Anomalously cold temperatures observed at the base of the gas hydrate stability zone on the U.S. Atlantic passive margin

In situ measurements to depths of approximately 415 m below sea floor in methane hydrate-bearing sediments on the U.S. Atlantic passive margin indicate that temperatures at the bottom simulating reflector (BSR) are anomalously low by 0.5-2.9 degrees C if the BSR marks the base of gas hydrate stability (BGHS). Several hypotheses may explain the occurrence of the BSR at inappropriate pressure-temperature (P-T) conditions. (1) If the BSR does not mark the BGHS, then P-T conditions need not be sufficient to dissociate gas hydrate at this depth. (2) The BSR may lie at nonequilibrium P-T conditions due to incomplete readjustment in response to upper Pleistocene-Holocene climate change. However, the occurrence of the Blake Ridge BSR at an overly shallow depth cannot be easily explained by realistic combinations of pressure-driven deepening (sea-level rise) and temperature-driven shoaling (bottom water temperature changes). (3) The properties of sediments or pore fluids may inhibit the stability of gas hydrate. In particular, capillary forces arising in the fine-grained, montmorillonite-rich sediments of the Blake Ridge may lead to shoaling of the BSR in this setting.