INORGANIC GEOCHEMISTRY

Thirty-seven interstitial water samples were collected from Hole 1167A and analyzed according to the procedures outlined in "Inorganic Geochemistry" in the "Explanatory Notes" chapter (Table T5). The sampling protocol required one 5- to 15-cm-long whole-round interval from each section of core to 60 mbsf, one whole-round interval per core to 100 mbsf, and one whole-round interval every two to three cores to the bottom of the hole. The shallowest sample was taken from 1.45 mbsf and the deepest from 432.65 mbsf, ensuring coverage of diagenetic processes throughout the complete cored section.

An ~2% increase occurs in both salinity and chlorinity near the seafloor between 10 and 50 mbsf (Fig. F26). Salinity increases downhole from 35.0 at the seafloor to 35.5 at ~22 mbsf and remains at 35.5 to 50 mbsf. Chlorinity increases from 560 to 572 mM over the same interval. It is likely that the interstitial waters above ~50 mbsf represent high-salinity last-glacial seawater, although in situ hydration of clay minerals cannot be discounted with the current data. Both salinity and chlorinity decrease in parallel from 100 to 325 mbsf (34.5-34.0 and 560-540 mM, respectively). Chlorinity then increases to 433 mbsf (541-552 mM), whereas salinity remains constant.

Sulfate parallels salinity and chlorinity profiles between 10 and 50 mbsf, increasing from 28.8 mM near the seafloor to 29.8 mM at ~19 mbsf. From 20 to 433 mbsf, sulfate decreases in a stepped profile to 24 mM at 433 mbsf. As sulfate is not exhausted as an organic matter oxidant, CO₂ reduction does not occur at Site 1167 (cf. Site 1165).

Ammonium increases linearly from near-surface values of 0 µM to a maximum of ~80-90 µM between 60 and 100 mbsf. Below 100 mbsf, ammonium decreases steadily to ~60 µM at 433 mbsf. These low ammonium concentrations (an order of magnitude less than ammonium at Site 1165) reflect limited sulfate reduction. Two additional products of sulfate reduction, alkalinity and phosphate, behave nonconservatively. Alkalinity decreases linearly from seafloor values of ~3 mM to 1.3 mM at ~40 mbsf and remains relatively constant from 100 mbsf to the base of the hole. Similarities in the downhole profiles of alkalinity, potassium, lithium, and magnesium and the inverse trend shown by calcium (see "Magnesium, Calcium, Strontium, and Sodium") suggest that alkalinity is responding to an unidentified diagenetic silicate mineral reaction. Seafloor concentrations of phosphate (6.0 µM) rapidly decrease to zero at 10 mbsf. Aside from a small excursion to 2.1 µM between 46 and 49 mbsf, phosphate concentrations remain below the detection limit to the base of the core.

Dissolved manganese decreases rapidly from 25 to 15 µM in the first 10 m of the core. Below 10 mbsf, manganese concentrations increase to 20 µM at 22 mbsf and then decrease to ~17 µM at ~30 mbsf, where they remain more or less constant to the base of the core. High manganese concentrations between 20 and 30 mbsf are the product of the reduction of manganese oxides during the oxidation of OC. Comparison of the phosphate and manganese profiles (Fig. F26) suggests that phosphate may be adsorbed onto the surface of manganese (or iron) hydroxides.

Dissolved magnesium concentrations decrease rapidly from 54.5 mM at the seafloor to ~40 mM at 50 mbsf. Between 50 and 433 mbsf, magnesium decreases from 40 to 30 mM, with a small change in gradient at ~100 mbsf. Dissolved calcium concentrations show an inverse trend, increasing rapidly from 11 mM at the seafloor to 24 mM at 50 mbsf. Between 50 and 433 mbsf, calcium concentrations increase from 24 to 28 mM. A small change in gradient is again apparent at ~100 mbsf. The strong negative linear correlation (Ca²⁺/Mg²⁺ = -1) observed throughout the core is likely due to silicate reconstitution reactions. The general downhole decrease in sodium concentration may reflect the albitization of plagioclase, with the associated release of calcium to the interstitial waters. Clay mineral reactions (e.g., smectite formation) may partially account for the magnesium decrease. Dissolved strontium concentrations increase downhole from ~100 µM at the seafloor to ~200 µM at 335 mbsf (Fig. F26). The strontium profile is consistent with a minor downhole increase in calcium carbonate reconstitution reactions.

Dissolved potassium decreases from ~12 to ~2 mM between the seafloor and 100 mbsf (Fig. F26), with a marked change in slope at ~50 mbsf. Below 100 mbsf, potassium concentrations remain at ~2 mM to 433 mbsf. Lithium decreases from ~30 µM at the seafloor to ~5 µM at ~40 mbsf. Below ~40 mbsf, lithium concentrations increase nonlinearly to ~10 µM at 335 mbsf. The profiles suggest that both potassium and lithium are participating in the same diagenetic reactions as calcium and magnesium. The overall low lithium concentrations argue against the alteration of volcanic material as a major contributing factor.

Dissolved silica concentrations are high from the seafloor to 5 mbsf (568-673 µM) because of the dissolution of siliceous microplankton in the near-surface sediments (see "Biostratigraphy and Sedimentation Rates"). Immediately below this zone, silica values return to ~200 µM, approximating the concentrations found in modern undersaturated bottom seawater. Silica concentrations increase gradually downhole to ~300 µM at 433 mbsf.