(Table 1) Interstitial water chemistry at DSDP Leg 76 Holes

Low salinities and strong opposing fluctuations between chlorinity and interstitial water d18O and dD characterize the region of gas-hydrate occurrence at Site 533. In addition to the hydrate recovered from 243 m sub-bottom, thin, diffuse layers of hydrate composing about 10% of the total pore space may have been cored around 290 m. Decomposition during recovery could account for the high oxygen isotope value (+2.80 per mil) and low chlorinity (17.21 g/kg) observed at this depth. High alkalinity and dissolved ammonia (71 meq/l and 33 mM, respectively) indicate substantial microbial activity.Chlorinity gradually increases with depth in the 1600-m-thick sediment column at Site 534, perhaps due to deepseated alteration-hydration reactions or to the presence of nearby evaporites. d18O and dD decrease with depth and are well correlated, suggesting that a common mechanism is fractionating both.