Interstitial water from 24 samples was collected at Site 1261: 22 from Hole 1261A (2.95–638.31 mbsf) and 2 from Hole 1261B (619.37–648.00 mbsf). The samples from both holes were taken to constitute a single depth profile. However, slight differences in lithology may cause minor breaks in concentration-depth gradients of some chemical parameters. Chemical constituents were determined according to the procedure outlined in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Alkalinity was not determined in five samples because of low yields of interstitial water. Results of the chemical analyses are presented in Table T17.
Pore water chemistry at Site 1261 is dominated by processes in two depth regimes: the rapidly sedimented Unit I (65 m/m.y.) (see "Sedimentation Rates") and the organic matter–rich black shale sequence of Santonian–Cenomanian age (lithostratigraphic Unit IV [~564–654 mbsf]). Unlike the previous sites drilled on Demerara Rise, the pore water sulfate profile at Site 1261 approaches zero at ~200 mbsf in the middle Pliocene section of Subunit IB (see Fig. F15F). At the base of Subunit IC and in Unit II, sulfate returns to slightly higher values (<1.5 mM) before zero concentrations are attained a second time in and below Subunit IIIA. These observations suggest that the organic matter–rich Unit IV provides a suitable substrate for ongoing microbial activity at depth and that sulfate reduction is occurring in the upper 200 m of the sedimentary column. Most of the hydrogen sulfide produced by sulfide reduction at Site 1261 must be trapped as iron sulfide because we detected no smell of H2S gas. The second decrease of residual interstitial sulfate may be related to methane diffusing upward from the black shales. The source for metabolic activity is possibly anaerobic methane oxidation (Borowski et al., 1999; Boetius et al., 2000) because higher alkalinities are found above the black shales (Fig. F15E).
The reducing character of the clay-rich and therefore Fe-rich Unit I sediments is also demonstrated by measurable quantities of dissolved Fe (Fig. F15O). Below 300 mbsf, dissolved Fe concentrations are below the detection limit. The same is true for Mn, where only trace quantities are detected in lithostratigraphic Units II–IV (Fig. F15P). Therefore, both redox-sensitive metals not only reflect the generally reducing character of the sediments but also the difference between the clay contents of Unit I and the remaining units. We interpret the low Mn content of Unit IV to indicate complete syndepositional removal under highly reducing conditions (see "Inorganic Geochemistry," in the "Site 1257" chapter; "Inorganic Geochemistry," in the "Site 1258" chapter; "Inorganic Geochemistry," in the "Site 1259" chapter; and "Inorganic Geochemistry," in the "Site 1260" chapter).
Sulfate depletion is accompanied by an increase in ammonium (Fig. F15G), a common respiration product of organic matter consumption. Ammonium concentrations are high throughout lithostratigraphic Units II–IV and decrease from ~100 mbsf in Subunit IB toward the sediment/seawater interface. The highest ammonium concentrations are attained in the black shales and imply that Unit IV is characterized by continuing microbial activity.
The complete absence of sulfate in two different depth intervals promotes another phenomenon typically seen in organic matter–rich sediments, the mobilization of Ba. Increases in Ba concentrations (Fig. F15H) are governed by barite solubility (Church and Wolgemuth, 1972) and are prone to even slight contamination by seawater sulfate. For this reason, we have marked one data point with a question mark (interval 207-1261B-11R-2, 147–157 cm) because we suspect some contamination with seawater sulfate from drilling fluid. Assuming that the sulfate content (1.4 mM) originates from a seawater admixture, we estimate the level of contamination to be ~5%. The highest dissolved Ba levels attained in Unit IV are >250 µM. The second maximum at ~250 mbsf is smaller in magnitude but closely follows near-zero sulfate levels in this depth interval. It is noteworthy that elevated Ba levels are still present in Subunit IIIB, where authigenic barite crystals of millimeter to centimeter scale are frequently observed (see "Lithostratigraphy"). Similar sedimentological and mineralogical relationships are reported elsewhere (Brumsack, 1986; Torres et al., 1996).
The downhole interstitial water concentration profile for Mg (Fig. F15K) shows an S-shaped trend: a pronounced decrease toward 200 mbsf (the depth interval where sulfate reaches zero values), increasing concentrations down to Subunit IIIB, followed by a decrease in Unit IV. The Mg profile resembles the K profile, which is also indicated by the good correlation between Mg/Cl and K/Cl ratios (Fig. F16). We assume that the Mg distribution is influenced more by clay mineral interaction than dolomitization processes, except for Unit IV, where dolomite formation cannot be excluded. From the sediment/water interface to the top of lithostratigraphic Unit IV, the downhole interstitial water concentration profile for Ca appears totally decoupled from Mg (Fig. F15I). Instead, the pore water Ca depth profile more closely resembles the Sr profile, particularly in the upper 250 mbsf (Subunits IA and IB) (Fig. F15J), and a good linear correlation of the two elements is shown in Figure F17. Between ~300 and 400 mbsf, Sr concentrations continue to increase downhole whereas Ca concentrations remain essentially constant. We interpret the decoupling of these two parameters in terms of carbonate precipitation supported by the low alkalinity values and poor preservation of nannofossils and foraminifers documented within and below Unit II (see "Biostratigraphy") (Fig. F15E). Chloride normalization shows essentially the same trend (Fig. F15S).
The Li profile (Fig. F15L) shows seawater concentrations down to 290 mbsf (the base of clay-rich Subunit IB). Below this depth, values dramatically increase to 300 µM in Unit III and remain essentially constant to the base of Unit IV. Li/Cl ratios show the same trend (Fig. F15U). We have no firm explanation for this dissolved Li profile or for those seen at the other Leg 207 sites. Speculative alternatives for Site 1261 include the following:
The B profile shows a slight decrease in clay-rich Unit I down to 370 mbsf (Fig. F15M). Below this depth interval, B concentrations increase in Unit II and show minimum values in Unit III, which is also characterized by elevated clay contents. Within the black shales (lithostratigraphic Unit IV), B concentrations attain maximum values. The B profile seems to reflect adsorption processes by clays (Brumsack and Zuleger, 1992). A relationship to biogenic Si may be deduced from the similar shape of the B profile.
Dissolved Si concentrations decrease with depth down to ~370 mbsf (Fig. F15N), reflecting the absence of biogenic Si in Unit I (see "Biostratigraphy"). A strong increase in Si is seen in the upper part of lithostratigraphic Unit II, whereas values are lower in Unit III. Whether this is because of opal-A/opal-CT transformation (Dixit et al., 2001) or the absence of biogenic Si remains unanswered. Generally, the Si profile reflects the abundance of siliceous tests and may be highly variable because of chert or silicate mineral formation.
A prominent feature of the interstitial water chemistry at Site 1261 that is similar to Sites 1257 and 1259 is the increase in Cl concentration with depth to >60% relative to standard seawater (Fig. F15A–F15C). This increase is paralleled by Na, but the Na/Cl ratio decreases toward Unit IV, from seawater values of 0.86 to below 0.82 (Fig. F15V). The maximum salt content is located at the base of Unit IV at ~648 mbsf. Unfortunately, we were unable to retrieve an interstitial water sample from below the black shale sequence, but based on our findings at Site 1257 we assume that Unit IV may act as an aquifer for the brine. We cannot exclude that a fraction of the Ca, Sr, and Li (Fig. F15S–F15U) is also associated with this brine because their element/Cl ratios are essentially constant in the lowermost samples.
In summary, the interstitial water profiles from this site primarily reflect ongoing organic matter diagenesis and microbial activity in the rapidly deposited lithostratigraphic Unit I and the black shales, carbonate diagenesis, and the dissolution of biogenic silica. A small influence of ash alteration cannot be excluded, but further shore-based isotopic studies are required to rule out this possibility. In contrast to the findings at Site 1257 and in agreement with Site 1259, we cannot completely rule out the existence of underlying, deeper-seated evaporite sequence, as suggested at Site 144 by Waterman et al. (1972).