Gas Hydrate Stability Zone in Marine Sediments

Gas hydrates form wherever appropriate physieal conditions exist and eoneentrations of low moleeular weight gases, mostly methane, exeeed saturation. The P/T factors for the presence of methane hydrates (Fig. 

3) are present in marine sediments as shown by the phase boundary in Fig. 14.4. The dashed line shows a typical temperatnre profile throngh the water eolnnm in the Atlantie Ocean. Near snrfaee temperatures are too warm and pressures too low for methane hydrate to be stable. Below the major thermocline there is change in the temperature gradient, and the temperature profile interseets the phase boundary at 450 m water depth, which defines the npper limit for methane hydrate stability in that part of the ocean. If methane is sufficiently abundant, methane hydrate wonld form. However, since the density of hydrate is around 0.913 g em (Sloan 1998) ary eiyslalline hydrate that may form in the water eolnnm (e.g. at sites of methane diseharge) will rise due to its relative buoyancy and it will dissociate when it reaches depths above its stability field. However, if methane hydrate forms within the sediment pore space, it will be bound in plaee. If water temperatnres are colder the npper limit for methane hydrate is shallower. This hmit of 

Consistent with inferences based on methane concentration measured on cores collected at in situ pressures, gas hydrate in these sediments is only present between 45 and 134 meters, in what is known as the gas hydrate occurrence zone (GHOZ). 

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