THE EARLY TOARCIAN (JURASSIC) ANOXIC EVENT - STRATIGRAPHIC, SEDIMENTARY, AND GEOCHEMICAL EVIDENCE

Detailed paleogeographic studies in the Southern Alps suggest that an oxygen-minimum model is appropriate for interpreting conditions on Tethyan continental margins. Advection of manganese in this low-oxygen layer may explain the local occurrence of carbonate ores of this element. Carbon-sulfur and iron-sulfur ratios, although showing considerable scatter, suggest that bottom waters at this time were locally euxinic, containing free hydrogen sulfide. This Oceanic Anoxic Event was preceded by significant faunal turnover of ammonites in Tethys and accompanied by widespread extinction of benthos in northern Europe in response to the lateral spread of anoxic bottom waters during transgression. Similar changes may be recognized in other parts of the world. Other anoxic events may have taken place during the Jurassic, but documentation is as yet meagre. Furthermore, models for such phenomena remain largely speculative, although upwelling and increased planktonic productivity, commencing in pre-Toarcian time, are favored for the falciferum-Zone event documented here.-from Autho