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Science 11 April 2008:
Vol. 320. no. 5873, pp. 214 - 218
DOI: 10.1126/science.1152860
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Reports

Determining Chondritic Impactor Size from the Marine Osmium Isotope Record

François S. Paquay,1* Gregory E. Ravizza,1 Tarun K. Dalai,1{dagger} Bernhard Peucker-Ehrenbrink2

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.

1 Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822–2225, USA.
2 Department of Marine Chemistry and Geochemistry, Woods Hole Oceano-graphic Institution, Woods Hole, MA 02543, USA.

{dagger} Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India.

* To whom correspondence should be addressed. E-mail: paquay{at}hawaii.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)