CRETACEOUS/TERTIARY AND PERMIAN/TRIASSIC BOUNDARY EVENTS COMPARED

Abstract

The Cretaceous/Tertiary () and Permian/Triassic () events differ in many more aspects than they are similar. This alone makes it unlikely that they are parts of a common web of periodic mass extinction events (MEE). Unlike the , most MEE have no Ir anomalies and where (in the comparatively complete section of the at Gartnerkofel, Austria) weak Ir concentrations are found, their ratios to siderophile elements (Co, Ni, Cr) are far less than chondritic. In contrast, ratios to chalcophile elements (As, Sb, Zn) never approach chondritic, even in peaks and locality differences suggest local depositional processes that may also affect the siderophiles. Profiles of isotopes (δ13Ccarb, , and possibly δ18Ocarb) across these boundaries also differ radically. Whereas the profile of surface ocean δ18Ocarb has been shown to be mainly a drop in primary productivity, the profile is interpreted at least in part to be a consequence of a large and accelerating oxidation of previously stored Corg, with greatly differing consequences for climate. These and other differences of proximate forcings for these two MEE lead us to a hypothesis that such external exigencies were ever (intermittently and severally) with us, but were effective only when the biotic web had already internally unravelled in some crucially strained patch. Gaia became Antigaia!