We collected interstitial water from 38 samples at Site 1258: 24 from Hole 1258A (1.45–361.53 mbsf), 12 from Hole 1258B (133.80–448.27 mbsf), and 2 from Hole 1258C (462.52–483.55 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. An incomplete data set exists for Sample 207-1258A-37R-2, 110–120 cm, and no data exist for Sample 207-1258B-43R-2, 110–120 cm, because of low and zero interstitial water yields, respectively (Table T20).

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 from total sulfur by ICP-AES. Calibration and quality control were performed using appropriate calibration solutions and spiked International Association for the Physical Sciences of the Ocean (IAPSO) seawater standards. 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 T20 and Figure F20.

Black Shales as a Diagenesis Bioreactor

Interstitial water chemistry at Site 1258 is dominated by the presence of black shales and associated organic matter–rich sediments. These sediments are of Turonian–Albian age (lithostratigraphic Unit IV [~390–450 mbsf]). Sulfate concentrations approach zero at the top of lithostratigraphic Unit IV (see Fig. F20F), and the gradient from the top of Unit IV to the sediment/water interface is almost linear. These observations suggest the following:

  1. Almost 100 m.y. after Unit IV was deposited, the organic matter–rich sediments continue to provide a suitable substrate for ongoing microbial activity.
  2. Sulfate reduction is of minor importance at shallower depth intervals.
  3. The resulting downhole profile is controlled by the existence of one major stratigraphic sink (Unit IV) and simple compensatory downward diffusion from the sediment/seawater interface.

We interpret this situation to reflect minimal accumulation of sediments younger than middle Eocene at Site 1258 (Fig. F10). Unfortunately, the two samples that we collected from the Albian phosphatic calcareous claystones at the base of Hole 1258C (lithostratigraphic Unit V) do not lie far enough below Unit IV to determine whether the black shale sequence is also supplied by the diffusion of sulfate from below (Fig. F20F) as is the case at Site 1257.

At Site 1258, we detected no smell of hydrogen sulfide during core-splitting procedures on the catwalk. Occasionally, however, an H2S smell was detected from interstitial water whole rounds taken from lithostratigraphic Unit III during the routine scraping undertaken to remove contaminated material prior to squeezing. It is possible that pyrite formation in the Unit III sediments is triggered by hydrogen sulfide diffusion upward from the Unit IV sediments, which are most likely iron limited. Shore-based sulfur isotopic studies will help to test this hypothesis.

The reducing character of the sedimentary column also is seen in remarkably well-constrained profiles showing elevated manganese and iron concentrations (Fig. F20O, F20P). Peak concentrations of ~8 然 manganese and 50 然 iron are attained at ~60–80 and 110 mbsf, respectively. These results represent a somewhat expanded version of the classic interstitial water chemically reductive sequence of Froelich et al. (1979) and are consistent with the dominant role played by the existence of an old deep-seated bioreactor (Unit IV). In fact, the interstitial water iron profile shows a second, smaller peak (~15 然) at ~300 mbsf. We note the broad association between the two interstitial water iron peaks and the two stratigraphic intervals at the site with sediments having a distinct red coloration (see "Lithostratigraphy").

In contrast to lithostratigraphic Units II and III, only very low interstitial water concentrations of the redox-sensitive metals are attained in lithostratigraphic Unit IV (Mn = <1 然 and Fe = 5 然) with the exception of one questionable data point in our iron profile (Fig. F20O). The same is true of Unit V. A similar association between organic matter–rich sediments and low interstitial water concentrations of manganese and iron was observed at Site 1257. Our favored working hypothesis for this observation is that these redox-sensitive metals were completely remobilized during or shortly after the host organic matter–rich units were deposited, implying conditions of severe synsedimentary oxygen depletion. Alternatively, the low interstitial water manganese concentrations observed in Units IV and V reflect the formation of manganese-rich carbonate phases (e.g., ankerite/rhodochrosite). Shore-based chemical analysis of the interstitial water "squeeze cakes" will provide a definitive test of these two competing hypotheses.

As expected, sulfate depletion in interstitial water samples is accompanied by increases in ammonium (Fig. F20G), consistent with organic matter consumption. As at Site 1257, ammonium concentrations peak in lithostratigraphic Unit IV and decrease almost linearly toward the sediment/seawater interface. Significantly, ammonium concentrations in Unit V are slightly lower than those in the overlying black shale sequence, supporting our interpretation that Unit IV is the main "microbial bioreactor" influencing interstitial water chemistry at the site.

The complete absence of sulfate in Unit IV at Site 1258 most probably promotes the same two phenomena inferred at other Leg 207 sites: (1) mobilization of barium and (2) formation of dolomite.

Increases in barium concentration (Fig. F20H) are governed by barite solubility (Church and Wolgemuth, 1972) and are prone to even slight contamination by seawater sulfate. All cores at Site 1258 were taken using RCB drilling technology, so minor seawater contamination cannot be excluded. However, the convincing form of the downhole interstitial water barium profile, together with extensive sedimentological evidence, indicates intense barium mobilization from near the base of the black shale sequence at Site 1258. Authigenic barite crystals of millimeter to centimeter scale 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 magnesium profile is nearly linear from the sediment/water interface to the basal sediments at Site 1258. In contrast, the downhole interstitial water calcium profile is nonlinear. Calcium concentrations peak at ~2 times seawater values at ~275–300 mbsf and, despite decreasing to lower values deeper in the section, remain comparatively high (~1.5 times seawater) downhole to the basal sediments at the site (Fig. F20I). These observations indicate the existence of a significant sink for magnesium and a source for calcium in the sulfate-depleted black shale interval, most probably dolomite formation.

The nonlinearity of the calcium profile from the top of Unit IV to the sediment/water interface indicates carbonate diagenesis in sediments of Late Cretaceous and Paleogene age, as deduced from associated trends in alkalinity (Fig. F20E). The alkalinity and calcium concentration depth profiles show particularly pronounced structure in lithostratigraphic Subunits IIB and IIC and Unit III, where two pronounced maxima are seen in both calcium and alkalinity. The first maxima is between ~260 and 310 mbsf and the second at ~400 mbsf, corresponding to the lower Paleocene and Maastrichtian chalks (Subunits IIB and IIC) and the top of the black shale sequence (Unit IV), respectively. Interstitial water calcium and alkalinity concentrations are significantly lower in the intervening claystones of Unit III. This general covariation between these two parameters and sediment CaCO3 content (see Fig. F4) implies that these interstitial water gradients are locally controlled by ongoing carbonate dissolution and reprecipitation reactions or recrystallization. Our interpretation of these nonlinear depth gradients in terms of carbonate diagenetic reactions is consistent with reports of particularly poor preservation of calcareous microfossils in strata of Maastrichtian age (see "Lithostratigraphy" and "Biostratigraphy"). On the other hand, the downhole interstitial water strontium and ammonium profiles, which are widely thought to be sensitive proxies for recrystallization of biogenic carbonate and organogenic bicarbonate (e.g., Baker et al., 1982), respectively, show minimal local control. Instead, we see a near-linear downhole increase in both parameters, indicating sources at depth and simple diffusion to the sediment/water interface (Fig. F20J). One interpretation of the decoupled behavior between these parameters (calcium and alkalinity vs. strontium and ammonium) is that carbonate diagenesis in the Upper Cretaceous chalks (Unit III) is dominated by calcite precipitation fed by chemical diffusion from below rather than dissolution or in situ recrystallization. In fact, the form of the strontium interstitial water depth profile (Fig. F20J) suggests that the main locus of carbonate recrystallization most likely lies below the strata that we drilled at Site 1258. We interpret the simple diffusion-dominated strontium and ammonium profiles to reflect minimal accumulation of sediments younger than middle Eocene age at Site 1258 (see Fig. F10). Similar findings have been reported from Blake Nose, where Eocene-age sediments crop out at the seafloor (Rudnicki et al., 2001). Maximum interstitial water strontium concentrations and average linear depth gradients are noticeably lower at Site 1258 than at Site 1257 (~5 times vs. ~10 times seawater and ~1 然/m vs. ~5 然/m, respectively) and modest relative to many Deep Sea Drilling Project/ODP sites (Rudnicki et al., 2001). These observations suggest that Site 1258 pore fluids are not in communication with a once aragonite-rich source as hypothesized at Site 1257 (see "Inorganic Geochemistry" in the "Site 1257" chapter).

Dissolved silica concentrations at Site 1258 are significantly higher in sediments of Eocene age (~600–800 然 [~20–200 mbsf]) than in those of Paleocene–Cenomanian age (~400–600 然 [~200–450 mbsf]) and decrease sharply to even lower values from the middle of the black shale sequence into the basal Albian claystones (~200 然) (Fig. F20N). These patterns undoubtedly track the abundance of biogenic silica and its transformation to opal-CT and possibly chert, but alteration of volcanic ash may also play a role (see "Lithostratigraphy" and "Biostratigraphy"). As is the case at Site 1257, the clay-rich Albian sediments below the black shale sequence appear to serve as a sink for potassium (Fig. F20D).

Low Salinity/Chlorinity Anomalies

Arguably the most remarkable feature of the interstitial water chemistry at Site 1258 is the presence of low salinity and chloride concentration anomalies between ~300 mbsf and the base of the site (Fig. F20A–F20C). These anomalies are in contrast to our findings at Sites 1257, 1259, and 1261, where significant increases in salinity and chloride concentration indicate the presence of a brine in the black shale sequence of lithostratigraphic Unit IV. The most pronounced of the low chloride concentration anomalies at Site 1258 are paralleled by sodium, and the average Na/Cl ratio at the Site 1258 is 0.85, which is very close to the IAPSO seawater value of 0.86. Minima in salinity and Cl concentration (26 psu and 465 mM, respectively) are attained in the deepest sample analyzed (interval 207-1258C-34R-2, 135–150 cm [483.55 mbsf]), and from the latter we calculate a 17% freshening relative to the sample taken from the shallowest depth at the site. Low-chlorinity anomalies such as those seen in Figure F20 are not easy to interpret with confidence on the basis of shipboard data alone. The presence of significant concentrations of methane in headspace gas analyses (>50,000 ppmv) (see "Interstitial Gas Contents" in "Organic Geochemistry") are consistent with the anomalies that were caused by dissociation of gas hydrates. Unfortunately, estimates of the geothermal gradient for the outer Demerara Rise are not available to us. If we use the value estimated for Ceara Rise (47蚓/km) (Curry, Shackleton, Richter, et al., 1995) and make the assumptions that all of any hydrate-stored gas is methane and the salinity is 35 psu, then using the hydrate program CSMHYD (Sloan, 1998), we estimate that Cretaceous strata at Site 1258 are near the base of the gas hydrate stability zone.

Alternative explanations for the salinity and chloride anomalies are clay dehydration reactions and dilution by meteoric water. The former possibility seems unlikely given the lithology at Site 1258, but the latter possibility cannot be excluded even though the nearest landmass is located nearly 400 km away. Lithium interstitial water concentrations show significant elevation at Site 1258 (Fig. F20L), suggesting that the association between high lithium concentrations and the brines of the black shale sequence at Site 1257 may be coincidental, but the cause of these significant lithium anomalies remains enigmatic.

In summary, the interstitial water chemistry profiles from Site 1258 primarily reflect ongoing organic matter diagenesis in the black shales, carbonate diagenesis, and the dissolution of biogenic silica. In sharp contrast to our findings at Sites 1257, 1259, and 1261, where lithostratigraphic Unit IV appears to act as an aquifer for fluids of relatively high salinity, we observe pronounced low salinity and chlorinity anomalies at Site 1258. We hypothesize these anomalies to be caused by either gas hydrate dissociation or dilution by meteoric water.