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

Significance Past “Oceanic Anoxic Events” (OAEs) represent important carbon cycle perturbations that offer the opportunity to study Earth’s response to extreme climate warming. A fundamental limitation for understanding OAEs is quantifying the timing and total extent of ocean anoxia. We present a quantitative account of global redox conditions for OAE 2 (∼94 million years ago), using a high-resolution record of uranium isotopes combined with a biogeochemical model. We present new evidence for two discrete intervals of globally extensive anoxia that were coupled to enhanced terrestrial weathering, within the typically defined OAE interval. These anoxic intervals were separated by ocean reoxygenation and the temporary recovery of the carbon cycle.