THERMODYNAMIC CONDITIONS FOR THE STABILITY OF GAS HYDRATE IN THE SEA-FLOOR

Suitable pressures and temperatures for methane hydrate are found over most of the seafloor but thermodynamic equilibrium imposes an additio nal condition on the concentration of dissolved gas. We quantify the t hermodynamic conditions for hydrate stability using a simulated anneal ing algorithm to minimize the free energy of a mixture of methane gas and seawater. The equilibrium state includes a description of the comp osition of all stable phases as a function of pressure, temperature, a nd salinity. When the hydrate phase is stable, we find that the equili brium concentration of dissolved gas (solubility) decreases sharply wi th temperature. The gas solubility is also lowered for typical values of salinity in seawater. Since lower solubilities reduce the amount of gas required to form hydrate, the presence of salts in seawater can a ctually promote hydrate formation. Changes in salinity that accompany hydrate formation add a thermodynamic degree of freedom, which permits a three-phase zone to develop, where hydrate, seawater, and free gas coexist over a range of temperatures at a constant pressure. We apply our calculations to determine the location of stable phases in the sea floor. The calculated profile of gas solubility permits hydrate to cry stallize directly from dissolved gas in seawater. Diffusion of gas alo ng the gradient in the equilibrium concentration implies a continual t ransport of gas through the hydrate layer into the overlying ocean. In order to maintain hydrate in the seafloor sediments, a persistant sou rce of methane is required to overcome the losses due to diffusion. Ra tes of hydrate growth and loss are estimated using simple models of ph ysical conditions in marine sediments.