Figure 1. Methane hydrate stability conditions for the pure methane-pure water and pure methane-seawater (S = 33.5 ppt) systems over the pressure range of 2.56-12.0 MPa. Data for the pure methane-pure water system are from this study as well as those compiled in Sloan . Data for "synthetic" seawater (S = 35 ppt) are from Dholabhai et al. . As shown in this figure, at any given pressure, the temperature at which methane hydrate dissociates; in the pure methane-seawater system is depressed by approximately -1.1 °C relative to the pure methane-pure water system. Note the pressure axis is reversed in order to reflect increasing pressure with depth along continental margins, and that the temperature axis is expressed in °C (rather than K) for convenience. The depth axis assumes a hydrostatic pressure gradient of 0.010 Mpa/m.