SEARCHING FOR EVIDENCE OF LATE DEVONIAN LOCALIZED ANOXIA IN THE GREAT BASIN REGION (SOUTHERN NV, USA): RETHINKING CAUSES OF THE LATE DEVONIAN MASS EXTINCTION

The Late Devonian is well known for one of the “big five” mass extinctions, which is thought have been caused in part by volcanism, global cooling, and/or widespread anoxia. The main biotic crisis occurs at the Frasnian-Famennian boundary (FFB) where positive carbon isotopic (δ¹³C) excursions are reported around the globe and are interpreted to record anoxia. These geochemical data, however, only provide insight into the global carbon cycle and cannot constrain whether anoxia was localized to shallow environments. Additional geochemical data that preserves a more local signal, such as I concentrations (I/Ca+Mg) and Fe-speciation, would be necessary to demonstrate anoxia persisted during this time. The I/(Ca+Mg) proxy is used to assess water column anoxia when I/(Ca+Mg) values are low (~0), if not reset by diagenesis. Fe speciation is used to quantify the reactive Fe content (e.g., Fe-bearing sulfide, oxide, and carbonate minerals) as a way to constrain the redox state of the water column during the time of deposition. Fe speciation is a primarily applied to fine-grained, siliciclastic sedimentary rocks, but in some instances this proxy can be used to study carbonates.

In this study we sampled a Late Devonian carbonate succession in southern Nevada, USA (Bactrian Mountain) to test whether global and local evidence for anoxia exists before the FFB. We measured δ¹³C trends from bulk carbonate and bulk organic matter to correlate with published trends, but we also sampled these carbonates for I/(Ca + Mg) and Fe speciation and in order to examine local redox conditions. Based on δ¹³C trends, evidence for anoxia occurs above the FFB. Preliminary I/(Ca+Mg) values during this anoxic interval are zero and return to non-zero values after the peak δ¹³C excursion. A total of 27 samples were measured for Fe speciation, which have low total Fe (Fe_T) values (<0.13 wt. %), but relatively high reactive Fe (Fe_HR) to total Fe (Fe_HR/Fe_T) values. While Fe_HR/Fe_T values >0.38 can be used as an indicator of water column anoxia, however, empirical evidence indicates that samples with low Fe_T yield non-diagnostic Fe_HR/Fe_T. In general, global and local proxy evidence for anoxia indicates that water column anoxia in this section post-dates the FFB. This suggests that anoxia may not have played as important of a role in the biotic crisis as previously thought.