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|a During the Paleocene-Eocene Thermal Maximum (PETM, ~55 Ma), marine and terrestrial carbon isotope values exhibit a negative shift of at least 2.5‰, indicative of massive destabilization of marine methane hydrates, releasing ~1100 gigatonnes of methane carbon. The cause of the hydrate destabilization is unknown but has been speculated to be warming due to a change from high-latitude to low-latitude deepwater formation. Here, we present results from a numerical ocean model indicating that a sudden switch of deepwater formation from southern to northern high latitudes caused mid-depth and deep-ocean warming of 3-5°C. The switch is caused by a slow increase in the intensity of the atmospheric hydrologic cycle, as expected under increasing temperatures and consistent with PETM sedimentary evidence. Deepened subduction prior to the thermohaline circulation switch causes warming of 1-4°C in limited areas at thermocline through upper intermediate depths, which could destabilize methane hydrates gradually and at progressively greater depths. The switch itself occurs abruptly, with up to 5°C warming resulting everywhere in the deep ocean.
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