Determining Chondritic Impactor Size from the Marine Osmium Isotope Record. 2008. F. S. Paquay et al. Science 320: 214-218.
National Geographic News:
Scientists working on the technique used chemical signatures in seawater and ocean sediments to study the dino-killing impact that occurred at the end of the Cretaceous period, about 65 million years ago.
In what could be a major scientific puzzle, the team's new size estimate for the dino-killing meteorite is a mere 4 to 6 kilometers across. The most recent computer models predicted a size of 15 to 19 kilometers across.
Abstract: 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.