Mesozoic to Cenozoic bathymetric and gateway changes reduced Atlantic Ocean sensitivity to deoxygenation

Abstract. The magnitude of past deoxygenation events depended on multiple factors, likely including the background state of the ocean-atmosphere system. In this study, we investigate how restricted paleogeography and high atmospheric pCO2 may have preconditioned the Mesozoic Atlantic Ocean, relative to the Cenozoic, to severe oxygen depletion. To do so, we simulate the background redox state for the moderate Paleocene-Eocene Thermal Maximum (PETM) with the Earth System Model IPSL-CM5A2 and compare it to the severe Cretaceous Oceanic Anoxic Event 2 (OAE2), with additional simulations illustrating the impact of the Tasman Gateway on post-PETM oxygenation. The deep Atlantic is, as expected, more oxygenated in the pre-PETM run due to lower Paleocene pCO2 and ventilation through a deep Equatorial Atlantic Gateway. Yet, counterintuitively, simulated pre-PETM and pre-OAE2 deoxygenation at intermediate depths is remarkably similar, suggesting that increased background oxygenation was not the only limit on oxygen loss during the PETM. Instead, we propose that the deepening of the Equatorial Atlantic Gateway between OAE2 and the PETM isolated the low oxygen zone from the seafloor at intermediate depths, disrupting the positive biogeochemical feedbacks associated with reducing sediments and thus increasing the oxygen inventory’s resilience against severe perturbations. Our simulations also suggest that the opening of the Tasman Gateway and Cenozoic pCO2 evolution drove additional oxygenation through the onset of deep-water formation in the Atlantic sector of the Southern Ocean.