VOLCANIC ACTIVATION OF OCEANIC ANOXIC EVENT 2 AND THE ROLE SULFUR PLAYS IN REGULATING THE MARINE CARBON CYCLE

Several lines of evidence suggest that Cretaceous oceans experienced discrete, short-lived (<1 Ma) episodes of widespread anoxia. These episodes, termed oceanic anoxic events (OAEs), record significant increases in organic carbon burial and are marked by positive carbon isotope excursions in both marine carbonate and organic carbon. While it is widely held that these events were driven by enhanced primary production within surface waters, the mechanistic details underpinning the initiation, maintenance and termination of OAEs remain equivocal. Here, we provide new sulfur isotope measurements from rocks deposited in the Cretaceous Western Interior Seaway of North America before, during and after OAE2 (~94.5 Ma)—one of the more pronounced and studied Cretaceous OAEs. These data support a new hypothesis that OAE2 was triggered by a massive addition of volcanic sulfur, ultimately manifesting as oceanic sulfate, which facilitated increased organic carbon remineralization via bacterial sulfate reduction, enhanced nutrient recycling, and a global increase in primary production. OAE2 persisted until sulfate levels were sufficiently reduced via pyrite burial to inhibit nutrient recycling, and in so doing, returned marine primary productivity to background levels. Furthermore, our findings suggest that fluctuating marine sulfate levels played an important role in regulating the exogenic carbon cycle for much of Earth history.