Smith Lecture - Dr. Maya Gomes, John Hopkins University

It is commonly thought that the advent of vertical bioturbation at the beginning of the Paleozoic resulted in a decrease in pyrite burial. This assumption has played an important role in reconstructions of atmospheric oxygen levels in the early Paleozoic because sulfate reduction and subsequent pyrite burial results in a net increase in oxygen in the ocean-atmosphere system. Much of our knowledge of how bioturbation impacts pyrite burial comes from modern ocean settings with bioturbation with high mixing intensity. However, the mixing intensity recorded in early Paleozoic is much lower than the modern ocean. In order to better constrain how the advent of bioturbation impacted pyrite burial, we investigated sedimentary sulfur isotope geochemistry of seasonally anoxic settings over a range of low mixing intensities. We show that pyrite burial is greatest under conditions of moderate sulfate limitation in mildly bioturbated sediments. We integrated these results into a model of Phanerozoic biogeochemical cycling (i.e., the Carbon-Oxygen-Phosphorus-Sulfur-Evolution or COPSE model) in order to investigate the implications of these results for atmospheric oxygen reconstructions. The results show a greater increase in oxygen in the early Paleozoic than previous reconstructions, specifically during time periods associated with major evolutionary radiations – the Cambrian explosion and Great Ordovician Biodiversification event. Thus, enhanced pyrite burial under mild bioturbation may have played a role in early Paleozoic oxygenation with implications for the diversification of life.