Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis contends that an extraterrestrial object exploded over North America at 12.9 ka, initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At 6 of the 10 original sites (excluding the Carolina Bays), elevated concentrations of various “impact markers” were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleowetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.In 2007, an interdisciplinary team of scientists proposed a startling hypothesis. An extraterrestrial body, possibly a comet, exploded over North America approximately 12.9 ka, significantly altering climate, ecosystems, human populations, and faunal assemblages across the Northern Hemisphere (1). The team reported evidence for a possible extraterrestrial (ET) impact event at multiple archeological and paleontological sites in North America and a site in Belgium. At each site, they identified physical and chemical evidence, collectively called impact markers, in sediments dating to 12.9 ka^* that they attributed to an ET impact event. Lines of evidence used to support the hypothesis are varied and include elevated levels of iridium in bulk and magnetic sediments, a spike in the abundance of magnetic spherules and titanomagnetite grains, fullerenes that contain ET helium, the presence of polycyclic aromatic hydrocarbons, carbon spherules, and nanodiamonds associated with glass-like carbon.The Younger Dryas impact hypothesis is intriguing because it addresses three fundamental issues in Quaternary science: the cause of the Younger Dryas cooling, the extinction of large herbivores and their predators in the New World, and the demise of the Clovis Paleoindian culture. Traditionally, these phenomena have been attributed to disparate causes. The prevailing theories are that the rapid return to cold conditions during the Younger Dryas resulted from freshening of the North Atlantic and a slowing of the Atlantic meridional overturning circulation (2); megafaunal extinctions were caused by human predation and overkill, climate change, or some combination thereof (3); and Clovis simply evolved into other Paleoindian cultures (4). In contrast, proponents of the impact hypothesis maintain that an ET airburst or surface impact contributed to all three of these phenomena simultaneously (1).Shortly after the impact hypothesis was proposed, critics began questioning whether some of the markers were necessarily related to an ET impact event (5–11), whereas others argued against the hypothesis after they were unable to find supporting evidence in other depositional settings (12, 13) or existing datasets (4, 14). Still others raised concerns over issues of replicability and laboratory protocols used in the original study (15). Here, we provide evidence that supports an alternative hypothesis for the accumulation of some of the markers in late Quaternary sediments. We suggest that, rather than resulting from an ET impact event, elevated levels of iridium in bulk and magnetic sediments, magnetic spherules, and titanomagnetite grains are the result of differential accumulation and concentration of naturally occurring, noncatastrophic levels of dust^† in depositional settings, characterized by slow sedimentation rates, low-energy deposition, and biogeochemical reactions occurring at the sediment–water interface. In other words, apparent “spikes” of some of the markers are due to geomorphic processes common to wetland systems, likely via the accumulation and concentration of dust, rather than a catastrophic impact event.At 6 of the 10 original sites (excluding the Carolina Bays), Firestone et al. (1) found markers just below or within organic-rich, dark-colored sedimentary layers called “black mats.” Black mats are common features in Pleistocene and Holocene wetland deposits worldwide and are formed in shallow marsh environments (16, 17). They are typically comprised of organic-rich silt and small amounts of clay and/or sand, and vary in age, composition, organic content, and thickness. In addition, they often contain the remains of invertebrate fauna (gastropods, ostracodes), diatoms, pollen, plant macrofossils, and other indicators of past environmental conditions.The combination of wet ground and dense plant cover allows marshes and wetlands to act as efficient dust traps, particularly in arid environments where atmospheric dust concentrations are relatively high (18). Wetland systems are low-energy, relatively stable environments and, because of their nature and position on the landscape, they often preclude or minimize the input of clastic fluvial sediments. Thus, black mats and related wetland sediments typically contain higher concentrations of dust than other depositional environments. Moreover, wetland sediments are subject to redoxymorphic reactions associated with diverse and abundant microbial ecosystems, which may serve to further concentrate nonreactive components of dust (19)To determine if the accumulation of markers might result from processes common to wetland systems rather than an ET impact event, we revisited several black mats and paleowetland sequences in the southwestern United States and the Atacama Desert of northern Chile (Fig. 1). Our research group investigated these sites previously to reconstruct late Quaternary climatic and hydrologic change in arid settings in the Americas (20–23). Examination of deposits in these regions allowed us to evaluate the occurrence of markers in black mats of different ages and in different hemispheres, including localities far removed from the presumed impact location and its fallout.Open in a separate windowFig. 1.Generalized location of study sites in the American Southwest and the Atacama Desert of northern Chile.     

Structural insights into the biogenesis and biofilm formation by the Escherichia coli common pilus

Bacteria have evolved a variety of mechanisms for developing community-based biofilms. These bacterial aggregates are of clinical importance, as they are a major source of recurrent disease. Bacterial surface fibers (pili) permit adherence to biotic and abiotic substrates, often in a highly specific manner. The Escherichia coli common pilus (ECP) represents a remarkable family of extracellular fibers that are associated with both disease-causing and commensal strains. ECP plays a dual role in early-stage biofilm development and host cell recognition. Despite being the most common fimbrial structure, relatively little is known regarding its biogenesis, architecture, and function. Here we report atomic-resolution insight into the biogenesis and architecture of ECP. We also derive a structural model for entwined ECP fibers that not only illuminates interbacteria communication during biofilm formation but also provides a useful foundation for the design of novel nanofibers.     

Influence of menopause on adipose tissue clock gene genotype and its relationship with metabolic syndrome in morbidly obese women

Menopausal women exhibit a loss of circadian coordination, a process that runs parallel with a redistribution of adipose tissue. However, the specific genetic mechanisms underlying these alterations have not been studied. Thus, the aim of the present study was to determine whether the development of menopause induces an alteration of the genes that control biological rhythms in human subcutaneous (SAT) and visceral (VAT) adipose tissue, and whether changes in clock gene expression are involved in the increased risk of developing metabolic syndrome (MetS), which is frequently associated with menopause. To this end, VAT and SAT biopsies were taken in pre- (n = 7) and postmenopausal (n = 7) women at similar hours in the morning. RNA was extracted, and a microarray analysis was made. Data were confirmed by quantitative real-time polymerase chain reaction. Western blot and immunohistochemical analysis were also performed. When clock gene expression was compared between both groups of women, data in SAT showed that expression of the core clock gene period3 was significantly higher in postmenopausal women, while casein kinase-1δ, E1A-binding protein and cAMP-responsive element were preferentially expressed in the premenopausal group. In VAT, period2 (PER2) and v-myc myelocytomatosis viral oncogene expressions were significantly higher in the postmenopausal group. Western blot analysis indicated that PER2 and PER3 protein expression was also increased in postmenopausal women. In addition, several genes, including PER2, were differentially expressed depending on whether or not the patient met the MetS criteria. We conclude that menopause transition induces several changes in the genotype of the adipose tissue chronobiological machinery related to an increased risk of developing MetS.

Electronic supplementary material

The online version of this article (doi:10.1007/s11357-011-9309-2) contains supplementary material, which is available to authorized users.