Terrestrial versus extraterrestrial causes of extinction events

We have been using platinum group element (PGE) concentrations, 187Os/188Os, 3He/4He, and other geochemical indicators to distinguish between terrestrial and extraterrestial causes of mass extinction events at major geologic boundaries. Work on the Cretaceous-Tertiary (K-T) boundary (Ravizza & Peucker-Ehrenbrink, 2003, Science 302, 1396-1398) clearly indicates that the main phase of the Deccan volcanism preceded the K-T boundary impact by about 100,000 years. The short marine residence time of Os that allows for a quick (~100,000 yr) recovery of the marine Os isotope system after large extraterrestrial impacts makes the marine Os isotope system a very sensitive recorder of multiple impact events, even if the record has gaps that are shorter in duration than the recovery time of the system. The fact that marine sediments record only a single excursion to unradiogenic values around the K-T boundary does not support the interpretation by Gerta Keller and coworkers that multiple impacts affected Earth at the end of the Cretaceous.

Geochemical data for the Paleocene-Eocene (P-E) boundary (Schmitz et al., 2004, EPSL 225, 1-17) do not support the comet-impact scenario proposed by Dennis Kent and coworkers. While we cannot exclude a major impact of a very dust-poor comet or a achondrite meteorite with low PGE concentrations, we consider it more likely that the environmental crisis at the P-E boundary was caused by explosive basaltic volcanism in the North Atlantic region that may have triggered breakdown of gas hydrates and rapid release of methane (a powerful greenhouse gas) to the atmosphere.

Studies of two European sections across the end-Permian extinction event 251 Ma, the largest in Earth s history, also do not support claims of an extraterrestrial impact. While we cannot exclude an impact of a PGE-poor extraterrestrial body (ice-rich comet or fractionated, PGE-poor asteroid), the absence of clear PGE anomalies in the sedimentary record supports a home-grown cause for this event - possibly the effusion of the Siberian Traps (Koeberl et al., 2004, Geology 32, 1053-1056).