Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ¹³C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca. 94 Mya) is especially notable for globally-distributed changes in calcareous invertebrate and plankton populations. While the presence of organic-rich facies is consistent with locally anoxic environments in many cases, determining the global extent of anoxia is more problematic. To address this issue, we investigate uranium isotope (δ²³⁸U) compositions of upper Cretaceous (Cenomanian-Turonian) open marine platform carbonates from southern Mexico as a proxy for global seawater redox conditions. These data are complementary to previous δ²³⁸U studies across OAE 2 in both black shales and pelagic carbonates, which have yielded variable results that reflect both global redox and local depositional processes. In Morelos Formation carbonates, a significant and well-defined negative δ²³⁸U excursion down to a nadir of −0.6‰ is recorded over an ∼40 m interval. This is consistent with the expansion of marine anoxia, pointing to an areal extent of anoxic seawater of about 1–10% of the global seafloor (or ∼5 to 50 times the modern value). Importantly, based on biostratigraphically-controlled estimates of sediment accumulation rates, the δ²³⁸U anomaly precedes the δ¹³C excursion by a median of ∼45 to 51 kyr (95th percentile confidence interval, CI) or ∼105 to 120 kyr (95th percentile CI) depending on how the onset of the δ²³⁸U anomaly is estimated. These results, along with previously reported thallium isotope and trace metal data, suggest that anoxic expansion preceded carbon cycle perturbation. This observation further increases estimates of the duration of OAE 2, implying that widespread ocean anoxia may have lasted >900 kyr.