Joint Oceanographic Institutions, United States

The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, the dissolved Cl (super -) concentrations and delta D and delta (super 18) O values of the interstitial water reflect changes in the salinity and the isotopic compositions of seawater from the Last Glacial Maximum to the present. The presence of disseminated gas hydrates, which is well identified by discrete low Cl (super -) anomalies within the gas hydrate stability zone, is accompanied by high delta D and delta (super 18) O values of the freshened fluids. This is consistent with incorporation of heavy isotopes into the gas hydrate lattice, which is also apparent in the signals observed at the ridge summit. Here, massive gas hydrate formation in the upper 20 meters below seafloor leads the formation of brines with dissolved Cl (super -) concentrations as high as 1400 mM. The interstitial waters sampled near massive gas hydrates at the ridge summit are extremely depleted in D and (super 18) O. Clay mineral dehydration within the deep prism results in a progressive decrease in Cl (super -) and delta D with depth. Dehydration temperature estimates based on those data likely suggest a progressive increase in the temperature of isotopic fractionation between clay and water with distance from the prism toe. The oxygen isotope data probably reflect the combined effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement; however, there are not enough data to constrain the relative contribution of these processes to the observed signals.