BIOGEOCHEMICAL CONSEQUENCES OF A SEDIMENTARY MIXED LAYER COLLAPSE IN THE WAKE OF THE END-PERMIAN MASS EXTINCTION

The end-Permian mass extinction (EPME) was the most devastating mass extinction in the Phanerozoic, resulting in a permanent shift in the marine biosphere, setting the stage for Mesozoic ecology and evolution. One group of animals most profoundly impacted by the EPME was shallow marine bioturbators. The Early Triassic trace fossil record shows evidence for a significant reduction in trace fossil diversity, size, and depth, which is evidence for a severe reduction in bioturbation intensity. Although some complex, deep bioturbation behaviors persisted locally, the collapse in bioturbation intensity after the EPME likely led to a significant reduction in the sedimentary mixed layer globally. A collapse of an Early Triassic sedimentary mixed layer is hypothesized to have had major consequences on sulfur, organic carbon, and oxygen biogeochemical dynamics. However, hypotheses about the biogeochemical impact of the loss of bioturbation and the sedimentary mixed layer in the Early Triassic have not been quantitatively tested. We used biogeochemical reactive-transport models to identify the broad consequences of a mixed layer collapse. We modeled organic carbon, oxygen, sulfate, and hydrogen sulfide sediment profiles for four scenarios: an oxic pre-extinction scenario with a fully developed mixed layer and three euxinic extinction scenarios with fully developed layer, a complete mixed layer collapse, and a maintenance of 10% of the pre-extinction mixed layer depth. We find that the effect of the extent of the mixed layer collapse versus the shift from oxic to euxinic conditions varies between chemical species. The largest change in the oxygen profile is due to the shift from oxic to euxinic conditions, with only minor differences in the oxygen profiles between the three euxinic mixed layer scenarios. In contrast, the largest shift in the hydrogen sulfide profiles occurs when the mixed layer is severely reduced, with only minor differences between a full mixed layer collapse and the maintenance of a small mixed layer. In future work, we will test whether the rate of the mixed layer collapse has a major effect on sedimentary sulfur cycling. Ultimately, our results will provide important quantitative biogeochemical insights into the consequences of an extinction of bioturbators after the EPME.