated topic in Quaternary science. Australia lost >90% of its larger species by around 40 thousand years (ka) ago, but the relative importance of human impacts and increased aridity remains unclear. Resolving the debate has been hampered by a lack of sites spanning the last glacial cycle. Here we report on an exceptional faunal succession from Tight Entrance Cave, southwestern Australia, which shows persistence of a diverse mammal community for at least 100 ka leading up to the earliest regional evidence of humans at 49 ka. Within 10 millennia, all larger mammals except the gray kangaroo and thylacine are lost from the regional record. Stable-isotope, charcoal, and small-mammal records reveal evidence of environmental change from 70 ka, but the extinctions occurred well in advance of the most extreme climatic phase. We conclude that the arrival of humans was probably decisive in the southwestern Australian extinctions, but that changes in climate and fire activity may have played facilitating roles. One-factor explanations for the Pleistocene extinctions in Australia are likely oversimplistic.     

Ecological consequences of early Late Pleistocene megadroughts in tropical Africa

Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416-16421]. This resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an approximately 125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (approximately 35-15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa.     

Late quaternary extinction of a tree species in eastern North America

Widespread species- and genus-level extinctions of mammals in North America and Europe occurred during the last deglaciation [16,000-9,000 yr B.P. (by (14)C)], a period of rapid and often abrupt climatic and vegetational change. These extinctions are variously ascribed to environmental change and overkill by human hunters. By contrast, plant extinctions since the Middle Pleistocene are undocumented, suggesting that plant species have been able to respond to environmental changes of the past several glacial/interglacial cycles by migration. We provide evidence from morphological studies of fossil cones and anatomical studies of fossil needles that a now-extinct species of spruce (Picea critchfieldii sp. nov.) was widespread in eastern North America during the Last Glacial Maximum. P. critchfieldii was dominant in vegetation of the Lower Mississippi Valley, and extended at least as far east as western Georgia. P. critchfieldii disappeared during the last deglaciation, and its extinction is not directly attributable to human activities. Similarly widespread plant species may be at risk of extinction in the face of future climate change.     

The early rise and late demise of New Zealand’s last glacial maximum

Recent debate on records of southern midlatitude glaciation has focused on reconstructing glacier dynamics during the last glacial termination, with different results supporting both in-phase and out-of-phase correlations with Northern Hemisphere glacial signals. A continuing major weakness in this debate is the lack of robust data, particularly from the early and maximum phase of southern midlatitude glaciation (∼30–20 ka), to verify the competing models. Here we present a suite of 58 cosmogenic exposure ages from 17 last-glacial ice limits in the Rangitata Valley of New Zealand, capturing an extensive record of glacial oscillations between 28–16 ka. The sequence shows that the local last glacial maximum in this region occurred shortly before 28 ka, followed by several successively less extensive ice readvances between 26–19 ka. The onset of Termination 1 and the ensuing glacial retreat is preserved in exceptional detail through numerous recessional moraines, indicating that ice retreat between 19–16 ka was very gradual. Extensive valley glaciers survived in the Rangitata catchment until at least 15.8 ka. These findings preclude the previously inferred rapid climate-driven ice retreat in the Southern Alps after the onset of Termination 1. Our record documents an early last glacial maximum, an overall trend of diminishing ice volume in New Zealand between 28–20 ka, and gradual deglaciation until at least 15 ka.According to the Milankovitch orbital theory of glaciation, variations in northern high-latitude summer insolation are responsible for glacial–interglacial cycles (1, 2). On this basis, it is commonly assumed that climatic changes in the Northern Hemisphere (NH) constitute the principle forcing mechanism for glaciation in the Southern Hemisphere (SH) (3). However, in recent times, this view has been challenged by studies showing that at least some glacial signals in the SH have no NH correlative or precede events in the NH by up to 1.5 ka (4–7). Although some of these patterns can be explained by an extended bipolar seesaw model, other features, including the decreasing expression of hemispheric antiphasing away from the polar regions, indicate that additional forcing mechanisms must be involved (8).A compilation of glacial records from the NH indicates that midlatitude ice sheets in North America and northern Eurasia began a major expansion phase between 33–29 ka (9). From this time onward, relative sea level reconstructions indicate a steady increase in global ice volume, until most ice sheets had reached a near-maximum extent by 26.5 ka, followed by 6–7 ky of equilibrium conditions until the onset of final retreat around 20–19 ka (9). For the midlatitude SH, specifically New Zealand (NZ), paleoecologic data (10), supported by dated glaciofluvial aggradation sequences (11), have also suggested an onset of mountain glacier growth around 30–28 ka (12). Contrary to NH ice volume trends, however, recent records from NZ may indicate that maximum glaciation in the Southern Alps occurred before 28 ka, several thousand years before the NH ice maximum (13–15). A second aspect in the debate centers on determining the precise onset of last glacial maximum (LGM) retreat (a period referred to as Termination 1 and dated to ∼19–10 ka) and the mode of this deglaciation (i.e., models of collapse versus slow recession) in NZ. Together, these issues are responsible for ongoing controversy about the evolution of the last glacial-to-interglacial transition in the SH midlatitudes and the relative importance of interhemispheric climate forcing versus regional insolation and/or synoptic forcing of SH glaciation.     

From the Cover: Antarctic climate signature in the Greenland ice core record

A numerical algorithm is applied to the Greenland Ice Sheet Project 2 (GISP2) dust record from Greenland to remove the abrupt changes in dust flux associated with the Dansgaard-Oeschger (D-O) oscillations of the last glacial period. The procedure is based on the assumption that the rapid changes in dust are associated with large-scale changes in atmospheric transport and implies that D-O oscillations (in terms of their atmospheric imprint) are more symmetric in form than can be inferred from Greenland temperature records. After removal of the abrupt shifts the residual, dejumped dust record is found to match Antarctic climate variability with a temporal lag of several hundred years. It is argued that such variability may reflect changes in the source region of Greenland dust (thought to be the deserts of eastern Asia). Other records from this region and more globally also reveal Antarctic-style variability and suggest that this signal is globally pervasive. This provides the potential basis for suggesting a more important role for gradual changes in triggering more abrupt transitions in the climate system.     

Younger Dryas cooling and the Greenland climate response to CO2

Greenland ice-core δ¹⁸O-temperature reconstructions suggest a dramatic cooling during the Younger Dryas (YD; 12.9–11.7 ka), with temperatures being as cold as the earlier Oldest Dryas (OD; 18.0–14.6 ka) despite an approximately 50 ppm rise in atmospheric CO₂. Such YD cooling implies a muted Greenland climate response to atmospheric CO₂, contrary to physical predictions of an enhanced high-latitude response to future increases in CO₂. Here we show that North Atlantic sea surface temperature reconstructions as well as transient climate model simulations suggest that the YD over Greenland should be substantially warmer than the OD by approximately 5 °C in response to increased atmospheric CO₂. Additional experiments with an isotope-enabled model suggest that the apparent YD temperature reconstruction derived from the ice-core δ¹⁸O record is likely an artifact of an altered temperature-δ¹⁸O relationship due to changing deglacial atmospheric circulation. Our results thus suggest that Greenland climate was warmer during the YD relative to the OD in response to rising atmospheric CO₂, consistent with sea surface temperature reconstructions and physical predictions, and has a sensitivity approximately twice that found in climate models for current climate due to an enhanced albedo feedback during the last deglaciation.     

Two terrestrial records of rapid climatic change during the glacial-Holocene transition (14,000- 9,000 calendar years B.P.) from Europe

Two independent multidisciplinary studies of climatic change during the glacial-Holocene transition (ca. 14,000-9,000 calendar yr B.P.) from Norway and Switzerland have assessed organism responses to the rapid climatic changes and made quantitative temperature reconstructions with modern calibration data sets (transfer functions). Chronology at Krakenes, western Norway, was derived from calibration of a high-resolution series of 14C dates. Chronologies at Gerzensee and Leysin, Switzerland, were derived by comparison of delta18O in lake carbonates with the delta18O record from the Greenland Ice Core Project. Both studies demonstrate the sensitivity of terrestrial and aquatic organisms to rapid temperature changes and their value for quantitative reconstruction of the magnitudes and rates of the climatic changes. The rates in these two terrestrial records are comparable to those in Greenland ice cores, but the actual temperatures inferred apply to the terrestrial environments of the two regions.     

Global climate evolution during the last deglaciation

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO₂ and CH₄ to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.During the interval of global warming from the end of the Last Glacial Maximum (LGM) approximately 19 ka to the early Holocene 11 ka, virtually every component of the climate system underwent large-scale change, sometimes at extraordinary rates, as the world emerged from the grips of the last ice age (Fig. 1). This dramatic time of global change was triggered by changes in insolation, with associated changes in ice sheets, greenhouse gas concentrations, and other amplifying feedbacks that produced distinctive regional and global responses. In addition, there were several episodes of large and rapid sea-level rise and abrupt climate change (Fig. 2) that produced regional climate signals superposed on those associated with global warming. Considerable ice-sheet melting and sea-level rise occurred after 11 ka, but otherwise the world had entered the current interglaciation with near-pre-Industrial greenhouse gas concentrations and relatively stable climates. Here we synthesize well-dated, high-resolution ocean and terrestrial proxy records to describe regional and global patterns of climate change during this interval of deglaciation.Open in a separate windowFig. 1.(A) Climate simulation of the Last Glacial Maximum 21,000 y ago using the National Center for Atmospheric Research Community Climate System Model, version 3.0 (141). Sea-surface temperatures are anomalies relative to the control climate. Also shown are continental ice sheets (1,000-m contours) (149) and leaf-area index simulated by the model (scale bar shown). (B) Same as A except for 11 ka.Open in a separate windowFig. 2.Climate records and forcings during the last deglaciation. The oxygen-isotope (δ¹⁸O) records from Greenland Ice Sheet Project Two (GISP2) (150) (dark-blue line) and Greenland Ice Core Project (GRIP) (151) (light-blue line) Greenland ice cores shown in (A) (placed on the GICC05 timescale; ref. 57) document millennial-scale events that correspond to those first identified in northern European floral and pollen records. LGM, Last Glacial Maximum; OD, Oldest Dryas; BA, Bølling–Allerød; ACR, Antarctic Cold Reversal; YD, Younger Dryas. (B) Oxygen-isotope (δ¹⁸O) record from European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (152) (dark-green line) and deuterium (δD) record from Dome C (45) (light-green line) Antarctic ice cores, placed on a common timescale (2). (C) Midmonth insolation at 65°N for July (orange line) and at 65°S for January (light-blue line) (153). (D) The combined radiative forcing (red line) from CO₂ (blue dashed line), CH₄ (green dashed line), and N₂O (purple dashed line) relative to preindustrial levels. CO₂ is from EPICA Dome C ice core (1) on Greenland Ice Core Chronology 05 (GICC05) timescale from ref. 2, CH₄ is from GRIP ice core (154) on the GICC05 timescale, and N₂O is from EPICA Dome C (155) and GRIP (156) ice cores on the GICC05 timescale. Greenhouse gas concentrations were converted to radiative forcings using the simplified expressions in ref. 157. The CH₄ radiative forcing was multiplied by 1.4 to account for its greater efficacy relative to CO₂ (158). (E) Relative sea-level data from Bonaparte Gulf (green crosses) (159), Barbados (gray and dark-blue triangles) (160), New Guinea (light-blue triangles) (161, 162), Sunda Shelf (purple crosses) (163), and Tahiti (green triangles) (164). Also shown is eustatic sea level (gray line) (165). (F) Rate of change of area of Laurentide Ice Sheet (LIS) (166) and Scandinavian Ice Sheet (SIS) (SI Appendix). (G) Freshwater flux to the global oceans derived from eustatic sea level in E. (H) Record of ice-rafted detrital carbonate from North Atlantic core VM23-81 identifying times of Heinrich events 1 and 0 (167). (I) Freshwater flux associated with routing of continental runoff through the St. Lawrence and Hudson rivers (filled blue time series) with age uncertainties (168). Also shown is time series of runoff through the St. Lawrence River during the Younger Dryas (solid blue line) (142).Between the LGM and present, seasonal insolation anomalies arising from the combined effects of eccentricity, precession, and obliquity were generally opposite in sign between hemispheres (Fig. 2C), whereas variations in annual-average insolation were symmetrical about the equator. At the LGM, seasonal insolation was similar to present, whereas subsequent changes in obliquity and perihelion caused Northern-Hemisphere seasonality to reach a maximum in the early Holocene.CO₂ concentrations started to rise from the LGM minimum approximately 17.5 ± 0.5 ka (1). The onset of the CO₂ rise may have lagged the start of Antarctic warming by 800 ± 600 years (1), but this may be an overestimate (2). CO₂ levels stabilized from approximately 14.7–12.9 ka, and then rose again from about 12.9–11.7 ka, reaching near-interglacial maximum levels shortly thereafter. CH₄ concentrations also began to rise starting at approximately 17.5 ka, with a subsequent abrupt increase at 14.7 ka, an abrupt decrease at about 12.9 ka, followed by a rise at approximately 11.7 ka (3). Changes in N₂O concentrations appear to follow changes in CH₄ (4). The combined variations in radiative forcing due to greenhouse gases (GHGs) is dominated by CO₂, but abrupt changes in CH₄ and N₂O modulate the overall structure, accentuating the rapid increase at 14.7 ka and causing a slight reduction from 12.9–11.7 ka (Fig. 2D).Freshwater forcing of the Atlantic meridional overturning circulation (AMOC) is commonly invoked to explain past and possibly future abrupt climate change (5, 6). During the last deglaciation, the AMOC was likely affected by variations in moisture transport across Central America (7), salt and heat transport from the Indian Ocean (8), freshwater exchange across the Bering Strait (9), and the flux of meltwater and icebergs from adjacent ice sheets (6). The first two factors largely represent feedbacks on AMOC variability. Freshwater exchange across the Bering Strait began with initial submergence of the Strait during deglacial sea-level rise. Highly variable fluxes from ice-sheet melting and calving and routing of continental runoff (Fig. 2 E–I) also directly forced the AMOC, but uncertainties in the sources of several key events remain (SI Appendix).     

Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 y

The dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles, ocean circulation through its influence on atmospheric circulation, and sea-surface temperatures. However, few records from the summer North American Monsoon system are available to test for a synchronous response with other global monsoons to shared forcings. In particular, the monsoon response to widespread atmospheric reorganizations associated with disruptions of the Atlantic Meridional Overturning Circulation (AMOC) during the deglacial period remains unconstrained. Here, we present a high-resolution and radiometrically dated monsoon rainfall reconstruction over the past 22,000 y from speleothems of tropical southwestern Mexico. The data document an active Last Glacial Maximum (18–24 cal ka B.P.) monsoon with similar δ¹⁸O values to the modern, and that the monsoon collapsed during periods of weakened AMOC during Heinrich stadial 1 (ca. 17 ka) and the Younger Dryas (12.9–11.5 ka). The Holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation. We conclude that the Mesoamerican monsoon responded in concert with other global monsoon regions, and that monsoon strength was driven by variations in the strength and latitudinal position of the Intertropical Convergence Zone, which was forced by AMOC variations in the North Atlantic Ocean. The surprising observation of an active Last Glacial Maximum monsoon is attributed to an active but shallow AMOC and proximity to the Intertropical Convergence Zone. The emergence of agriculture in southwestern Mexico was likely only possible after monsoon strengthening in the Early Holocene at ca. 11 ka.     

Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice

The observed sea surface temperature in the Southern Ocean shows a substantial warming trend for the second half of the 20th century. Associated with the warming, there has been an enhanced atmospheric hydrological cycle in the Southern Ocean that results in an increase of the Antarctic sea ice for the past three decades through the reduced upward ocean heat transport and increased snowfall. The simulated sea surface temperature variability from two global coupled climate models for the second half of the 20th century is dominated by natural internal variability associated with the Antarctic Oscillation, suggesting that the models’ internal variability is too strong, leading to a response to anthropogenic forcing that is too weak. With increased loading of greenhouse gases in the atmosphere through the 21st century, the models show an accelerated warming in the Southern Ocean, and indicate that anthropogenic forcing exceeds natural internal variability. The increased heating from below (ocean) and above (atmosphere) and increased liquid precipitation associated with the enhanced hydrological cycle results in a projected decline of the Antarctic sea ice.     

Atmospheric composition 1 million years ago from blue ice in the Allan Hills,Antarctica

Here, we present direct measurements of atmospheric composition and Antarctic climate from the mid-Pleistocene (∼1 Ma) from ice cores drilled in the Allan Hills blue ice area, Antarctica. The 1-Ma ice is dated from the deficit in ⁴⁰Ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12-m section at the base of a 126-m ice core. The 1-Ma ice appears to represent most of the amplitude of contemporaneous climate cycles and CO₂ and CH₄ concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion (ppb), respectively. These concentrations, together with measured δD of the ice, are at the warm end of the field for glacial–interglacial cycles of the last 800 ky and span only about one-half of the range. The highest CO₂ values in the 1-Ma ice fall within the range of interglacial values of the last 400 ka but are up to 7 ppm higher than any interglacial values between 450 and 800 ka. The lowest CO₂ values are 30 ppm higher than during any glacial period between 450 and 800 ka. This study shows that the coupling of Antarctic temperature and atmospheric CO₂ extended into the mid-Pleistocene and demonstrates the feasibility of discontinuously extending the current ice core record beyond 800 ka by shallow coring in Antarctic blue ice areas.Ice cores serve as a critical archive of past environmental conditions, providing constraints on global atmospheric composition and the climate of polar regions (1). Reconstructions of atmospheric CO₂ and CH₄ from air trapped in ice cores dating as far back as 800 ka indicate a link between greenhouse gases and global climate in the form of 100-ky glacial cycles (Fig. 1). These climate cycles are recorded in proxy records from deep sea sediments reflecting variations in ocean temperature and continental ice volume (2). Deep sea records indicate that the 100-ky glacial cycle developed only ∼900,000 y ago [the mid-Pleistocene transition (MPT)] (3). Before this time and going back to 2.8 Ma, glacial cycles lasted, on average, 40 ky (4). The origins of both the 100- and 40-ky glacial cycles, their links to orbital forcing, and changes in atmospheric greenhouse gases are debated. Extending ice core records to earlier times would advance our understanding of links between greenhouse gases, climate, and causes of the MPT.Open in a separate windowFig. 1.Records of (A) CH₄, (B) CO₂, and (C) δD from the Allan Hills BIA (Site 27; black line and black symbols between 115 and 250 ka) compared with records from Vostok/EPICA Dome C (green, red, and blue lines) (11, 18–20). The range of gas and ice properties in the 1-Ma ice from Site BIT-58 is shown to the right (Tables S1–S4). Boxes around the 1-Ma data indicate an age uncertainty of ±89 ky (SE) for n = 6 measurements of ice below 117 m assuming an external reproducibility (1σ) of ±213 ky (Materials and Methods has additional details). D shows the stacked benthic foraminiferal δ¹⁸O record (4), and E shows a record of deep ocean temperature based on foraminiferal Mg/Ca (17). ppb, parts per billion.One archive for extending ice core records beyond 800 ky is blue ice areas (BIAs), outcrops of glacial ice brought to the surface by ice flow guided by bedrock topography (5). These records are likely to be stratigraphically complex because of deformation associated with ice transport but may also contain the oldest easily accessible ice on the planet. In the Allan Hills, Antarctica, the antiquity of shallow ice is documented by terrestrial ages of englacial meteorites exposed on the surface by ablation (6). These ages cluster between 100 and 400 ky, with a small number that extend to 1 Ma and a single meteorite yielding an age of 2.2 Ma (7).Here, we present the first, to our knowledge, direct snapshots of atmospheric composition during the MPT from an ice core drilled at Site BIT-58 (8) in the Allan Hills BIA (Fig. S1). We date ice and trapped gases directly, taking advantage of the slow leak of ⁴⁰Ar into the atmosphere from the decay of ⁴⁰K in Earth’s interior (the ⁴⁰Ar_atm geochronometer). The observed increase in the ⁴⁰Ar/³⁸Ar ratio of the atmosphere is small, resulting in uncertainties for a single sample of ±213 ky (9). Measurements of Ar isotope ratios date a 12-m section at the base of Site BIT-58 to ∼1 Ma (Fig. 2). We report on measurements of CO₂ and CH₄ concentrations, the δ¹⁸O of paleoatmospheric O₂, δD, and deuterium excess (d) in the 1-Ma ice from Site BIT-58. This work provides a direct window into atmospheric composition and Antarctic climate during the mid-Pleistocene.Open in a separate windowFig. 2.Measured δ¹⁸O_atm, δD_ice, ⁴⁰Ar_atm ages, CO₂, and CH₄ concentrations of the 126-m ice core from Site BIT-58. Error bars on ⁴⁰Ar_atm ages of ±213 ky represent 1σ uncertainties associated with repeat measurements of Princeton Air (n = 67).     

Brief Report: Methane release from carbonate rock formations in the Siberian permafrost area during and after the 2020 heat wave

Anthropogenic global warming may be accelerated by a positive feedback from the mobilization of methane from thawing Arctic permafrost. There are large uncertainties about the size of carbon stocks and the magnitude of possible methane emissions. Methane cannot only be produced from the microbial decay of organic matter within the thawing permafrost soils (microbial methane) but can also come from natural gas (thermogenic methane) trapped under or within the permafrost layer and released when it thaws. In the Taymyr Peninsula and surroundings in North Siberia, the area of the worldwide largest positive surface temperature anomaly for 2020, atmospheric methane concentrations have increased considerably during and after the 2020 heat wave. Two elongated areas of increased atmospheric methane concentration that appeared during summer coincide with two stripes of Paleozoic carbonates exposed at the southern and northern borders of the Yenisey-Khatanga Basin, a hydrocarbon-bearing sedimentary basin between the Siberian Craton to the south and the Taymyr Fold Belt to the north. Over the carbonates, soils are thin to nonexistent and wetlands are scarce. The maxima are thus unlikely to be caused by microbial methane from soils or wetlands. We suggest that gas hydrates in fractures and pockets of the carbonate rocks in the permafrost zone became unstable due to warming from the surface. This process may add unknown quantities of methane to the atmosphere in the near future.     

Glacial forcing of central Indonesian hydroclimate since 60,000 y B.P.

The Indo-Pacific warm pool houses the largest zone of deep atmospheric convection on Earth and plays a critical role in global climate variations. Despite the region’s importance, changes in Indo-Pacific hydroclimate on orbital timescales remain poorly constrained. Here we present high-resolution geochemical records of surface runoff and vegetation from sediment cores from Lake Towuti, on the island of Sulawesi in central Indonesia, that continuously span the past 60,000 y. We show that wet conditions and rainforest ecosystems on Sulawesi present during marine isotope stage 3 (MIS3) and the Holocene were interrupted by severe drying between ∼33,000 and 16,000 y B.P. when Northern Hemisphere ice sheets expanded and global temperatures cooled. Our record reveals little direct influence of precessional orbital forcing on regional climate, and the similarity between MIS3 and Holocene climates observed in Lake Towuti suggests that exposure of the Sunda Shelf has a weaker influence on regional hydroclimate and terrestrial ecosystems than suggested previously. We infer that hydrological variability in this part of Indonesia varies strongly in response to high-latitude climate forcing, likely through reorganizations of the monsoons and the position of the intertropical convergence zone. These findings suggest an important role for the tropical western Pacific in amplifying glacial–interglacial climate variability.Three major zones of deep atmospheric convection energize the Earth’s moisture and energy budgets: tropical Africa, the Amazon, and the Indo-Pacific. Convection over the Indo-Pacific warm pool (IPWP) is by far the largest of these, and exerts enormous influence on global climate through its role in coupled ocean–atmosphere circulation (1, 2) and its influence on the concentration of atmospheric water vapor, which is the Earth’s most important greenhouse gas (3). Despite the region’s importance, variations in Indo-Pacific hydroclimate on orbital timescales remain poorly constrained.Climate models and theory predict that Indo-Pacific hydrology responds strongly to, and interacts with, glacial–interglacial climate variations (4–6). This prediction is partly borne out by terrestrial sedimentary records that suggest widespread drying across the IPWP during the Last Glacial Maximum (LGM; refs. 7, 8) between 19,000 and 26,000 y ago (9). Unfortunately, many of these records are relatively short and discontinuous, limiting their utility to detect the relationship between regional climate change and global forcing. New, long, high-resolution oxygen isotopic (δ¹⁸O) records from speleothems from northern Borneo paint a very different picture of Indo-Pacific paleoclimate, suggesting that orbital-scale changes in regional convection are dominantly controlled by changes in equatorial insolation driven by orbital precession (10). On the other hand, marine sedimentary runoff records from southern Java imply little change in IPWP hydrology at glacial–interglacial timescales (11). Given this disagreement, new records—especially long proxy records that respond strongly to precipitation—are needed to understand the response of Indo-Pacific climate to glacial–interglacial climate change and forcing.Indonesia lies at the center of the IPWP and has thousands of lakes, the sediments of which represent a largely untapped archive of the region’s hydrologic history. Here we present a 60 thousand y (ky) B.P. record of IPWP hydrology from the sediments of Lake Towuti, located on the island of Sulawesi in central Indonesia (Fig. 1). Lake Towuti is the largest tectonic lake in Indonesia, and at 205 m depth, its sediments preserve perhaps the longest and most continuous terrestrial record of climate available from the region. In 2007–2010, we recovered 13 sediment piston cores from Lake Towuti; here we focus on the most continuous radiocarbon-dated stratigraphy from core TOW10-9B (Material and Methods and Fig. S1 and Tables S1 and S2).Open in a separate windowFig. 1.(Upper) A map of Indonesia showing the location of Lake Towuti and regional records discussed in the text. (Lower) A regional map showing the location of Lake Towuti (2.5°S, 121.5°E) within central Sulawesi.     

Younger Dryas "black mats" and the Rancholabrean termination in North America

Of the 97 geoarchaeological sites of this study that bridge the Pleistocene-Holocene transition (last deglaciation), approximately two thirds have a black organic-rich layer or “black mat” in the form of mollic paleosols, aquolls, diatomites, or algal mats with radiocarbon ages suggesting they are stratigraphic manifestations of the Younger Dryas cooling episode 10,900 B.P. to 9,800 B.P. (radiocarbon years). This layer or mat covers the Clovis-age landscape or surface on which the last remnants of the terminal Pleistocene megafauna are recorded. Stratigraphically and chronologically the extinction appears to have been catastrophic, seemingly too sudden and extensive for either human predation or climate change to have been the primary cause. This sudden Rancholabrean termination at 10,900 ± 50 B.P. appears to have coincided with the sudden climatic switch from Allerød warming to Younger Dryas cooling. Recent evidence for extraterrestrial impact, although not yet compelling, needs further testing because a remarkable major perturbation occurred at 10,900 B.P. that needs to be explained.