CONCLUSIONS

Simultaneous solution of the transport equations for pore-water Cl^- and ²H may be used to construct characteristic curves that are independent of the assumed rate of pore-water migration. Minimum in situ concentrations of hydrate can be determined by combining these curves with the results of Cl^- and ²H measurements on water from hydrate decomposition experiments and pore water obtained by squeezing of sediments, provided the system is transport dominated. This is not a serious restriction because formation of hydrate in deep-sea sediments seems to be closely associated with fluid flow. The method, which is based on analysis of distances in the two-dimensional Cl^--²H space, generates in situ Cl^- concentrations that may be used to estimate rates of fluid flow. It is also possible to evaluate whether the minimum hydrate-amount estimates are representative of the actual hydrate concentrations by considering the distribution of pore-water compositions relative to the characteristic curves.

At Site 996 this method results in hydrate concentrations from 0% to 31% of pore space, with a mean hydrate concentration of 10%. The highest hydrate concentration was determined for cores with visual detection of hydrate-filled veins as much as 0.5 cm thick and 3-4 cm wide that could be traced about 30-40 cm along the cores.

The rate of upward fluid migration could be constrained to 0.125-0.5 m/k.y., with a mean value of 0.35 m/k.y. Mass balance calculations showed that decomposition of hydrate below the BSR is insufficient to account for the fluxes of water and CH₄, and that the fluxes of Cl^- and H₂O are one to two orders of magnitude higher than the integrated rates of hydrate formation.