INORGANIC GEOCHEMISTRY

Fifty-seven interstitial water samples were gathered from Hole 1082A over a depth range from 1.4 to 586 mbsf. Whole-round samples were sampled at a frequency of one sample per section for the upper 55 mbsf, one sample per core from 55 to 107 mbsf, and every third core thereafter to total depth (Table 11). As with Site 1081, also a deeply drilled site in the same general region as Site 1082, we are able to relate the interstitial water chemistry to a variety of diagenetic processes that reflect not only remineralization of organic matter in the shallower sediments, but also compositional changes in the deeper sediments. As an overall comparison between the two sites, the interstitial water chemistry at Site 1082 records the greater supply of organic matter, along with a pronounced contrast in many dissolved species distributions between lithostratigraphic Units I and II.

Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 17) through the upper ~100 mbsf record the degradation of organic matter. Sulfate is completely consumed within the upper 25 mbsf, which is approximately twice as quickly as at Site 1081. Through this uppermost depth interval, alkalinity values increase sharply, and the concentration of ammonium also shows the greatest relative increase. Alkalinity and ammonium eventually reach maxima that are two to three times those observed at Site 1081.

After reaching a broad maximum from ~90 to 250 mbsf, alkalinity decreases steadily to a minimum value of 16.8 mM at the bottom of the hole. Through this deeper interval, ammonium concentrations also reach maximum values before decreasing toward the boundary between lithostratigraphic Subunit IC and Unit II. This decrease most likely indicates cation exchange reactions during clay diagenesis. Moreover, within lithostratigraphic Unit II, ammonium concentrations increase dramatically to values greater than the maximum observed stratigraphically higher in lithostratigraphic Subunit IB. Because TOC concentrations in Unit II are lower than those in Subunits IA and IB (see "Organic Geochemistry" section, this chapter), this increase in dissolved ammonium is surprising. It may be related to the smaller amount of clay in the nannofossil ooze of Unit II, compared with the nannofossil clay of Subunit IC (see "Lithostratigraphy" section, this chapter). Presumably, the smaller proportional amount of clay decreases the amount of ammonium that can be removed by ion exchange reactions.

Concentration profiles of Ca²⁺, Mg²⁺, and Sr²⁺ reflect processes of carbonate dissolution and precipitation (Fig. 18). Dissolved Sr²⁺ increases monotonically to the middle of lithostratigraphic Subunit IB at ~280 mbsf, before increasing the rate of increase to the bottom of the sequence. The deeper gradual increase records continual dissolution of biogenic calcite. Within the uppermost nearly 50 mbsf, dissolved Ca²⁺ and Mg²⁺ concentrations decrease by essentially the same amount (~8 mM), suggesting that dolomite is precipitating and removing dissolved Ca²⁺ and Mg²⁺ in equal proportions. The dissolved Ca²⁺ profile at Site 1082 is very similar to that previously observed at Site 1081, whereas the decrease in dissolved Mg²⁺ at Site 1082 is more gradual than that at Site 1081. Because dissolved Ca²⁺ is increasing through the sequence from 50 mbsf to the bottom of the sequence at both sites, the contrast in dissolved Mg²⁺ between the two sites likely reflects differences in Mg²⁺ uptake by clay minerals.

Dissolved silica increases in concentration rapidly from representative bottom-water values to a maximum of ~1200 µM at 50 mbsf (Fig. 19), recording the dissolution of biogenic opal. A slight minimum (~1000 µM) occurs at 100 mbsf, corresponding to a low in the first-order abundance index of diatom distributions (see "Biostratigraphy and Sedimentation Rates" section, this chapter). From 100 to nearly 520 mbsf, dissolved silica concentrations increase gradually. Interestingly, dissolved silica does not show a maximum within lithostratigraphic Subunit IB (nannofossil and diatom-rich clay; see "Lithostratigraphy" section, this chapter), despite the greater potential source of biogenic silica in this subunit. Dissolved silica decreases in concentration through lithostratigraphic Unit II, the nannofossil ooze, likely reflecting the decrease in diatom abundance through this portion of the sequence (see "Biostratigraphy and Sedimentation Rates" section, this chapter).

Dissolved phosphate concentrations increase with depth within Subunit IA, recording the remineralization of organic matter. The slight inflection change in the increase found between ~20 and 40 mbsf corresponds with a similar pattern in the alkalinity profile and may reflect a change in organic matter concentration or composition. After reaching a sharply defined maximum of 161 µM at 69 mbsf, dissolved phosphate decreases relatively smoothly to ~400 mbsf, below which concentrations are <10 µM. Within the upper reaches of lithostratigraphic Subunit IB, there appears to be a zone of increased dissolved phosphate removal (shaded region in Fig. 19).

The concentration of dissolved Na⁺ increases from seawater values to maximum values at depth (Fig. 20). This increase may be recording the release of Na⁺ from clay minerals. The concentration of K⁺ (Fig. 20) shows some variability in the uppermost 50 mbsf before remaining essentially constant to ~300 mbsf, below which the concentration of dissolved K⁺ decreases dramatically. This decrease only broadly corresponds with the lithologic boundary between lithostratigraphic Subunits IB and IC. This broad pattern in dissolved K⁺ is similar to that observed at Site 1081, although at Site 1081 there was no relationship among the lithostratigraphic unit distributions. Because this decrease in dissolved K⁺ occurs over a depth range similar to the one in which ammonium concentrations decrease, we interpret these concentration profiles as reflecting deep ion exchange reactions with clay.

Salinity is greater at Site 1082 than at Site 1081, reflecting the greater amounts of alkalinity, ammonium, Mg²⁺, silica, phosphate, Na⁺, and K⁺, particularly through the uppermost 300 mbsf (Fig. 21). Concentrations of dissolved Cl^– record several maxima and minima through the uppermost 50 mbsf, perhaps recording variations in bottom-water content during glacial (and interglacial?) periods. From 130 mbsf to the bottom of the hole, dissolved Cl^– concentrations steadily increase.