IRIDIUM ANOMALY AT THE BOUNDARY OF EXTINCTION OF THE EDIACARIAN FAUNA

Abstract

There are data indicating that geochemical anomalies (particularly of elevated iridium concentrations) in the sections through sedimentary rocks are confined to boundaries marked by considerable changes in animal life in the Phanerozoic, suggesting that biotic crises are related to global events in the environment. To ascertain the geochemical features of the sedimentary rocks corresponding to the boundary of the Vendian biotic crisis, samples were collected from the Upper Vendian sections in the region around the Dnestr River. The contents of rare and disseminated elements in 31 samples (whole rocks) from boreholes and natural outcrops were determined by instrumental epithermal neutron-activation analyses. The hypothesis advanced can explain the selective character of extinctions, which usually encompass groups of animals with a more active metabolism, differences in the times of extinction of different taxa, and also the different reactions of animals and plants to changes in the environment. In the Vendian period, the level of extinction of the Ediacarian fauna coincided in time with the flourishing of the earliest multicellular plants (vendotaenids) - an argument in favor of the proposed hypothesis that the Vendian biotic crisis was related to a change in gas composition of the atmosphere and hydrosphere caused, in turn, by global activation of vulcanism at the end of the Redkinian age.