INORGANIC GEOCHEMISTRY

Sixteen interstitial water samples were collected at Holes 1078A, 1078B, and 1078C over a depth range from 1.4 to 150.3 mbsf (Table 9) to provide information about diagenetic reactions occurring in the Angola Basin. In addition to the changes in interstitial water composition caused by organic carbon degradation, carbonate dissolution and reprecipitation, and other chemical reactions, the strong decrease in sedimentation rate from ~60 cm/k.y. at depths below ~15 mbsf to much lower values in the uppermost sequence (>24 cm/k.y.; see "Biostratigraphy and Sedimentation Rates" section, this chapter) has a profound effect on the diagenetic reactions occurring at Site 1078. On all profiles, the chemical distributions at Site 1077, which are broadly representative of the sites drilled in the Congo Basin, are provided for interbasinal comparison purposes.

Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 26) reflect the degradation of organic matter, as has been observed at Sites 1075, 1076, and 1077. Of particular interest at Site 1078, however, is the influence of the change in sedimentation rate at ~15 mbsf. Below this depth, there is a marked change in slope in the alkalinity gradient, and sulfate has been almost completely consumed. These profiles show the effect of the decrease in sedimentation rate through time; below 15 mbsf, where sedimentation occurred more rapidly, faster burial of organic matter resulted in complete depletion of sulfate and the accumulation of alkalinity, whereas above 15 mbsf the chemical profiles are dominated by diffusion from seawater. Thus, the 15-mbsf depth is a profound boundary at Site 1078, as will be seen in other profiles as well.

Dissolved ammonium increases monotonously to maximum values at 60-70 mbsf and remains constant from that depth to the bottom of Hole 1078C. These concentrations are very high compared with those documented at the Congo Basin. The clear change in behavior of the distribution of ammonium at 75 mbsf does not correspond to a horizon or depth interval of particular interest of which we are aware, and its cause at this point remains unknown.

The effect of the change in sedimentation rate at 15 mbsf is also observed in the profiles of elements that respond to carbonate dissolution and reprecipitation, such as Ca²⁺, Mg²⁺, and Sr²⁺ (Fig. 27). Through the 0-15 mbsf depth range, Ca²⁺ concentrations decrease by 6 mM, whereas Mg²⁺ concentrations decrease by 3 mM and Sr²⁺ by 26 µM. Because there is a greater molar decrease in Ca²⁺ than in Mg²⁺ and there is an associated decrease in Sr²⁺, the process of Ca²⁺ dissolution (release of Sr²⁺) and dolomite precipitation (decrease in Ca²⁺ and Mg²⁺) cannot be solely responsible, and an additional sink for both the Ca²⁺ and Sr²⁺ must exist. We postulate that this sink is finely disseminated diagenetic apatite. Small amounts of diagenetic apatite that would be undetectable by XRD could cause the simultaneous decrease in Ca²⁺ and Sr²⁺ while not affecting the phosphate profile (which will be dominated by the source provided by organic matter degradation, as discussed below). Regardless of which phase(s) is involved in the consumption of dissolved Ca²⁺ and Sr²⁺, its precipitation is essentially confined to the uppermost 15 mbsf.

Below this depth, Ca²⁺ concentrations remain essentially constant, Mg²⁺ concentrations decrease (perhaps recording uptake by clays), whereas Sr²⁺ stays essentially constant (with noise) after an increase from 10 to 40 mbsf.

Dissolved silica increases in concentration very rapidly through the uppermost 5-6 m of sediment (Fig. 28), recording the dissolution of biogenic opal. Below a minor maximum at ~40 mbsf, concentrations decrease slightly, most likely recording authigenic clay mineral formation. The concentrations of dissolved silica are lower than at Site 1077, reflecting the generally low abundances of diatoms in this sequence (see "Biostratigraphy and Sedimentation Rates" section, this chapter).

Dissolved phosphate increases very rapidly to a maximum value of ~150 µM within the uppermost 15 mbsf. This increase, which occurs through the same depth range as the increase in alkalinity and the consumption of sulfate is caused by degradation of organic matter. If authigenic apatite is forming through this depth interval, as hypothesized above, the consumption of dissolved phosphate by such authigenesis is much less than its production during organic destruction.

Concentrations of dissolved Na⁺ steadily increase with depth downcore (Fig. 29), most likely reflecting cation exchange reactions involved with authigenic clay formation. Compared with Site 1077 in the Congo Basin, however, the release of Na⁺ is much greater, which potentially reflects contrasts in detrital mineralogy. The profile of dissolved K⁺ decreases through the uppermost 15 mbsf, before increasing with depth. The reasons behind the K⁺ decrease in the uppermost 15 mbsf remain unknown. Although K⁺ is commonly associated with clay exchange reactions along with Na⁺, based on the shipboard data we are unable to interpret the K⁺ decrease occurring in the uppermost 15 mbsf within the context of the other reactions, discussed above.

The profiles of salinity and Cl^- also show marked gradients in the uppermost 15 mbsf (Fig. 30). Salinity values decrease, which most likely reflects the strong removal of sulfate over the same depth range, before increasing to a maximum value of 36 at 70 mbsf. Below 15 mbsf, the profile of salinity closely follows that of ammonium. Dissolved Cl^-, however, increases through the upper 15 mbsf before decreasing to minimum values at depth. It is unclear at this point whether the initial increase in Cl^- is caused by a glacial ice volume signal, as we hypothesize is the case for the distributions at Site 1075, or is related to diagenetic processes operating through this portion of the sedimentary succession.