Twelve interstitial-water samples were collected at Site 899 from 84.4 to 346.7 mbsf. Whole-round samples were collected from the first two cores and from every third core thereafter. Interstitial-water samples spanned lithostratigraphic Unit I through Subunit IIB. Results from the shipboard interstitial-water analyses are presented in Table 17.
The concentration of sulfate remains nearly constant (between 18.5 and 19.9 mM) from 84.4 to 255.3 mbsf, where concentrations begin to decrease slightly to a minimum of 15.5 mM in the deepest sample (Fig. 49A). Sulfate has been depleted with respect to seawater, indicating that sulfate reduction probably occurred in the shallower sediments. The absence of a negative gradient in the upper cores indicates that the zone of sulfate reduction was not sampled (i.e., it lay at a shallower depth).
Values of alkalinity remain nearly constant between 84.4 and 123.0 mbsf and increase to a maximum of 9.4 mM at 227.3 mbsf (Fig. 49B). This increase in alkalinity results from anaerobic organic carbon degradation (Gieskes, 1974, 1983). Alkalinity decreases below 227.3 mbsf to about 7 mM at 290.3 mbsf.
The concentration of ammonia generally decreases with depth from a value of ~500 然 at 84.4 mbsf to 157 然 at 346.7 mbsf (Fig. 49C). Fluctuations in the profile indicate alternating zones of production and removal. Ammonia concentrations are elevated with respect to oxic seawater in the first interval sampled; thus, the observed negative gradient below 84.4 mbsf suggests that a zone of ammonia production was present above the shallowest interval sampled at this site.
Concentrations of dissolved iron increase from 43 然 at 84.4 mbsf to a maximum of 164 然 at 123.0 mbsf (Fig. 49D). Below the maximum, concentrations of iron decrease to nearly zero between 179.0 and 255.3 mbsf, but show a slight increase at 290.3 mbsf.
Concentrations of dissolved manganese generally increase downhole (with the exception of the sample from Section 149-899A-14R-2 at 209.5 mbsf) from 4 然 at 84.4 mbsf to 45 然 at 290.3 mbsf (Fig. 50A).
Concentrations of calcium increase with depth from values near those for present-day bottom water at 84.4 mbsf to 26.8 mM at 346.7 mbsf (Fig. 50B). Concentration of magnesium remains near bottom- water values (54-57 mM) from 84.4 to 178.9 mbsf (Fig. 50C). Below 178.9 mbsf, concentration of magnesium decreases to about 44 mM at 232.0 mbsf and remains low to 346.7 mbsf.
Concentrations of potassium generally decrease downhole, from a maximum of 9.9 mM at 92.6 mbsf to a minimum of 4.3 mM at 346.7 mbsf, with some narrow zones of release and removal indicated by fluctuations in the profile (Fig. 50D).
Concentrations of silica increase with depth to a maximum value of greater than 1200 然 at 209.4 mbsf. Alternating zones of release and removal have produced another peak at 290.3 mbsf (Fig. 51). Concentrations of silica decrease below 290.3 mbsf to about 580 然 in the bottom two samples (318.7-346.7 mbsf).
Concentrations of chloride show a slight enrichment with respect to bottom water from 84.4 to 227.3 mbsf, but show slight depletion with respect to bottom water between 232.0 and 346.7 mbsf (Table 17).
Concentrations of sodium increase from the interval sampled at 84.4 mbsf to a broad maximum between 209.4 and 227.3 mbsf (Table 17). Concentrations of sodium decrease below 227.3 mbsf to values near those of typical bottom-water concentrations.