332-5 Ironing Out OAE1a: Reconstructing Water-Column Redox Conditions in the Vocontian Basin

Abstract:

Oceanic Anoxic Event 1a (OAE1a; ~120.5 Ma) represents a major perturbation to the Early Cretaceous carbon cycle, characterized by enhanced organic carbon burial, widespread black shale deposition, and global marine deoxygenation. Although OAE1a has traditionally been interpreted as an interval of widespread euxinic (anoxic and sulfidic) conditions, recent geochemical evidence suggests that ferruginous (anoxic and iron-rich) conditions may have been more extensive than previously recognized. Distinguishing between these redox states is important because ferruginous and euxinic environments exert fundamentally different controls on nutrient availability and marine biogeochemical cycling. However, the distribution and evolution of these redox states during OAE1a remain poorly constrained, limiting our understanding of their roles in Earth-system biogeochemical feedbacks.

Here, we present iron-speciation data from the Les Sauzeries core (Vocontian Basin) to reconstruct local bottom-water redox conditions across OAE1a. As a semi-restricted basin with terrigenous input and limited ventilation, the Vocontian Basin provides a setting where bottom-water redox conditions may be more closely linked to changes in weathering-derived iron fluxes than more open, abyssal environments. Sequential iron extractions quantify highly reactive iron phases, including carbonate-associated iron, ferric oxides, magnetite, and pyrite iron and are integrated with phosphorus-speciation data and existing carbon isotope data to evaluate changes in redox conditions and nutrient cycling. These results provide new constraints on the temporal evolution of ferruginous and euxinic conditions in the western Tethyan and clarify how redox heterogeneity influenced biogeochemical cycling during OAE1a.

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