We collected interstitial water from 30 samples at Site 1257: 12 from Hole 1257A (1.45–275.81 mbsf), 11 from Hole 1257B (52.75–225.30 mbsf), and 7 from Hole 1257C (157.50–233.87 mbsf). The samples from all three 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. Alkalinity was not determined in three samples because of low interstitial water yields.
Alkalinity, chloride, ammonium, and silica were determined by standard shipboard procedures (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter). The major ions Na, K, Mg, and Ca were analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) after 50-fold sample dilution with deionized water. The minor components Li, B, Si, Fe, Mn, and Sr were determined by ICP-AES from 10-fold diluted interstitial water samples. From the minor component dilution, we determined sulfate as total sulfur by ICP-AES. Details of the methods, including the emission lines used for analysis, are given in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Results of the chemical analyses are presented in Table T19 and in Figure F16.
Interstitial water chemistry at Site 1257 is dominated by the presence of Santonian–Cenomanian black shales and associated organic matter–rich sediments (lithostratigraphic Unit IV [~270–320 mbsf]). The downhole dissolved sulfate concentration profile at the site approaches zero at the top of Unit IV (see Fig. F16F), suggesting that this organic matter–rich unit provides a suitable substrate for ongoing microbial activity at depth. The sulfate gradient from the top of Unit IV to the sediment/water interface is almost linear, suggesting that sulfate reduction is of minor importance at shallower depths. We interpret this observation to reflect minimal accumulation of sediments younger than middle Eocene age (see Fig. F7). A faint smell of hydrogen sulfide was noticeable in lithostratigraphic Unit III (above the black shales and associated organic matter–rich sediments). It is possible that pyrite formation in these sediments is triggered by hydrogen sulfide diffusion upward from the sediments in Unit IV, which are most likely iron limited. Shore-based sulfur isotopic studies will help to test this hypothesis. The reducing character of the sedimentary column is also seen in slightly elevated manganese concentrations (Fig. F16P). Nevertheless, only very low concentrations (<1 µM) are attained in Unit IV. Presumably these sediments lost most of their manganese during or shortly after they were deposited, an interpretation that would indicate deposition under conditions of severe oxygen depletion. Alternatively, the low interstitial water manganese concentrations observed in Unit IV reflect the formation of manganese-rich carbonate phases (e.g., ankerite/rhodochrosite), which are common in organic matter–rich sediments. Shore-based chemical analysis of the interstitial water "squeeze cakes" will provide a definitive test of these two competing hypotheses.
Significantly, sulfate concentrations in lithostratigraphic Unit V (claystones and siltstones of Albian age) are slightly higher than those in the overlying black shale sequence, suggesting that Unit IV is the main "microbial bioreactor" influencing interstitial water sulfate, which is supplied from both above and below. Sulfate depletion is accompanied by increases in ammonium (Fig. F16G), a common respiration product of organic matter consumption. Ammonium concentrations peak in lithostratigraphic Unit IV and decrease almost linearly toward the sediment/seawater interface. In the very shallow subsurface, ammonium must be oxidized because concentrations are close to the detection limit in the two uppermost interstitial water samples taken. In lithostratigraphic Unit V (claystones and siltstones of Albian age), ammonium concentrations still remain at high levels, reflecting ongoing organic matter metabolization.
The complete absence of sulfate in Unit IV most probably promotes the occurrence of two other phenomena typically seen in organic matter–rich sediments: (1) mobilization of barium and (2) formation of dolomite. Increases in barium concentrations (Fig. F16H) are governed by barite solubility (Church and Wolgemuth, 1972) and are prone to even slight contamination by seawater sulfate. All cores from below ~60 mbsf were taken using RCB drilling technology, so minor seawater contamination cannot be completely excluded. However, sedimentological evidence points toward intense barium mobilization in the sedimentary sequence at Site 1257. Authigenic millimeter- to centimeter-scale barite crystals are frequently observed in the overlying Upper Cretaceous chalks (lithostratigraphic Unit III) (see "Lithostratigraphy"). Similar sedimentological and mineralogical relationships are reported elsewhere (Brumsack, 1986; Torres et al., 1996).
The downhole interstitial water concentration profile for magnesium is linear from the sediment/water interface to the middle of lithostratigraphic Unit IV, where concentrations reach their minimum before increasing to significantly higher values in the underlying Albian sediments (Fig. F16I, F16K) (~200 mbsf). In direct contrast, the downhole interstitial water calcium profile is nonlinear and calcium concentrations peak in the middle of Unit IV before decreasing to significantly lower values in the underlying Albian sediments (Fig. F16I). These observations indicate the existence of a significant sink for magnesium and source for calcium in the sulfate-depleted black shale interval, more likely formation of dolomite rather than alteration of volcanogenic ash.
The nonlinearity of the calcium profile from the top of Unit IV to the sediment/water interface indicates calcite precipitation in Paleogene sediments, as deduced from the associated consumption of alkalinity. This interpretation is consistent with reports of poor preservation of calcareous planktonic foraminifers and nannofossils of early Eocene age (see "Biostratigraphy"). The downhole interstitial water strontium profile at Site 1257 shows a near-linear increase from the sediment/water interface to high values (>1000 µM) that are maintained from the middle of lithostratigraphic Unit IV (~200 mbsf) to the deepest sample analyzed (~275 mbsf) (Fig. F16J). The shape of this strontium profile contrasts with the typical ooze–chalk transition "mid-depth maximum" (Baker et al., 1982) and suggests that carbonate diagenesis in the Paleogene strata, as inferred from the calcium and alkalinity profiles, is dominated by calcite precipitation rather than dissolution or in situ recrystallization. We interpret this observation to result from minimal accumulation of sediments younger than middle Eocene age (see Fig. F7). Similar findings have been reported from Blake Nose, where post-Eocene sedimentation is also absent (Rudnicki et al., 2001). Instead, carbonate dissolution and recrystallization reactions appear to be focused in sediments from the middle of Unit IV and possibly below. In fact, the relatively high strontium values and steep downhole gradients observed (>10 times seawater values and ~5 µM/m, respectively) suggest a sediment source (possibly aragonite-rich shallow-water carbonate sediments) may underlie the drilled section and continue to exert significant control on the Site 1257 interstitial water strontium profile. In this context, the pronounced negative linear relationship between magnesium and strontium is noteworthy (Fig. F17A). The maxima in lithium and boron (Fig. F16L, F16M) may be associated with this process, even though neither element is significantly enriched in carbonate sediments.
Dissolved silica concentrations increase with depth, reflecting the presence of biogenic silica (Fig. F16N). At greater depth, decreases in dissolved silica may also be associated with zeolite and, possibly, chert formation. Below the black shale unit, low silica concentrations may be attributed either to the absence of such material or to transformation of biogenic silica into opal-CT or even quartz (Gieskes, 1983; Dixit et al., 2001). The clay-rich unit below the black shales also serves as a sink for potassium (Fig. F16D), which is taken up by clay minerals during later diagenesis.
Another very prominent feature of the interstitial water chemistry at Site 1257 is the increase in chloride concentration with depth (Fig. F16A–F16C). This increase is paralleled by sodium, leading to an average Na/Cl ratio of 0.85, which is very close to the International Association for the Physical Sciences of the Oceans (IAPSO) seawater value of 0.86. It is important to note that the maximum in salt content is located at ~200 mbsf in lithostratigraphic Unit IV. The decrease in chloride below the black shale sequence indicates that the brine is not sourced from below the section drilled at Site 1257. Instead, the brine appears to be confined to Unit IV, which we thereby interpret to act as an aquifer for brine derived elsewhere. We cannot exclude the possibility that a fraction of the lithium and boron (Fig. F16L, F16M) is also associated with this brine. The very pronounced correlation of lithium and chloride (Fig. F17B) indicates a close association of these two parameters.
The interstitial water profiles from this site primarily reflect ongoing organic matter diagenesis in the black shales, carbonate diagenesis, and the dissolution of biogenic silica. In contrast to the previous interpretation of the existence of an underlying deep-seated evaporite sequence at Site 144 (Waterman et al., 1972), we believe that lithostratigraphic Unit IV may act as an aquifer for relatively high salinity fluids that are sourced elsewhere.