INORGANIC GEOCHEMISTRY

Sixteen interstitial water samples were gathered from Hole 1086A between 1.4 and 201.1 mbsf. Whole-round samples were sampled at a frequency of one sample per core to 96.6 mbsf and every third core thereafter to total depth (Table 9). As was the case at Site 1085, also located in the Cape Basin, the interstitial water chemistry at this site is controlled dominantly by the high carbonate and low organic carbon concentrations in the sediment, which results in extremely modest variations in chemical gradients of many dissolved species. The chemical gradients found at Site 1086 are even more gradual than those found at Site 1085.

Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 16) reflect the degradation of the low concentrations of organic matter. Alkalinity reaches a maximum value of only 17 mM at 125 mbsf and subsequently decreases to the bottom of the hole. Compared with other Leg 175 sites, this is an extremely deep stratigraphic position for the alkalinity maximum, with most alkalinity maxima at the previous sites being reached within the uppermost 50 to 100 mbsf. Sulfate is consumed by 180 mbsf, which is also very deep in comparison with the previous Leg 175 sites. This very weak gradient reflects both the low level of organic carbon (see "Organic Geochemistry" section, this chapter) as well as the low sedimentation rate (see "Biostratigraphy and Sedimentation Rates" section, this chapter). Ammonium reaches a maximum of only ~2800 µM at 125 mbsf. As with the alkalinity profile at this site, this concentration is very low compared with other Leg 175 sites and reflects the small amount of organic matter.

The concentration of dissolved Sr²⁺ increases essentially linearly to a maximum value of 312 µM at the bottom of the hole (Fig. 17). The overall increase downhole records the dissolution of biogenic calcite, which releases Sr²⁺ to the interstitial waters. The rate of increase of dissolved Sr²⁺ is strongly similar to that observed at Site 1085, reflecting the common influence of the high concentrations of biogenic calcium carbonate in the sediment from these two sites (see "Organic Geochemistry" section, this chapter).

Dissolved Ca²⁺ decreases by only 5 mM from the seafloor to 96.6 mbsf (Fig. 17). Compared with the decrease observed at Site 1085, this decrease is very gradual. Below 96.6 mbsf, dissolved Ca²⁺ remains essentially constant at ~6.5 mM. Dissolved Mg²⁺ decreases continually from the seafloor to total depth, with a low value of 30.5 mM. Through the interval in which Ca²⁺ decreases by 5 mM, Mg²⁺ decreases by 12 mM. This lack of correspondence in both extent and pattern of the respective decreases suggests that dolomitization is less important than at other Leg 175 sites. Rather, we attribute the decrease in Ca²⁺ to phosphate precipitation and the decrease in Mg²⁺ to clay mineral uptake, with only a minor effect from dolomitization.

Dissolved silica is present in interstitial waters from Site 1086 at concentrations greater than representative bottom-water values (Fig. 18), indicating dissolution of biogenic opal. The rapid increase through the uppermost 30 mbsf is also consistent with the fact that siliceous sponge spicules are abundant and serve as a source of dissolved silica. Although the concentration of dissolved silica is relatively low throughout the entire sequence in the absolute sense, a maximum of ~600 µM is found at 30 mbsf with a rapid decrease of values below that depth. This pattern of dissolved silica is consistent with the lack of siliceous microfossils below 35 mbsf, and thus a source of dissolved silica is missing from this deeper portion of the sequence.

Dissolved phosphate concentrations increase only very slightly with depth and reach a maximum of only 21 µM at 49 mbsf (Fig. 18). The maximum itself is very broad and diffuse. Toward the deeper portion of the sequence, dissolved phosphate concentrations decrease to values <10 µM, most likely reflecting authigenic phosphate precipitation.

Dissolved Na⁺ increases rapidly through the uppermost 30 mbsf and is relatively constant to the bottom of the hole (Fig. 19). Concentrations of dissolved K⁺ are relatively constant through the uppermost 68 mbsf and then decrease to the bottom of the hole. The concentrations of both of these cations are lower at Site 1086 than at Site 1085.

Salinity decreases slightly through the sequence (Fig. 20), recording the total decreases in the dissolved species described above, particularly alkalinity, Mg²⁺, and K⁺. Concentrations of dissolved Cl^– record an initial increase to a maximum of ~557 mM (30–40 mbsf) before decreasing to the bottom of the hole. This initial increase in dissolved Cl^– may reflect changes in bottom-water chemistry associated with ice-volume variations through glacial periods. Both the salinity and Cl^– profiles are very similar to those observed at Site 1085.