332-4 Tracking the Marine Sulfate Reservoir Through OAE1a

Abstract:

One of the largest negative seawater sulfate sulfur isotope (δ³⁴S) shifts of the last 130 million years (~-5‰) coincides with Oceanic Anoxic Event 1a (OAE1a; the Selli Event). Although an excursion of this magnitude has highly significant implications for biogeochemical cycling in the Early Aptian, the specific drivers of the event remain unresolved. One hypothesis attributes the shift to the addition of ³⁴S-depleted sulfur associated with large igneous province (LIP) volcanism, whereas another invokes a decrease in global pyrite burial resulting from extensive sulfate removal through evaporite deposition linked to the opening of the South Atlantic Ocean. However, these processes affect marine sulfate concentrations differently:  increased volcanism enhances sulfate delivery to the ocean, whereas evaporite deposition removes sulfate through calcium sulfate precipitation. From an Earth-systems perspective, this distinction matters because seawater sulfate levels regulate pathways of organic carbon remineralization and thus influence nutrient recycling, ocean redox, methane production, and alkalinity generation, all of which affect carbon-cycle feedbacks and climate.

This study presents new δ³⁴S records for pyrite and sulfurized organic matter from a section that captures OAE1a in southeastern France (Vocontian Basin, Western Tethys). We integrate these records with redox-sensitive trace-metal and iron and phosphorus speciation data to reconstruct changes in local redox conditions and sulfur cycling during OAE1a. Together, these data help constrain local sulfate availability and provide insight into the state of the global marine sulfate reservoir and its influence on biogeochemical cycling during the event.

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