(Table 1) Chemisity of interstitial water at DSDP Leg 66 holes

During Leg 66, a transect was drilled across the Middle America Trench off Mexico. In this report, interstitial water analyses for Sites 487 (on oceanic crust), 490, 491, and 492 (continental slope apron), and 493 (upper continental slope) are presented. All sites are characterized by high sedimentation rates, and as a result sulfate reduction and methane generation processes are very important. At Sites 490 through 492 decreases in chlorinity are evidence of the presence of gas hydrates. At Site 493 a sharp increase in dissolved calcium and the complete depletion of dissolved magnesium just above basement indicate that weathering of continental crust is still an ongoing process.