INORGANIC GEOCHEMISTRY

We collected interstitial water from 32 samples at Site 1259, 28 from Hole 1259A (2.55–488.33 mbsf) and 4 from Hole 1259C (525.65–549.22 mbsf). One sample taken from Hole 1259B (interval 207-1259B-25R-1, 89.0–100.0 cm) is not considered in the following discussion, as interstitial water could not be retrieved. 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 three samples because of low yields of interstitial water. Results of the chemical analyses are presented in Table T19.

Black Shales as a Diagenesis Bioreactor

Interstitial water chemistry at Site 1259 is dominated by the black shales and associated organic matter–rich sediments of Santonian–Cenomanian age (lithostratigraphic Unit IV [~493–549 mbsf]). The downhole pore water concentration profile of sulfate at Site 1259 approaches zero slightly above the top of Unit IV (see Fig. F24F), suggesting that this organic matter–rich unit provides a suitable substrate for ongoing microbial activity at depth. The pore water sulfate gradient from the base of Unit III to the sediment/water interface is essentially linear, suggesting that sulfate reduction is of minor importance at shallower depth intervals. We assume that methane diffusing upward from the black shales may provide the source for metabolic activity, possibly anaerobic methane oxidation (Borowski et al., 1999; Boetius et al., 2000). In contrast to the situation at nearby Site 1257, the smell of hydrogen sulfide was not noticeable within or above the black shales. It is possible that the intense pyritization observed in Unit III sediments is triggered by hydrogen sulfide production and immediate trapping in this zone. In particular, the Paleocene section (Subunit IIIA) is clay rich and therefore iron rich. This assumption is further supported by high dissolved iron concentrations further upward (Fig. F24O), with maximum values >100 ÁM in the chalky Subunits IIA–IIC. Even though chalks are generally iron poor, the absence of trace quantities of hydrogen sulfide may explain the high values.

The reducing character of the sedimentary column is also seen in slightly elevated Mn concentrations (Fig. F24P). Nevertheless, only very low concentrations (<1 ÁM) are attained in Unit IV. Presumably these sediments lost most of their Mn during or shortly after they were deposited, an interpretation that would indicate deposition under conditions of severe oxygen depletion. Alternatively, the low interstitial water Mn concentrations observed in Unit IV reflect the formation of Mn-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.

Sulfate depletion is accompanied by increases in ammonium (Fig. F24G), a common respiration product of organic matter consumption. Ammonium concentrations peak in lithostratigraphic Unit IV and decrease linearly toward close to 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 pore water samples taken. The close relation between sulfate depletion and ammonium buildup is illustrated by the linear inverse correlation shown in Figure F25.

The complete absence of sulfate in Unit IV most probably promotes two other phenomena typically seen in organic matter–rich sediments: (1) mobilization of Ba and (2) formation of dolomite. Increases in Ba concentration (Fig. F24H) are governed by barite solubility (Church and Wolgemuth, 1972) and are prone to even slight contamination by seawater sulfate. The highest dissolved Ba levels attained in Unit IV are >300 ÁM and exhibit a smooth profile. For this reason, we assume that seawater contamination is minimal at this site. It seems noteworthy to mention 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 shows three different trends: an almost linear decrease from the sediment/water interface toward the P/E boundary (Hole 1259A [367 mbsf]) (see "Lithostratigraphy"), constant or slightly increasing values in the clay-rich Unit III, and decreasing values from the center of Unit IV to the base of the black shales (Fig. F24K). It seems surprising that clay-rich Unit III particularly shows no sign of Mg removal with depth, even though clays may represent an important Mg sink. Because salinity increases are evident at Site 1259 (see "Lithostratigraphic Unit IV as an Aquifer for Brines"), we have normalized Mg values to chloride. The downcore profile of the Mg/Cl ratio (Fig. F24Q) is quite linear, suggesting Mg removal throughout the sedimentary column. The same is essentially true for K, an element that is incorporated into clay minerals during burial diagenesis. The K profile (Fig. F24D) exhibits a stepwise decrease downcore and, again, rather constant values in Unit III. When normalized to Cl, a smooth downcore decrease is evident (Fig. F24R). The pronounced positive correlation between the Mg/Cl and K/Cl ratios (Fig. F26) suggests a common removal process, most likely incorporation into clay minerals.

In direct contrast, the downhole pore water concentration profile for Ca is nonlinear and shows a clear break at ~300 mbsf, close to the P/E boundary (Fig. F24I). Below this interval, an almost linear Ca increase toward the base of Unit IV is evident. Also, a clear relationship between Ca and alkalinity is not visible immediately. Alkalinity buildup is noticeable downcore to ~300 mbsf (Fig. F24E). Below this depth, significantly lower values are observed, most likely resulting from carbonate precipitation. Again, Cl normalization shows a much clearer picture (Fig. F24S), with increasing Ca/Cl ratios in Unit I to Subunit IIB, decreasing values in Subunit IIC, and essentially constant values down to the base of Unit IV. The decrease in Ca/Cl ratio parallels the drop in alkalinity, indicating carbonate precipitation. The poor preservation of nannofossils and foraminifers in this interval supports this assumption (see "Biostratigraphy").

The downhole pore water Sr profile at Site 1259 shows a steady increase from the sediment/water interface to increasingly high values (>1600 ÁM) in the deepest sample analyzed (~549 mbsf) (Fig. F24J). The shape of this pore water Sr profile contrasts with the typical ooze–chalk transition "middepth maximum" (Baker at el., 1982) and suggests that carbonate diagenesis in the Paleogene strata as inferred from the Ca and alkalinity profiles is dominated by calcite precipitation rather than dissolution or in situ "recrystallization." Even when normalized to Cl, Sr shows an almost linear increase downcore (Fig. F24T), suggesting a source (possibly aragonite-rich shallow-water carbonate sediments) underlying the section drilled.

Li concentrations (Fig. F24L) increase steadily down to 400 mbsf. Below this depth interval, a linear increase exists. Li/Cl ratios, by contrast, are almost constant below 300 mbsf (Fig. F24U), suggesting a source in the overlying sedimentary column. There is a striking linear correlation between Ca/Cl and Li/Cl ratios in the upper 270 m of the sedimentary column (Fig. F27), suggesting a common source. Whether carbonate recrystallization or ash alteration is dominating cannot be ruled out without additional isotopic investigations.

The B profile shows slight increases down to 300 mbsf (Fig. F24M), possibly resulting from B desorption processes (Brumsack and Zuleger, 1992). Below this interval, B adsorption onto clay minerals is indicated.

Increases in dissolved Si concentrations with depth to ~300 mbsf reflect biogenic Si (Fig. F24N), particularly in Subunit IIB. At greater depth, decreases in dissolved Si may also be associated with chert or silicate mineral formation. The depth profiles of B and Si concentrations are strikingly similar, suggesting clay mineralogy may influence both chemical parameters. Below the black shale unit, low Si concentrations may be attributed either to the absence of clays or to transformation of biogenic Si into opal-CT or even quartz (Dixit et al., 2001).

Lithostratigraphic Unit IV as an Aquifer for Brines

Another prominent feature of interstitial water chemistry is an increase in Cl concentration with depth of >60% relative to the International Association for the Physical Sciences of the Ocean (IAPSO) value (Fig. F24A–F24C). This increase is paralleled by Na, but the Na/Cl ratio decreases toward Unit IV, from seawater values of 0.86 to below 0.80 (Fig. F24V). It is noteworthy to mention that the maximum in salt content is located at the base of Unit IV at ~549 mbsf. Unfortunately, we were unable to retrieve an interstitial water sample from below the black shales, but we assume that this unit may act as an aquifer for the brine. We cannot exclude that a fraction of the K, Ca, Sr, Mg, and Li (Fig. F24D, F24I–F24L) is also associated with this brine (Figs. F26, F27).

In summary, the interstitial water profiles from this site primarily reflect ongoing organic matter diagenesis in the black shales, carbonate diagenesis, the dissolution of biogenic Si, and, possibly, a minimal influence of ash alteration. In contrast to the findings at Site 1257, we cannot completely rule out the existence of underlying deeper-seated evaporites, as suggested for Site 144 by Waterman et al. (1972).

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