INORGANIC GEOCHEMISTRY

Shipboard interstitial water (IW) analyses were performed on 15 of the 27 whole-round samples taken from Hole 1169A. The balance of the samples was archived for shore-based investigations. The whole-round samples were taken at the frequency of three per core in the upper ~70 mbsf, one per core from 70-100 mbsf, and one every third core to total depth. All results on IW geochemistry are reported in Table T13 and Figures F10, F11, F12, F13, and F14. Although the sediment cores were heavily disturbed during drilling, the geochemical results on IW exhibit patterns that are comparable to those obtained at Site 1168.

Chloride, Sodium, and Salinity

The conservative parameters salinity (not shown in figure), chloride (Cl-), and sodium (Na+) exhibit very little change within the upper 250 mbsf (Fig. F10). Salinity decreases downward to 34 from the near-seafloor concentration of ~35; maximum dilution relative to seawater is only 3%. Chloride varies from 556 to 564 mM and exhibits a maximum at ~25-50 mbsf. Sodium ranges between 470 and 488 mM and remains conservative with respect to Cl-. Within the analytical precision of shipboard instrumentation, the Na+ concentrations show no change with depth from the normal seawater sodium concentration of 480 mM.

The ~1.4% increase in chloride within the upper ~25-50 mbsf relative to the uppermost sample, which was also observed in Hole 1168, may be attributed to a salinity increase during the last glacial maximum as proposed by McDuff (1985) and Schrag et al. (1996).

Sulfate, pH, and Alkalinity

Titration alkalinity values from the uppermost pore waters are ~4.0 mM and slightly increase to a maximum of 6.8 mM at ~200 mbsf (Fig. F11). The pH decreases from ~7.5 at the top to <7.3 at 30 mbsf and subsequently increases to 7.5 between 40-100 mbsf (Fig. F11). From that depth downward, the pH continuously decreases to <7.4. Sulfate concentrations decrease from near-seawater values (26-27 mM) at the top of the hole to a minimum of ~18 mM at ~200 mbsf (~33% decrease), thus never being completely removed (Fig. F11).

The downcore change in sulfate and alkalinity is most likely caused by organic matter remineralization. Interstitial water SO42- concentrations at Site 1169 do not exhibit the same amount of depletion through comparable depths at Site 1168; the degree of sulfate reduction is half that observed at Site 1168, although TOC concentrations have comparable concentrations. The downcore change in alkalinity is similar at both holes, although values are generally lower by ~1-2 mM at Site 1169.

Strontium, Calcium, and Lithium

Strontium (Sr2+) concentrations gradually increase with depth from ~141 然 near the seafloor to ~700 然 at 200 mbsf (Fig. F12). A Sr2+ concentration decline to ~600 然 was observed from 200 mbsf to the base of the hole. Calcium (Ca2+) concentrations remain near seawater values for the upper ~100 mbsf of the hole (Fig. F12). A distinct maximum in Ca2+ concentrations (12.8 mM) exists at ~200 mbsf. Lithium (Li+) concentrations remain relatively constant with depth, varying between 8 and 17 然 in the uppermost ~170 mbsf (Fig. F12). Below, Li+ concentrations steadily increase, reaching 97 然 at ~220 mbsf.

The pronounced maximum in Sr2+ concentrations at ~200 mbsf within the calcareous lithostratigraphic Unit I compares to the Sr2+ maximum at comparable depths in Hole 1168A, although absolute concentrations are higher in Hole 1168A. Recrystallization of biogenic calcite to diagenetic low-Mg calcite and dolomite and/or dissolution of biogenic calcite causes a release of dissolved strontium into pore waters (Manheim and Sayles, 1974; Baker et al., 1982). Dissolution and precipitation reactions were shown to control the Ca2+ profile at Site 1168. The increase in Li+ in the same interval may also be attributed to alteration of biogenic carbonate, although the abundance of Li+ in biogenic carbonates may not be sufficient to account for all the Li+ observed here. Alternatively, the early diagenesis of biogenic opal-A and ion-exchange reactions involving clay minerals may influence dissolved Li+ concentrations (Gieskes, 1983; DeCarlo, 1992).

Magnesium and Potassium

The downcore magnesium (Mg2+) and potassium (K+) profiles exhibit gradually decreasing concentrations (Fig. F13). Mg2+ concentrations are ~54 mM at the top of the hole and decrease by ~20% to ~44 mM at the base of Hole 1169A. K+ concentrations decrease by ~10% in the hole. The change in concentration of both elements is highly correlated (r = 0.81) and suggests the involvement of similar processes (e.g., basement alteration, ion-exchange reactions associated with clay minerals; see Gieskes, 1983; De Carlo, 1992).

Silica

Dissolved silica concentrations (H4SiO40) range from ~600 to 950 然 and exhibit a near-continuous increasing downward profile (Fig. F14). The highest concentrations (>900 然) are between ~90 and 190 mbsf. Below this, silica concentrations decrease to ~800 然.

Although dissolved silica concentrations in Hole 1169A are consistently higher by ~20%-30% than those observed in Hole 1168A, the downcore pattern of dissolved silica is similar for the uppermost sediment sections in both cores. In Hole 1168A most silica dissolution occurs in sediments rich in biogenic silica and IW-silica concentrations are a reflection of the opal content of the sediments. In Hole 1169A, we expect a similar relationship between dissolved silica and biogenic opal. The suspected opal increase in Hole 1169A matches the observation that carbonate concentrations in Hole 1169A are slightly lower in lithostratigraphic Subunits IA and IB compared to Hole 1168A.

Geochemical Zonation

The pore-water profiles allow us to differentiate the cored interval into two geochemical zones. Lithostratigraphic Subunit IA is divided into an upper Zone 1 covering the upper ~100 mbsf. This zone contains no gradient in Ca2+ and K+ concentrations, whereas dissolved silica is continuously decreasing. Zone 2, from ~100 to ~170 mbsf (the boundary between lithostratigraphic Subunits IA and IB), instead shows sharply increasing gradients in Ca2+ and K+ concentrations, whereas dissolved silica reaches a steady state.

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