Interstitial Water Studies, Leg 81

Abstract

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...

Interstitial Water and Bulk Calcite Chemistry, Leg 130, and Calcite Recrystallization

“…The relatively constant depth for the Sr maximum at these sites (Table 1) indicates that controls other than diffusional path length relative to sedimentation rate (Gieskes and Johnson, 1984) are important in determining the position of the maxima. The position of the Sr maximum, shallower and younger than the ooze/chalk boundary, indicates the importance of calcite diagenesis producing dissolved Sr above this boundary.…”

Site 841

“…The changes in Ca, Mg, K, and Na at Site 841 are much more pronounced than are usually observed in porewater-depth profiles, and are probably caused by the combined effect of chemical exchange with abundant volcanogenic material and high rates of sedimentation. Increased sedimentation rates cause diminished diffusive exchange with the overlying ocean water (Gieskes and Johnston, 1984), thus allowing steeper concentration gradients to be maintained. Nevertheless, quantitative estimates of the diagenetic processes at Site 841 await isotope measurements of pore waters and quantitative determinations of the secondary mineralogical phases (including clays, zeolites, sulfates, and other secondary minerals).…”

Hydrogeochemistry, ODP Leg 110: an Overview

“…First, sedimentation rates are important. Increased sedimentation rates cause a diminished diffusive exchange with the overlying ocean (Gieskes and Johnston, 1984). Thus, even with reaction rates of similar magnitude, increased depletions in magnesium will result if sedimentation rates are higher.…”