Addressing Factors that Control Near-Surface Gas Hydrate Stability with Time-Series Measurements

Abstract:

Gas hydrates are sensitive to pressure and temperature changes, based on their thermodynamic properties. In nature, this translates to changes in sealevel and/or ocean water temperature fluctuations. When hydrates outcrop the seafloor, however, they could also be sensitive to physical disturbances, such as earthquakes, and microbial processes (such as sulfate reduction and/or methane oxidation), both of which could lead to their dissolution. To address these factors controlling hydrate stability, we will present in situ methane, sulfate, and chloride concentrations over time, in pore-waters of shallow sediments near gas hydrates in seep systems. Datasets presented will include one 4-month time series from the Northern Gulf of Mexico, Mississippi Canyon 118, and two 9-month records from offshore Vancouver Island, Barkley Canyon and Bubbly Gulch at Bullseye Vent. We will address the following questions: Does regional scale oceanography affect methane flux from the hydrate-containing sediments, are microbial processes playing a role in hydrate stability, and what are in situ hydrate dissolution rates? We will also discuss challenges faced with collecting such data, and ways to move forward. We will show that in some systems, methane is nearly saturated within a few cm of the overlying water, thus stabilizing the hydrate. Yet in other systems, methane is undersaturated with respect to methane hydrate which suggests hydrates will dissolve. We will also present laboratory rates of hydrate dissolution to compare to those gained from the field.