Combined marine δ88/86Sr and 87Sr/86Sr record supports global anoxia as a cause for P/T mass extinction

The biggest mass extinction within the Phanerozoic Eon occurred at the Permian/Triassic (P/T) boundary and is characterized by up to 96% loss in species accompanied with the demise of the Paleozoic faunal community. Recently, five major mechanisms are consulted to explain the mass extinction in terrestrial and marine environment. However, a geochemical quantification of carbonate burial rates during this biotic turnover is still remaining. By extending the conventional radiogenic isotope system by a simultaneous measurement of radiogenic and stable strontium (Sr) isotopes (δ^88/86Sr) we are able to add new constraints to the seawater chemistry including quantitative information about the Sr output flux of the ocean, mainly represented by marine carbonates. Consequently, variations in δ^88/86Sr becoming a suitable tool to investigate in the global carbonate budget in earth's history including the biotic turnover of calcifying organisms at stratigraphic boundaries which are expected to have a large influence on Sr geochemistry and isotope composition of seawater. In order to examine variations in the marine Sr isotope composition through Earth's history paired ⁸⁸Sr/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr-ratios of 104 Phanerozoic brachiopods, belemnites and carbonate matrix samples are determined. Applying the fractionation factor of our carbonate recording phase we constructed a δ^88/86Sr_seawater record. The most noteworthy observation is the occurrence of the two global extrema in δ^88/86Sr values of the whole Phanerozoic within the last 20Ma years of the Paleozoic prior to the P/T boundary. This corresponds to an increase of δ^88/86Sr_seawater of 0.000249 Ma^-1 in the latest Permian indicating a strong disturbance in the Sr budget of the ocean. To interpret our observations and to compare them to existing scenarios for the P/T boundary we applied a one-box, isotope mass balance model of the oceanic Sr cycle connected to a mass balance model of Ca. Our results show a high strontium output flux from the ocean by marine carbonates and sulfates corresponding to an excess calcium output flux of 2.2x10¹⁹mol within the last 13Ma of the Permian period. This is explained by enhanced evaporate deposition and carbonate precipitation from hydrogen carbonate derived by Bacterial Sulfate Reduction (BSR) in an anoxic and stratified ocean. The model results support a kill mechanism triggered by an overturn of anoxic deep waters in a stratified ocean.