Methane under-saturated fluids in deep-sea sediments: Implications for gas hydrate stability and rates of dissolution

Deep-sea sediments contain Earth's largest reservoir of methane (CH4, 3000 GTons C) trapped within ice-like crystals known as gas hydrates. Understanding the controls on gas hydrate stability is critical because methane released from hydrate destabilization is hypothesized to be a powerful agent of past and potentially future climate change. Hydrates are stable under high pressure, low temperature, moderate salinity, and saturated gas conditions. Yet, the degree of gas saturation is rarely known in nature because in situ dissolved pore-water CH4 concentrations are rarely measured. Here, we report measurements of these concentrations in sediments immediately surrounding deep-sea gas hydrate deposits and show that pore-fluids are greatly under-saturated with respect to expected values for equilibrium with methane gas hydrate. This indicates that the hydrates are dissolving, even though they are found within the appropriate pressure and temperature stability field. However, dissolution rates calculated from the in situ CH4 data are significantly less than dissolution rates predicted for methane-under-saturated pore-water in direct contact with pure methane gas hydrate if equilibrium CH4 concentrations exist immediately adjacent to the hydrate surface. Diffusion-retarding factors found naturally in ocean sediments, such as oil coatings or biofilms, appear to enhance stability in outcropping hydrate deposits. The in situ seafloor evidence provided herein leads us to hypothesize that the stability of the worldwide hydrate deposits may be much greater than predicted from diffusion kinetics because biological (microbial excretion of slime or surfactants) and/or physical processes (oil coatings) effectively armor and stabilize exposed hydrate surfaces, substantially retarding their dissolution.