URANIUM ISOTOPES IN PLATFORM CARBONATES RECORD A MAJOR EXPANSION OF GLOBAL OCEAN EUXINIA ACROSS CRETACEOUS OAE 2

Ocean Anoxic Events (OAEs) are discrete periods in Earth history of decreased marine oxygen concentrations, often associated with volcanism, increased organic carbon burial in the oceans, and positive δ¹³C excursions in marine carbonates. One such event, OAE 2, occurred ~94 Mya and is associated with widespread deposition of black shale and an accompanying positive δ¹³C excursion recorded globally. Whereas the presence of black shale is indicative of locally anoxic environments, it is difficult to determine the global extent of anoxia as many seafloor environments are not preserved in the geologic record. Here, we investigate uranium concentrations and isotope compositions of marine carbonates from two OAE 2 localities—the Umbria-Marche Sequence of Italy and the Guerrero-Morelos Platform of Mexico. Uranium concentrations are exceptionally low in pelagic carbonates of Italy—initially precluding isotopic measurement—but an order of magnitude higher in platform carbonates of Mexico. δ²³⁸U values in Mexico reveal an initial negative excursion that could indicate a brief pulse of anoxia pre-OAE 2, which is immediately followed by re-oxygenation. At the start of OAE 2, we see a large, progressive negative U isotope excursion that peaks at −0.6‰, before a final return to modern carbonate δ²³⁸U values. This indicates a large expansion of global marine euxinia during OAE 2. In comparison to a previously published coeval pelagic OAE 2 section from Eastbourne (UK) that showed two negative δ²³⁸U excursions, the δ²³⁸U record of Mexico reveals a single pulse of marine euxinia across OAE 2 with less overall δ²³⁸U scatter. Importantly, the Mexico negative δ²³⁸U excursion is accompanied by a positive δ¹³C excursion with a discrete temporal offset (δ²³⁸U change precedes δ¹³C change by ~200 kyr based on section-specific estimates of sediment accumulation rate). This indicates that organic carbon burial associated with the positive δ¹³C excursion is a direct result of the expansion of marine euxinia represented by the negative δ²³⁸U excursion. This, in turn, strengthens the link between the carbon cycle and ocean redox across OAEs.