Re-thinking anoxia as a cause of the Late Devonian mass extinction: Carbon, sulfur, and iodine proxies for anoxia across the Frasnian-Famennian Boundary in the Great Basin region, USA

The Late Devonian biotic crisis has long been studied to better understand the cause(s) of the mass extinction that occurred during the Middle−Late Devonian period. Anoxia has been widely investigated as a driver of this biotic crisis based on the temporal overlap of the extinction of some marine groups and proxy evidence for anoxia from sections around the globe. This evidence for anoxia can be local, in the form of black shales and pyrite framboids, or global, in the form of uranium isotopes or indirectly as stable carbon (δ13C) and sulfur (δ34S) isotopes. Evidence for anoxia, however, is inconsistent between sections where either the timing of anoxia is not coeval or evidence conflicts such that some trends indicate anoxia persisted whereas other evidence indicates oxic conditions were present. To further complicate these interpretations, many Late Devonian sections are located within epeiric or epicontinental basins that appear to have experienced differential and possibly isolated circulation patterns compared to sections that were better exposed to the open ocean. If anoxia were a globally felt phenomenon during the Late Devonian that possibly served as a driver of the Late Devonian biotic crisis, we expect that both global and local proxy evidence for anoxia would be preserved along continental margins connected to the open ocean free of circulation restriction. In this study we sampled four Late Devonian sections in the Great Basin region of western USA to test for both global and local proxy evidence of anoxia across the Frasnian-Famennian Boundary (FFB). We report a single positive 1‰−2.4‰ δ13C excursion that is coeval with a 1.3‰−8‰ δ34S excursion, which varies in magnitude between sections from both shallow and deeper environments. This variability does not appear to be explained only by depth, as has been observed in other basins and times in Earth history, suggesting that other factors may be involved. These isotopic excursions occur above the FFB (with the exception of the Devils Gate section), suggesting that the global carbon and sulfur cycles were perturbed by increased organic and pyrite burial rates, respectively, after the biotic crisis associated with this horizon. Furthermore, iodine ratios [I / (Ca + Mg)] measured from these rocks suggest that these sections did not experience anoxic conditions locally. These new data suggest that marine anoxia expanded during this time and affected global carbon and sulfur cycles; however, anoxia was not likely present on continental margins and was more likely confined to periodically restricted or isolated epeiric continental basins where black shale deposition was a more dominant facies. The occurrence of these δ13C and δ34S excursions above the FFB may indicate that though anoxia expanded during the Late Devonian near the Frasnian−Famennian transition, it may not have been as significant as previously thought to explain the decline of biodiversity observed before and during this interval. Future work aimed at documenting in more detail the temporal and spatial extent of localized anoxia from other basins around the globe during this time will help to better resolve whether anoxia was a driver of the Frasnian−Famennian biotic crisis, or simply a consequence of a different unknown driver.