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Abstract:
Dissolved chloride concentrations higher than seawater were observed over a broad depth range in pore water profiles from International Ocean Discovery Program Site U1517 on the Hikurangi Margin. This Cl maximum is not associated with an 87Sr/86Sr anomaly, indicating that it is not caused by hydration reactions during ash alteration. We use a numerical modeling approach to examine possible causes for recent gas hydrate formation that can result in the observed Cl high. Our approach considers sedimentation, sea level, and bottom water temperature (BWT) changes due to glaciation as drivers for the downward migration of the base of gas hydrate stability and gas hydrate formation. The modeling results reveal that lowering of sea level during glaciation can allow methane hydrate dissociation followed by postglacial hydrate formation as sea level rises. However, BWT cooling of 2 °C during glaciation followed by warming during deglaciation would mostly counteract the impacts of sea level change. Bottom water cooling during glaciation is expected in this region and many locations worldwide. As a result, our simulations do not support the previous hypotheses of large‐scale gas hydrate dissociation due to sea level drop during glaciation, which have been proposed as triggers for widespread gas release and slope failure. Such a mechanism is only possible where BWT remains constant or increases during glaciation. Our simulations indicate that sedimentation constitutes the largest factor driving recent methane hydrate formation at Site U1517, and we suggest that sedimentation may play a larger role in gas hydrate dynamics along margins than previously recognized.
