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...