Geochemical anomalies near the Ordovician‐Silurian boundary, Northern Yukon Territory, Canada1

Abstract

Basinal strata spanning the Late Ordovician extinction have been determined for major and trace elements, and isotopes of C, O and S. A marked increase in organic C, and siderophile (i.e., Ni, V) and chalcophile (i.e., Zn, Cd, Hg, As, Sb, Se and Mo) elements is observed at a level that is no older than the base of the G. persculptus Zone. In sections for which isotopic data are available, these anomalies coincide with decreases in δ 13 C values, suggesting a major biomass reduction, and δ18O values, indicating a warming trend. At the same time, δ34S values in pyrite become highly positive indicating reduced bottom‐water conditions. These results suggest that a rapid inversion of stratified ocean water causing a brief but widespread episode of anoxic surface waters may have been the immediate cause of extinction. The ultimate cause of extinction is unknown, although the magnitude and apparent global synchroneity of the extinction suggest that it was of catastrophic effect. The chalcophile and siderophile e...