Airbursts/impacts by a fragmented comet or asteroid have been proposed at the Younger Dryas onset (12.80 ± 0.15 ka) based on identification of an assemblage of impact-related proxies, including microspherules, nanodiamonds, and iridium. Distributed across four continents at the Younger Dryas boundary (YDB), spherule peaks have been independently confirmed in eight studies, but unconfirmed in two others, resulting in continued dispute about their occurrence, distribution, and origin. To further address this dispute and better identify YDB spherules, we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry, morphologies, origins, and processes of formation. We investigated 18 sites across North America, Europe, and the Middle East, performing nearly 700 analyses on spherules using energy dispersive X-ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization. Twelve locations rank among the world's premier end-Pleistocene archaeological sites, where the YDB marks a hiatus in human occupation or major changes in site use. Our results are consistent with melting of sediments to temperatures >2,200°C by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere; they are inconsistent with volcanic, cosmic, anthropogenic, lightning, or authigenic sources. We also produced spherules from wood in the laboratory at >1,730°C, indicating that impactrelated incineration of biomass may have contributed to spherule production. At 12.8 ka, an estimated 10 million tonnes of spherules were distributed across ∼50 million square kilometers, similar to well-known impact strewnfields and consistent with a major cosmic impact event.Clovis-Folsom | lechatelierite | tektite | wildfires
The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835-12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of the YDB layer within the limits of dating uncertainty (∼100 y). The widespread distribution of the YDB layer suggests that it may serve as a datum layer.Younger Dryas | comet | Bayesian | radiocarbon | synchroneity A ccording to the Younger Dryas Impact Hypothesis (YDIH)(1), a major cosmic episode of multiple airbursts/impacts occurred at 12,800 ± 300 calendar years before 1950 (Cal B.P. represents calendar years before A.D. 1950, unless otherwise noted; 95% probability) or 12,950-12,650 Cal B.P. at 68% probability. This event produced the Younger Dryas boundary (YDB) layer, displaying peaks in a variable assemblage of spherules (glassy and/or magnetic-inferred to be impact ejecta and therefore, for simplicity, referred to below as impact-related spherules), high-temperature minerals and melt glass, nanodiamonds, charcoal, carbon spherules, glass-like carbon, aciniform carbon (soot), nickel, iridium, platinum, and osmium. The event may have triggered the Younger Dryas episode of abrupt climate change, contributed to the end-Pleistocene megafaunal extinctions, and initiated human population reorganization/ decline across the Northern Hemisphere (1-5). Because a temporally singular event is proposed, the YDIH requires dates on the YDB layer to be essentially isochronous across four continents within the limits of dating methods.In a test of synchroneity, it is ideal to have numerous, highly accurate, and precise dates to develop robust chronological models (6). The term "date" represents a measured value, and "age" refers to real or modeled calendar years. However, when developing high-precision chronologies, there are multiple challenges that are amplified in Pleistocene age deposits. Modern accelerator mass spectrometry (AMS) radiocarbon ( 14 C) measurements are typically very precise, with uncertainties of ±20 y to ±30 y at 11,000 14 C years B.P., but high precision does not mean high accuracy. Numerous problems can produce erroneous ages Significance A cosmic impact event at ∼12,800 Cal B.P. formed the Younger Dryas boundary (YDB) l...
Determining the impact of the Younger Dryas (YD) climate event on the unglaciated North American midcontinent has proved diffi cult due to a scarcity of suitable paleoclimate proxies. Here we present a well-dated carbon isotope (δ 13 C) record from a buried soil sequence in southwestern Missouri, which reveals a large isotopic excursion during the YD chronozone. In this region of the modern prairie-forest border, the δ 13 C signature of soil organic matter is a reliable indicator of past climatic change because δ 13 C values are controlled primarily by the relative abundance of C 3 and C 4 plants, which is tied to the environmental setting. Between ca. 13,200 and 11,900 yr ago, the abundance of C 4 grasses increased by upwards of 50% of the total biomass, indicating expansion of grassland most likely driven by increased aridity during this period. Environmental gradients in the midcontinent must have been very steep, because at the same time that a C 4 -rich prairie existed in southwestern Missouri, spruce forests grew in Iowa, Illinois, and Ohio.on June 5, 2015 geology.gsapubs.org Downloaded from
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