Os isotopes ͉ platinum group elements ͉ Clovis ͉ Pleistocene extinction ͉ meteorite P roxy records of millennial-scale climate variations during the most recent deglaciation from polar ice cores (1-3) as well as deep-sea and lacustrine sediments (4-10) display abrupt changes that are typically attributed to internal forcing of Earth's climate system. A striking example is the Younger Dryas (YD) cooling episode from 12.896 Ϯ 0.138 thousand years (ka) to 11.703 Ϯ 0.099 ka calendar years before AD 2000 (11) after the interstadial warming event Bølling-Allerød (BA) (14.692 Ϯ 0.186-14.075 Ϯ 0.169 ka). High-resolution stable ␦ 18 O and ␦D in H 2 O and the glaciochemical record from Greenland ice cores show that both the onset and the termination of the YD occurred abruptly, the former lasting slightly more than two centuries, whereas the latter transitioned into a new state in a few years (12). The most widely accepted interpretations of Earth's recent climate history place the origin and termination of the YD within Earth's complex network of feedback mechanisms (13-15). Proxies and model results favor a significant freshwater input into the North Atlantic reducing the formation of deep waters and weakening or shutting down of the meridional overturning circulation (16) as the primary cause of the YD cooling regardless of its source, timing, duration, volume, and path of melt water (17-21).

Decreases in the seawater 187Os/188Os ratio caused by the impact of a chondritic meteorite are indicative of projectile size, if the soluble fraction of osmium carried by the impacting body is known. Resulting diameter estimates of the Late Eocene and Cretaceous/Paleogene projectiles are within 50% of independent estimates derived from iridium data, assuming total vaporization and dissolution of osmium in seawater. The variations of 187Os/188Os and Os/Ir across the Late Eocene impact-event horizon support the main assumptions required to estimate the projectile diameter. Chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine osmium isotope record, suggesting that previously unrecognized impact events can be identified by this method.

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