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.

Interstitial water studies from sites drilled during a transect of the Walvis Ridge indicate that concentration increases in calcium and decreases in magnesium toward and into the basement. These trends can be understood principally in terms of reactions taking place in Layer 2 of the oceanic crust. At Site 525, however, some removal of magnesium occurs within the sediment column. Concentration maxima of dissolved strontium clearly indicate that carbonate recrystallization occurs throughout the carbonate sediments, and studies of the Sr/Ca ratio in carbonates indicate that in chalks and limestones recrystallization is essentially complete. Predictions of dissolved strontium maxima generally fail; this can be understood as removal of strontium in basal sediments and/or basalts.

Interstitial water studies on samples obtained during Leg 81 have revealed substantial changes with depth in dissolved calcium, magnesium, strontium, potassium, and lithium. Calcium and magnesium concentration changes result from alteration reactions, primarily in the basalts and also in the volcanic sediments overlying the basalts. Similarly, depletions in potassium are the result of reactions in volcanic sections of the drill holes. Lithium has its main source in the volcanic sediments. On the other hand, strontium-concentration maxima occur at the ooze-chalk boundary and are the result of carbonate recrystallization reactions. Distinct correlations between calcium gradients and extents of concentration maxima in strontium with sedimentation rates suggest (1) build up of calcium concentrations below diffusion communication depth and (2) increased rates of recrystallization of carbonate with rate of sedimentation. The latter process is probably related to the more "corrosive" character of the sediments as the result of organic carbon diagenesis. INTRODUCTIONDuring Leg 81 of the Deep Sea Drilling Project (DSDP), four sites were drilled in the vicinity of Sites 403 and 404 on the margins of the Rockall Plateau (Fig. 1). Our previous work (Ellis et al., 1979) at Sites 403-406 of Leg 48 revealed large concentration gradients in calcium and magnesium, but unfortunately no deep penetrations to igneous basement rocks were accomplished. Similarly Manheim et al. (1972) established concentration gradients in calcium and magnesium at Site 116, whereas one data point level at Site 117 (~223 m subbottom) also revealed a large increase in calcium and a decrease in magnesium. Sites 552-555 of Leg 81 did reach volcanic basement rocks and thus represent a far more complete picture of the concentration gradients of interstitial water constituents.In this chapter we present the data obtained both on D. V. Glomar Challenger and in our own laboratory on interstitial waters obtained during Leg 81. Studies of the oxygen isotope composition of the waters and the strontium isotope composition of dissolved strontium are presently underway, and these studies will in the future serve to constrain the mechanisms of the reactions postulated in this chapter as the cause of observed concentration changes. METHODS AND RESULTSShipboard analyses comprised determination of pH, alkalinity, salinity, chloride, calcium, and magnesium. In our shore laboratory we determined values of strontium, lithium, potassium, sulfate, ammonia, and silica. Methods used have been described by Gieskes (1974), Lawrence (1976), andGieskes et al. (1982).All the data are tabulated in Table 1 and are graphically represented in Figures 2, 3, 4 and 5 (Sites 552, 553, 554, and 555). DISCUSSIONBefore presenting a general discussion of the data obtained in this study it appears appropriate to describe Sedimentation rates of -20 m/m.y. usually are associated with only minor decreases in dissolved sulfate as a result of sulfate reduction processes (cf., Table 1).Conc...

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