Chemistry of interstitial-water samples from ODP Leg 105 sites (Table 1)

More than 100 interstitial-water samples from Sites 645, 646, and 647 were analyzed for major and minor ion chemistry. All sites display increases in calcium and decreases in magnesium with depth. The rate of change for these ions varies from site to site as a result of differences in rates of diffusion and sediment/water chemical reactions related to sediment lithology, physical properties, and redox conditions. The latter condition is reflected by the rate of sulfate reduction at each site. The lowest sulfate concentrations were observed at Site 645, where anomalously large depletions of calcium and magnesium are also recorded just below the sediment/water interface. Other dissolved ions, such as potassium, lithium, and strontium, also display overall anomalous depletions at Site 645. In contrast, concentration gradients of most cations at Sites 646 and 647, where dissolved sulfate is present, are more linear and may be primarily supported by diffusion between layer II basalts below and seawater above.
Interstial-water dD and d18O display concomitant decreases with depth at each site. A minor exception occurs at Site 645, where d18O values become more positive in deeper portions of the sequence. This, together with the fact that correlations between dD and d18O vary from site to site, suggests that these isotopes are dissimilarly affected by reactions involving interstitial waters and layer II basalt and/or sediments. Interstitial-water d18O may be more sensitive to low-temperature reactions with basalts and volcaniclastics, which results in an overall depletion. Sediment diagenesis at deeper depths may slightly enrich interstitial-water d18O. The effects of such processes on dD values may be negligible.