Gas Hydrate and Chloride Anomalies

It has long been recognized that the formation and decomposition of gas hydrate lead to changes in dissolved chloride concentration of marine pore fluids (e.g. Hesse and Harrison 1981). Gas hydrates, like normal ice, exclude salts from the crystal structure, thus increasing the salinity 

In situ chloride concentration in pore fluids of hydrate-bearing sediments also show enrichments relative to seawater in some natural systems. These occur when the geological setting supports formation of brines, or when gas hydrate forms so rapidly that the resulting excess ions do not have sufficient time to diffuse away. These scenarios are also discussed below. 

Estimating Gas Hydrate Abundance Using Dissolved Chloride Data 

Because gas hydrate is not stable at the temperature and pressure conditions that exist at the sea surface, most estimates of the in situ distribution and concentration of gas hydrate rely on a variety of proxies. Perhaps the most widely used of these proxies is based on the accurate measurement of dissolved chloride in the pore fluids. During core recovery, gas hydrate 

There is to date no reliable data on the amount of Cl sequestered by the hydrate cage because the physical separation of the water released by natural hydrate dissociation from pore water contamination can be very difficult. Suess et al. (2001) suggest that there may be residual chloride trapped within the hydrate pore space. Nevertheless, since this number is small and very poorly defined, most estimates of hydrate abundance in marine sediments assume that hydrate formation excludes all dissolved ions. 

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