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Mechanisms that drive cyclicity in marine sediment deposits during hothouse climate periods in response to Earth's orbit variations remain debated. Orbital cycles fingerprint in the oceanographic records results from the combined effect of terrestrial (e.g., Weathering derived nutrient supply, freshwater discharge) and oceanic (e.g., productivity, oxygenation) processes, whose respective contribution remains to be clarified. Here we investigate the effect of extreme orbital configurations on the oxygenation state of the ocean using marine biogeochemistry simulations with the IPSL‐CM5A2 Earth System Model under Cenomanian‐Turonian boundary conditions. Our simulations show that small changes in ocean ventilation triggered by orbitally induced variations in deep water formation have a strong impact on the spatial distribution of dissolved oxygen. This phenomena is amplified in enclosed and already poorly oxygenated basins, such as the proto‐Atlantic ocean, where up to 50% of the water volume become anoxic for some of the configurations. Plain Language Summary: Changes in the orbit of the Earth over time are responsible for periodic perturbations of the atmosphere and ocean dynamics that are ultimately being recorded in marine sediments through the occurrence of cyclicity. Origin of these cycles in past sedimentary records is not well understood because they are the result of the interactions between different processes that all exert a control on the nature of the deposited sediments. One of these processes is the oxygenation of the ocean that controls the potential for preservation of deposited organic matter. Here we focus on how variations in ocean dynamics modulate the oxygenation of the ocean during the late Cretaceous, using paleoclimate model simulations. We find that small changes in the oceanic circulation can lead to strong loss in oxygen in some part of the ocean, such as the Central Atlantic. Our results have implication for understanding the sedimentary record, the functioning of biogeochemical cycles and feedbacks within the climate system. Key Points: Small orbitally induced changes in ventilation produce large‐scale modification of O2 distribution in the late Cretaceous oceanDeoxygenation is strongly enhanced in already poorly oxygenated basins such as the proto‐Atlantic oceanVariations in deoxygenation strength occurs at all orbital frequency albeit with different amplitude [ABSTRACT FROM AUTHOR]

The Modern Ocean is characterized by the formation of deep‐water in the North Atlantic Ocean (i.e., NADW). This feature has been attributed to the modern geography, in which the Atlantic Ocean is a large basin extending from northern polar latitudes to the Southern Ocean, the latter enabling the connection of the otherwise isolated Atlantic with the Pacific and Indian Oceans. Sedimentary data date the establishment of the NADW between the beginning of the Eocene (∼49 Ma) and the beginning of the Miocene (∼23 Ma). The objective of this study is to quantify the impact of Miocene geography (∼20 Ma) on NADW using new simulations performed with the Earth System Model IPSL‐CM5A2. We specifically focus on the closure of the Eastern Tethys Seaway (ETS), dated between 22 and 14 Ma, which allowed the connection between the Atlantic and Indian Oceans, and on the Greenland Ice Sheet, whose earliest onset remains open to discussion but for which evidence suggest a possible existence as early as the Eocene. Our results show that the closure of the ETS does not appear to impact the establishment of NADW, because waters from the Indian Ocean do not reach the NADW formation zone when the seaway is open. Conversely, the existence of an ice sheet over Greenland strengthens the formation of NADW owing to topography induced changes in wind patterns over the North Atlantic, which in turn, results in a salinification of the North Atlantic and Nordic Seas, and in an increase in deep‐water formation. Key Points: Ocean circulation induced by Early Miocene paleogeography in the IPSL‐CM5A2 Earth System model is studiedNo clear impact of the closure of the Eastern Tethys Seaway is foundSignificant increase in North Atlantic Deep Water intensity results from ephemeral Greenland ice‐sheets during the Miocene [ABSTRACT FROM AUTHOR]

The opening of the Drake Passage (DP) during the Cenozoic is a tectonic event of paramount importance for the development of modern ocean characteristics. Notably, it has been suggested that it exerts a primary role in the onset of the Antarctic Circumpolar Current (ACC) formation, in the cooling of high‐latitude South Atlantic waters and in the initiation of North Atlantic Deep Water (NADW) formation. Several model studies have aimed to assess the impacts of DP opening on climate, but most of them focused on surface climate, and only few used realistic Eocene boundary conditions. Here, we revisit the impact of the DP opening on ocean circulation with the IPSL‐CM5A2 Earth System Model. Using appropriate middle Eocene (40 Ma) boundary conditions, we perform and analyze simulations with different depths of the DP (0, 100, 1,000, and 2,500 m) and compare results to existing geochemical data. Our experiments show that DP opening has a strong effect on Eocene ocean structure and dynamics even for shallow depths. The DP opening notably allows the formation of a proto‐ACC and induces deep ocean cooling of 1.5°C to 2.5°C in most of the Southern Hemisphere. There is no NADW formation in our simulations regardless of the depth of the DP, suggesting that the DP on its own is not a primary control of deepwater formation in the North Atlantic. This study elucidates how and to what extent the opening of the DP contributed to the establishment of the modern global thermohaline circulation. Key Points: A shallow opening of the Drake Passage induces strong changes in ocean properties and dynamicsA proto‐ACC is able to form during the Eocene under high levels of pCO2, but a strong ACC requires supplementary geographical changesNorth Atlantic Deep Water is probably not able to form before the separation of the Arctic and Atlantic Oceans [ABSTRACT FROM AUTHOR]