Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE 2), occurring ~94 million years ago, was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. It was typified by a rapid rise in atmospheric CO2, global warming, and marine anoxia, leading to the widespread devastation of marine ecosystems. However, the precise timing and extent to which oceanic anoxic conditions expanded during OAE 2 remains unresolved. We present a new record of global ocean redox changes during OAE 2 using a combined geochemical and carbon cycle modelling approach. We utilize a continuous, high-resolution record of uranium isotopes in pelagic and platform-carbonate sediments to quantify the global extent of seafloor anoxia during OAE 2. This new dataset is then compared to a dynamic model of the coupled global carbon, phosphorus and uranium cycles to test hypotheses for OAE 2 initiation. This unique approach highlights an intra-OAE complexity that has previously been under constrained, characterized by two expansions of anoxia separated by an episode of globally significant reoxygenation coincident with the ‘Plenus Cold Event’. Each anoxic expansion event was likely driven by rapid atmospheric CO2 injections from multi-phase Large Igneous Province (LIP) activity.