Twenty-seven interstitial-water samples were collected at Site 900 between 12.6 and 721.5 mbsf. Whole-round samples were collected from each core in the interval from 12.6 to 34.9 mbsf (Cores 149-9OOA-3R to -5R) and from every third core thereafter (when recovery allowed). Interstitial-water samples spanned lithostratigraphic Units I and II. Results from shipboard interstitial-water analyses are presented in Table 13.
Concentrations of sulfate decreased from 27.9 mM in the first sample, at 12.6 mbsf, to a minimum of 1.9 mM at 702.2 mbsf (Fig. 35A). The sulfate profile is slightly convex upward, suggesting that some sulfate reduction occurred within the sediment sequence.
Values of alkalinity decrease slightly from 12.6 to 54.1 mbsf and then increase to a maximum of 11.9 mM at 289.9 mbsf (Fig. 35B). The increase in alkalinity results from anaerobic organic carbon degradation (Gieskes, 1974, 1983). Alkalinity decreases between 289.9 and 465.7 mbsf and then increases to a peak of 11.7 mM at 465.7 mbsf. Alkalinity decreases below 407.7 mbsf to a minimum of 2.5 mM at 606.8 mbsf. Sample size limitations prevented the analysis of alkalinity in the deeper samples.
Concentrations of ammonia generally increase with depth from a concentration of 321 µM at 12.6 mbsf to a maximum of 698 µM at 435.5 mbsf and decrease slightly thereafter (Fig. 35C). The maximum is consistent with a zone of active sulfate reduction within the interval between the surface and about 450 mbsf. Fluctuations in the profile indicate alternating zones of production and removal (the two anomalously low ammonia values at 116 and 702 mbsf probably reflect analytical problems).
The dissolved manganese profile shows three distinct zones of release and removal that produced maxima at 12.6, 115.7, and 465.7 mbsf (Fig. 35D). The high concentrations of manganese in the first sample probably result from the reduction of manganese oxides during the oxidation of organic carbon. The two deeper maxima may result from the reduction of manganese oxides and/or dissolution of some manganese carbonate phase.
Concentrations of calcium are slightly depleted with respect to typical concentrations of bottom water in the first four samples from 12.6 to 54.1 mbsf and then increase linearly to 31.5 mM at 289.9 mbsf (Fig. 36A). Below 289.9 mbsf, concentrations of calcium increase slightly to reach 36.0 mM by 636.3 mbsf and then increase rapidly to a maximum value of 46.5 mM by 721.5 mbsf. The profile suggests zones of calcium release at about 300 and below 636 mbsf. The release of calcium probably reflects regions of carbonate recrystalization (Gieskes, 1983).
Concentrations of magnesium generally decrease with depth from near typical bottom water to a minimum of 17.3 mM at 702.2 mbsf (Fig. 36B). The profile shows two zones where the gradient is steep, below 115.7 and below 606.8 mbsf. The steep negative gradient suggests that magnesium removal is greatest in these zones. The removal of magnesium probably results from clay mineral alteration (Gieskes, 1983).
Concentrations of strontium increase from a minimum of 125 µM in the first sample at 12.6 mbsf to a maximum of 846 µM at 465.7 mbsf and remain fairly constant below that depth (Fig. 36C). The strontium profile is slightly convex up between the surface and the maximum at 465.7 mbsf, indicating release of strontium from the solids through this sequence. Strontium release is probably associated with recrystalization of carbonate phases (Gieskes, 1974).
Concentrations of potassium generally decrease downhole, with some narrow zones of release and removal indicated by fluctuations in the profile (Fig. 36D). Potassium values decrease from a maximum of 10.3 mM in the first sample at 12.6 mbsf to a minimum of 0.7 mM at 702.2 mbsf. The profile is fairly linear, suggesting removal of potassium by means of interaction with basement rock.
Concentrations of silica remain near typical bottom-water values from 12.6 to 77.1 mbsf and then increase to a broad maximum, reaching 1188 µM between 143.0 and 435.5 mbsf (Fig. 37). Concentrations of silica decrease below 435.5 mbsf to a minimum of 130 µM at 665.9 mbsf.
Concentrations of chloride remain fairly constant with respect to typical bottom water through most of the sediment column, with the exception of the last six samples, which are lower (Table 13). In contrast to chloride, sodium is generally depleted with respect to typical bottom water throughout the sediment column (Table 13).