INORGANIC GEOCHEMISTRY

Two sets of interstitial water samples were collected at Site 1077 (Table 10). First, 16 interstitial water samples were collected from Hole 1077A (1.5-203.9 mbsf) to provide coverage of diagenetic processes through the entire recovered sedimentary succession and to complete the three-site transect of the Congo Margin. The chemical distributions in interstitial waters at Site 1077 are very similar to those observed at Site 1075 and are only modestly distinct from those at Site 1076.

Second, a special high-resolution study, with interstitial water samples being gathered at a frequency of three per core, targeted the interval from ~100 to 130 mbsf with a total of nine additional samples. This depth range included a seismic reflector (see "Site Geophysics" section, "Site 1075" chapter, this volume) that potentially was caused by a thin (2-10 m in thickness) layer of methane hydrate. This study was intended to test whether hydrate was the cause of the seismic discontinuity.

As seen at both Sites 1075 and 1076, downcore profiles of alkalinity, sulfate, and ammonium (Fig. 18) reflect the degradation of organic matter. The general distribution of these interstitial water components records the shallow generation of alkalinity, in large part from consumption of sulfate, paired with a steady and consistent increase in ammonium. Although the deep increase in alkalinity found at Site 1076 was not observed at Site 1077, in all other aspects the behavior of these dissolved components records a chemical behavior intermediate in nature to those observed at Sites 1075 and 1076.

The strong gradients in Ca²⁺, Mg²⁺, and Sr²⁺ that initially occur at shallower depths at Site 1077 (Fig. 19) record carbonate dissolution and reprecipitation reactions. The increase in dissolved Sr²⁺ above values of deep bottom-water reflects biogenic calcite dissolution, whereas the decreases in dissolved Ca²⁺ and Mg²⁺ through the same interval most likely indicate precipitation of dolomite. Reflecting the commonality of processes operating at Sites 1077 and 1075, the chemical distributions in interstitial waters at these two sites are strongly similar. The only significant contrast between the two sites is a slight increase, at depth, in dissolved Sr²⁺ and Ca²⁺ at Site 1077, which collectively indicates a second phase of biogenic calcite dissolution.

Dissolved silica increases in concentration very rapidly through the uppermost 5 m of sediment (Fig. 20), recording the dissolution of biogenic opal. Concentrations continue to gradually increase at depth downcore. There appears to be no obvious relationship between this depth pattern and the distribution of diatoms in the sequence (see "Biostratigraphy and Sedimentation Rates" section, this chapter).

Dissolved phosphate increases very rapidly to a maximum value of ~250 µM within the uppermost 20 mbsf. This rate of increase is much sharper than that at Site 1075 or Site 1076, both also located in the Congo Basin. This is somewhat surprising, given that the increase in alkalinity and the consumption of sulfate at Site 1077 are approximately the same as at the other sites. This contrast may reflect differences in the organic matter-to-clay ratio, in the amount of diagenetic apatite that is forming, or in some other analogous solid phase variation that would serve to create a different production-to-removal balance. One potentially important indicator is glauconite; Site 1077 has appreciably greater quantities of this phase (see "Lithostratigraphy" section, this chapter), which is preferentially found in nutrient-rich upwelling regions. Although the TOC concentrations at Site 1077 are similar to those at Sites 1075 and 1076, the TOC value is a bulk measurement and therefore may not detect subtle variations in organic matter composition that are manifest in the inorganic solid phase mineralogy. An additional consideration is the fact that although the maximum in dissolved phosphate is notably greater at Site 1077, the concentrations at depth are approximately the same as at the other sites. Therefore, the removal of dissolved phosphate at Site 1077 is also greater. We postulate, therefore, that postcruise mineralogical studies will find greater concentrations of solid phase, phosphate-bearing minerals as well.

Concentrations of dissolved Na⁺ and K⁺ both steadily increase with depth downcore (Fig. 21), most likely reflecting cation exchange reactions involved with authigenic clay formation.

The presence of methane hydrate in sedimentary sequences is easily documented by interstitial water chemistry because the decomposition of the hydrate during recovery sharply decreases salinity and dissolved Cl^- concentrations. At Site 1077, salinity steadily decreases downcore, whereas the concentration of dissolved Cl^- increases relatively smoothly (Fig. 22). Neither of these profiles is characteristic of hydrate presence in the sedimentary sequence. The depth interval that was highlighted by seismic stratigraphic studies (see "Site Geophysics" section, "Site 1075" chapter, this volume) as a zone potentially bearing methane hydrate yielded no chemical evidence of the presence of methane hydrate, nor did any other portion of the sequence recovered from Site 1077.