INORGANIC GEOCHEMISTRY

Fifteen interstitial water samples were collected from Hole 1079A over a depth range from 1.4 to 120 mbsf (Table 8) at a typical frequency of one sample per core. On all profiles presented here, the chemical distributions at Site 1077, which are broadly representative of the sites drilled in the Congo Basin, and at Site 1078, the other Angola Basin site drilled during Leg 175, are provided for both inter- and intrabasinal comparison purposes. At Site 1079, the dominant factor affecting the interstitial water chemical profiles is the low level of organic carbon preserved in the sediments (see "Organic Geochemistry" section, this chapter). Also, evidence suggests a brine influence near the base of the hole, as shown in the chemical profiles of dissolved sulfate, Sr²⁺, Na⁺, and K⁺, as well as of salinity.

Downcore profiles of alkalinity, sulfate, and ammonium at Site 1079 (Fig. 16) are markedly different from those observed at the previous Leg 175 drill sites. Degradation of organic matter is the common process affecting alkalinity, ammonium, and sulfate at all the sites; at Site 1079 each of these components records relatively low rates of degradation. The alkalinity maximum is reached at ~110 mbsf, and the buildup of ammonium also is very slow, particularly in the upper portions of the section. Most significantly, the complete consumption of dissolved sulfate is achieved only at ~50 mbsf, which is the deepest stratigraphic position observed so far.

There are several possible causes for the contrast between these patterns and those observed at previous Leg 175 sites. The amount and/or nature of the organic matter can influence the rate of degradation of organic matter and hence the stratigraphic position of complete sulfate consumption; however, these aspects of the organic matter at Site 1079 are not significantly different from those at previous sites (see "Organic Geochemistry" section, this chapter). Sedimentation rate may also play an important role. In general, slow relative sedimentation rates allow the deeper diffusive resupply of sulfate from seawater, whereas in certain situations, relatively high sedimentation rates can act to preserve sulfate in interstitial waters provided the balance between sulfate addition by burial and sulfate depletion by organic degradation is in favor of the burial term. Because sedimentation rates at Site 1079 are, in fact, higher than those at all previous sites—except for Site 1078 (see "Biostratigraphy and Sedimentation Rates" section, this chapter)—we favor the latter option.

Concentrations of sulfate begin to increase in the deepest sections of the hole. As will be discussed later, this is evidence of a brine influence.

Within the uppermost 10 mbsf, concentrations of dissolved Mg²⁺, Ca²⁺, and Sr²⁺ (Fig. 17) are essentially constant (or perhaps show a slight increase). From 10 to 48 mbsf, all three components decrease in concentration. A paired decrease of Mg²⁺ and Ca²⁺ in this interval is consistent with dolomite precipitation; however, the decrease in dissolved Sr²⁺ concentration suggests that perhaps the precipitation of fluorapatite may also be occurring, as we hypothesize for Site 1078. Below ~50 mbsf, the downcore profiles of Ca²⁺, Mg²⁺, and Sr²⁺ exhibit a marked change. Concentrations of Ca²⁺ and Mg²⁺ remain essentially constant to the bottom of the hole, whereas the concentration of Sr²⁺ increases. Presumably, dissolution of biogenic carbonate is responsible for the increase of Sr²⁺, and Ca²⁺ does not show a similar increase because of formation of authigenic carbonates, or apatite, or both.

Dissolved silica increases in concentration very rapidly through the uppermost 4 mbsf of sediment (Fig. 18), recording the dissolution of biogenic silica. The concentration of silica continues to increase slightly at greater depths, but never reaches very high values. The concentrations of dissolved silica are closely similar to those at Site 1078 but are lower than those observed in the Congo Basin, reflecting the lower concentration of diatoms at this site (see "Biostratigraphy and Sedimentation Rates" section, this chapter).

Dissolved phosphate increases to a maximum value of ~250 µM within the uppermost 40 mbsf. The maximum occurs at greater depth here at Site 1079 than at the previous sites, which is consistent with the lower amount of organic matter available for degradation and the greater depth of sulfate consumption discussed previously.

Concentrations of dissolved Na⁺ steadily increase with depth downcore (Fig. 19), most likely reflecting cation exchange reactions involved with authigenic clay formation. Concentrations of dissolved K⁺ reach a minimum value at ~50 mbsf before increasing to maximum values at the bottom of the hole. There are at least two potential mechanisms causing these increases. First, as observed at Site 1078, the paired behavior of these elements is different than that observed in the Congo Basin sites, suggesting that differences in clay mineralogy between the basins may exist. Alternatively, as mentioned below, the distributions of both these elements are also suggestive of a deep evaporite brine source.

There are several characteristic chemical distributions in the deep interstitial waters recovered from Site 1079 that are suggestive of the influence of evaporite dissolution and formation or migration of brine. As mentioned above, the concentration of sulfate begins to increase again starting at 80 to 100 mbsf, where values increase above zero and provide a maximum of 7 mM at the bottom of the hole (Table 8). This increase in sulfate is consistent with an evaporite source. Along with this increase in sulfate, salinity and dissolved Cl^– also increase (Fig. 20). Other species also potentially sourced from a brine, including both Na⁺ and K⁺, also increase. Although we cannot definitively state that these increases are caused by evaporite dissolution, the data (particularly the sulfate) appear consistent with such a source.