THE ROLE OF OCEAN ANOXIA IN MODULATING EARTH SYSTEM RECOVERY FOLLOWING MASS EXTINCTION EVENTS

Evidence for persistent ocean anoxia has long been recognized in rocks deposited after the end-Permian and end-Triassic mass extinction events. More recently, detailed investigations of the fossil and geochemical records have demonstrated that these intervals of ocean anoxia are associated with delayed recovery of benthic marine ecosystems and instability in global biogeochemical cycles. However, the precise extent and duration of ocean anoxia has remained difficult to quantify because anoxia is largely known from local indicators, impeding efforts to further test the temporal and causal relationships between anoxia, carbon cycle perturbations, and Earth system recovery. Here we use uranium isotope (δ²³⁸U) data from shallow-marine Permian-Triassic and Triassic-Jurassic boundary sections to quantify the extent and duration of bottom water anoxia following the end-Permian and end-Triassic mass extinction events. In each case, δ²³⁸U exhibits a negative excursion across the extinction interval of approximately 0.5 permil, indicative of a global expansion in marine anoxia, followed by a gradual return to pre-extinction values during the next few million years. In both cases, the more negative δ²³⁸U values are associated with large perturbations to the global carbon and sulfur cycles as well as reductions in the taxonomic diversity and body sizes of benthic marine animals. These observations are also supported by numerical modeling, which suggests that shallow-marine anoxia, particularly a shallow and intense oxygen minimum zone (OMZ), can make the global carbon cycle more sensitive to forcing from factors such as variation in eustatic sea level, potentially explaining the coincidence of marine anoxia with large excursions in the carbon and sulfur isotope records. We hypothesize that the depth, thickness, and intensity of the OMZ served to modulate the biological and biogeochemical recoveries from these mass extinction events. Extrapolating these findings to Phanerozoic rock, fossil, and geochemical records, we further suggest that a gradual deepening of the OMZ can explain long-term trends toward fewer black shale deposits, larger animals, more complex marine ecosystems, and fewer and smaller carbon isotope excursions.