AN ALTERNATIVE MODEL FOR EARLY PALEOZOIC PYRITE BURIAL

A primary source of oxygen to the ocean and atmosphere is the reduction of sulfate to sulfide and its subsequent burial in sedimentary pyrite. Models of Paleozoic oxygenation typically assume that pyrite burial efficiency decreased monotonically as bioturbation evolved, largely because of higher rates of sedimentary sulfide reoxidation. To improve our understanding of Early Paleozoic pyrite burial under low-oxygen conditions, we sampled sediment cores from the Chesapeake Bay—an oxygen-stressed estuary with overlapping gradients in sulfate concentration, bioturbation, and organic carbon fluxes—and examined trends in pyrite accumulation under different geochemical conditions. We found that pyrite accumulation in the upper 0.5 m of sediment was maximized under intermediate sulfate-chloride ratios, not in strongly reducing, sulfate-depleted sediments. We infer from these data that reducing sediments with high microbial sulfate demand will quickly reduce the available reservoir of pore water sulfate, but that the supply of fresh sulfate to generate more pyrite will be limited. In contrast, mildly ventilated sediment columns can replenish sulfate to fuel more sulfate reduction, but excessive ventilation will introduce enough oxygen to suppress pyrite retention.

Our results lead to the hypothesis that initial ventilation of sediments, paired with the introduction of mild oxidizing power to the sediment column, may have temporarily increased pyrite burial fluxes under incipient bioturbation in the Early Paleozoic—thus providing a positive feedback to oxygen accumulation. To test this hypothesis, we compiled sulfur and organic carbon concentration data from the Sedimentary Geochemistry and Paleoenvironments (SGP) database. A set of >6500 fine-grained marine siliciclastic sulfur-carbon (S/C) ratios spanning the Ediacaran to the Cenozoic shows that median S/C values increased ~40% from the Ediacaran through the Ordovician, then declined from the Silurian onward. The variability of S/C ratios also declined permanently in the Silurian, suggesting a homogenization of the sedimentary carbon and sulfur cycles at that time. These data support the inference that bioturbation could have increased pyrite burial in the Early Paleozoic and contributed to ocean-atmosphere oxygenation.