INORGANIC GEOCHEMISTRY

Sixteen interstitial water samples were collected from Hole 1076A and analyzed (Table 10). The sampling protocol called for gathering one 10-cm-long whole-round interval from each core from the surface to ~100 mbsf and one 10-cm-long whole-round interval from approximately every third core thereafter. The shallowest sample was taken from 1.4 mbsf and the deepest from 202.2 mbsf, ensuring coverage of diagenetic processes throughout the complete section cored. Headspace samples (see "Organic Geochemistry" section, this chapter) were taken immediately adjacent to each interstitial water whole-round sample, thereby providing a comparable high-resolution data set for volatile hydrocarbons.

Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 17) reflect the degradation of organic matter. The high total organic carbon concentrations at this site (see "Organic Geochemistry" section, this chapter) are responsible for the very steep decline in dissolved sulfate as well as the sharp increase in alkalinity below the seafloor. Concomitant with the sulfate decrease and alkalinity increase, ammonium also increases rapidly through the uppermost 5-10 mbsf but continues to increase to the bottom of the hole, recording both shallow and deep degradation of organic matter. Below the initially rapid increase in alkalinity, there is a gradual increase to a broad maximum of ~35 mM from 30-70 mbsf, a pronounced minimum centered near 150 mbsf, and a clear increase in the deepest section of the sedimentary succession. The deep alkalinity increase will be discussed below in the context of the distributions of dissolved Ca²⁺, Mg²⁺, and Sr²⁺.

The depletion of dissolved sulfate within the uppermost 20 mbsf (Fig. 17) occurs at a shallower depth than was observed at Site 1075, reflecting the higher sedimentation rate at Site 1076. The effect of higher sedimentation rate is to hasten the isolation of the sedimented organic matter from overlying oxygenated seawater and, thus, to promote anaerobic degradation.

As at Site 1075, organic degradation also drives carbonate dissolution and precipitation, recorded in the distributions of dissolved Ca²⁺, Mg²⁺, and Sr²⁺ (Fig. 18). The strong decreases in Ca²⁺ and Mg²⁺ initially occur at depths shallower than at Site 1075. Likely phases responsible for the decreases in dissolved Ca²⁺ and Mg²⁺ include authigenic carbonates (e.g., dolomite), clays, and authigenic apatite. Keeping in mind that the uppermost three data points in the dissolved Sr²⁺ profile are within analytical precision, the dissolved Sr²⁺ in-creases over the uppermost 50 mbsf reflect diagenetic dissolution of biogenic calcite.

As we propose for Site 1075, there may be more than one distinct sink of dissolved Mg²⁺ at Site 1076. Extrapolating the deeper trend of Mg²⁺ depletion, presumably caused by authigenic clay mineral formation (at depths >50 mbsf), to stratigraphically shallower levels up to the bottom-water value of 53 mM (Fig. 18), we infer that the additional Mg²⁺ drawdown (see uppermost shaded region in Fig.18) is caused by dolomite formation. Stoichiometrically, the precipitation of dolomite should cause equal consumption of dissolved Ca²⁺ and Mg²⁺. The preliminary observations here suggest that the excess decrease of Mg²⁺ approximately balances the decrease in dissolved Ca²⁺ (note the different horizontal axis scales in Fig. 18). Postcruise mass balance calculations of the Ca²⁺, Mg²⁺, and Sr²⁺ budgets will provide further information about these hypothesized authigenic processes.

The increases in alkalinity (Fig. 17), dissolved Ca²⁺, and dissolved Sr²⁺ that occur below 130-150 mbsf suggest additional dissolution of biogenic calcite at this greater depth. Because there also appears to be a region of increased Mg²⁺ uptake through this depth range (see deeper shaded region in Fig. 18), dolomitization may also be occurring. This hypothesis is consistent with the observation that the Mg²⁺ gradient at Site 1076 (a decrease of 27 mM/200 m) is stronger than that at Site 1075 (19 mM/200 m) where we considered only authigenic clay mineral formation as a sink for Mg²⁺.

Dissolved silica concentrations increase very rapidly through the uppermost 5 m of sediment (Fig. 19), recording the dissolution of biogenic opal. Concentrations continue to gradually increase downcore. The maximum in dissolved silica at ~150 mbsf corresponds to an interval in which diatoms are rare or absent (see "Biostratigraphy and Sedimentation Rates" section, this chapter).

Dissolved phosphate increases very rapidly to a maximum value of ~110 µM within the uppermost 20 mbsf. The rate of increase is far greater than that observed at Site 1075; this is consistent with the sharper increase in alkalinity at Site 1076 and the sharp decrease in dissolved sulfate, as described above, all of which reflect intense degradation of organic matter in the shallowest buried sediments. The decrease in dissolved phosphate with depth reflects the presence of a phosphate sink, which is most likely the formation of authigenic apatite.

Concentrations of both dissolved Na⁺ and K⁺ increase with depth downcore (Fig. 20). The behavior of these components most likely reflects reactions with clay minerals throughout the sequence.

The initial downcore decrease of salinity through the upper 30 mbsf (Fig. 21) appears to be caused by the decrease in dissolved sulfate, Ca²⁺, and Mg²⁺ through this interval. The increase below 150 mbsf reflects the increase in many of the dissolved constituents through these deeper sections. The concentration of dissolved Cl^- increases relatively smoothly downcore. The cause of this increase remains unclear at this time, but the gradual increase at Site 1076 is different from the sharp increase at Site 1075 (for which we call on a diffusional glacial signal).

Neither the salinity nor the dissolved Cl^- profile suggests the presence of gas hydrate at any interval within the uppermost 200 mbsf. There is no chemical evidence of dilution by H₂O that would have been released by hydrate dissolution during recovery.