Eighteen interstitial-water samples were collected at Site 898 from 3 to 336 mbsf. Samples were collected at 10-m intervals in the top 50 m in Hole 898A, with two additional samples collected in the top 5 m from Hole 898B. Samples were collected at 30-m intervals between 50 and 336 mbsf. A slightly higher sampling density was used in the interval between 148 and 187 mbsf. Results from shipboard interstitial-water analyses are presented in Table 9.
Interstitial-water samples spanned lithologic Unit I through Subunit IIB. As was the case at Site 897, the most significant changes in interstitial-water chemistry occurred at the boundary between Unit I and Subunit IIA. However, this boundary occurred shallower in the sediment column at Site 898 than at Site 897.
Pleistocene and Pliocene sediments are dominated by turbidite sequences relatively rich in organic carbon. In contrast, Miocene through Eocene sediments are depleted of reactive carbon (see "Organic Geochemistry" section, this chapter).
Concentrations of sulfate decrease linearly from 26 mM in the first sample, at 3 mbsf, to near 0 mM by 71 mbsf (Fig. 22A). The gradient in concentration indicates downward diffusion of bottom-water sulfate to a zone of sulfate reduction (removal) between 71 and 152 mbsf. Sulfate concentrations remain near zero through Unit I (which extends down to 163 mbsf; see "Lithostratigraphy" section, this chapter), but increase below the boundary between Unit I and Subunit IIA (around 170 mbsf). The increase in sulfate concentrations below 170 mbsf indicate diffusion of sulfate from a deep reservoir to the base of the Pleistocene and Pliocene sequences. As was the case at Site 897, the deep sulfate reservoir has been preserved by the relatively slow sediment accumulation rates during the Miocene (see "Lithostratigraphy" section, this chapter). Below the sulfate minimum sulfate concentrations reach their maximum values (15-16 mM) between 296 mbsf and the deepest core sample at 336 mbsf.
Alkalinity increases linearly from about 4 mM at 3 mbsf to 16 mM at 40 mbsf (Fig. 22B). Alkalinity remains nearly constant between 44 and 101 mbsf and increases to a broad maximum between 129 and 156 mbsf. The increase in alkalinity above 156 mbsf results from organic carbon degradation through sulfate reduction (Gieskes, 1974, 1983). Below the zone of sulfate reduction, alkalinity generally decreases; however, alternating zones of removal and release are indicated by a small dip in concentration at 190 mbsf, followed by a peak at 237 mbsf.
Concentrations of ammonia increase linearly from a concentration of 447 然 at 3 mbsf to a broad maximum of greater than 3000 然between 101 and 129 mbsf (Fig. 22C). The production of ammonia in sediments results from anoxic carbon degradation; thus, the changes in the ammonia profile at this site are inverse to changes in the sulfate profile. The highest concentrations of ammonia can be observed in the Pleistocene and Pliocene sediments of Unit I, coincident with the sulfate minimum. Ammonia concentrations decrease below 129 mbsf to concentrations less than 800 然 at the deepest section cored (336 mbsf).
Concentrations of dissolved iron increase from 193 然 at 3 mbsf to a maximum of 272 然 by 6 mbsf (Fig. 22D). Below the maximum, iron concentrations decrease linearly to near zero between 71 and 101 mbsf, coincident with the shallowest zone of sulfate reduction. The maximum can be attributed to the release of iron caused by reduction to the soluble Fe+2 state during the decomposition of organic carbon. Iron concentrations increase below 100 mbsf to 70 然, but return to zero by 172 mbsf and remain low throughout the remainder of the cored sediments.
Concentrations of dissolved manganese increase from ~90 然 at 3 mbsf to a maximum of ~130 然 by 4 mbsf (Fig. 23A). Below the maximum, concentrations of manganese decrease to near 0 然 between 36 and 190 mbsf. The maximum can be attributed to the release of manganese as a result of reduction to the soluble Mn+2 state during the decomposition of organic carbon. Manganese concentrations remain near zero through the remainder of Unit I (~160 mbsf), but increase below 190 mbsf. The anomalously high concentrations at 172 mbsf are probably the result of sample contamination. Concentrations of manganese increase below 190 mbsf, resulting in a small peak of between 237 and 267 mbsf, followed by a zone of removal at about 300 mbsf.
In Unit I, concentrations of calcium show depletion (30%-60%), with respect to present bottom-water concentrations, down to about 160 mbsf (Fig. 23B). The depletion in calcium is coincident with the zone of maximum alkalinity and probably reflects precipitation of a carbonate phase. Concentrations of calcium increase below 160 mbsf to constant values of about 30 然 below 296 mbsf. The release of calcium between 160 and 336 mbsf probably reflects a region of silicate mineral alteration and/or carbonate dissolution.
Concentrations of magnesium show some depletion with respect to bottom water throughout the sediment column, with the greatest depletion (~30%) coinciding with the zone of calcium depletion (Fig. 23C). Concentrations of magnesium increase slightly below 160 mbsf.
Concentrations of strontium generally increase with depth from a minimum of about 80 然 near the sediment surface to a maximum of ~400 然 at 314 and 336 mbsf (Fig. 23D). The strontium profile is concave up between the surface and 172 mbsf, indicating removal, with some release occurring in Unit II, between 172 and 190 mbsf. The strontium profile also is concave up between 190 and 314 mbsf, again indicating removal. The concentration maximum at the base of the profile indicates a second zone of the release of strontium to the interstitial waters. Release of strontium is probably associated with recrystalization of carbonate phases (Gieskes, 1974).
In general, concentrations of potassium decrease through the sediment column, with some narrow zones of release and removal indicated by fluctuations in the gradient (Fig. 24A). Values of potassium decrease from a maximum of 12.5 mM near the seafloor to a minimum of about 6 mM below 300 mbsf. The slight enrichment in potassium (with respect to bottom water) near the seafloor may be the result of temperature-of-squeezing effects (Sayles et al., 1973). Possible artifacts in concentrations of potassium resulting from this effect could not be verified because of failure of the in-situ sampler.
Concentrations of silica generally increase with depth, but indicate alternating zones of release and removal that produced small peaks at about 70 and 160 mbsf and a broad high below 190 mbsf (Fig. 24B). Concentrations of silica are elevated (up to 700 然) with respect to bottom water through much of the Pleistocene sequences. Silica has been removed from the interstitial water at about 100 mbsf and again near the boundary between Unit I and Subunit IIA (~172 mbsf). Silica concentrations show release through the Miocene sequence, which resulted in a broad maximum approaching 1100 然 through the remainder of the cored sequence.
Concentrations of chloride show slight depletion with respect to seawater through Unit I (extending down to 163 mbsf) and return to near bottom-water values below (Table 9). In contrast to concentrations of chloride, concentrations of sodium show little variation with depth, with the exception of a single interval at 20 mbsf that is slightly depleted with respect of bottom water (Table 9).