INORGANIC GEOCHEMISTRY

Sixteen interstitial water samples were collected from Hole 1166A and analyzed according to the procedures outlined in "Inorganic Geochemistry" in the "Explanatory Notes" chapter (Table T4). The sampling protocol called for the gathering of one 10-cm-long whole-round interval from each core to total depth. The shallowest sample was taken from 1.45 mbsf and the deepest from 296.7 mbsf, ensuring coverage of diagenetic processes throughout the complete cored section.

Downcore profiles of sulfate and ammonium typically reflect the degradation of organic matter. Sulfate values decrease linearly (R² = 0.9939) downhole from an initial seafloor value of 28 to 8 mM at ~150 mbsf; however, sulfate values do not reach zero at this site (Fig. F27). Below 150 mbsf, sulfate values increase slightly to 14.2 mM at 287.4 mbsf and decrease again to 11.6 mM at 296.7 mbsf. Because sulfate does not become exhausted at this site and because there is no discernible methane in the sediment, we can conclude that coring did not penetrate the carbon dioxide (methanic) reduction zone (cf. Site 1165). Concomitant with decreasing sulfate, ammonium values increase from 117 µM near the seafloor to a maximum value of 1277 µM at 296.7 mbsf (Fig. F27). At both 75 and 150 mbsf, the downcore ammonium profile changes, possibly reflecting differing abundances of organic matter. Typically, alkalinity follows a trend similar to ammonium; however, at this site alkalinity appears to be more a function of mineral transformation than of sulfate reduction processes (see "Silica, Alkalinity, Potassium, and Phosphate").

Concentration profiles of calcium, magnesium, and strontium (Fig. F28) typically reflect processes of carbonate dissolution and precipitation. Generally, in interstitial waters the magnesium concentration is inversely related to calcium concentration. This pattern is preserved above ~75 mbsf, as calcium increases from 12 mM near the seafloor to 21 mM at 75.9 mbsf and magnesium decreases from 51 to 33 mM over the same interval. Below ~75 mbsf, both calcium and magnesium concentrations decrease to their minimum values of ~22 and 15 mM, respectively, near the base of the hole. Strontium shows a near-linear increase from 94 µM immediately below the seafloor to 301 µM at 296.70 mbsf. Lithium shows clear highs of 26 and 68 µM at the sea floor and 287.40 mbsf, respectively, but is routinely <11 µM throughout the rest of the hole. Manganese reaches a high of 123.1 µM near the seafloor, then decreases to 15.4 µM at 38.64 mbsf before declining slowly to ~2 µM at 296.70 mbsf.

Downhole profiles of silica, alkalinity, potassium, and phosphate at Site 1166 decrease rapidly from relatively high concentrations near the seafloor to minimum values between 50 and 100 mbsf (Fig. F29). Silica concentration in the interstitial waters just below the mudline is 750 µM, which corresponds directly to a silica-rich interval in the sediment (see "Biostratigraphy and Sedimentation Rates"). Between the seafloor and 100 mbsf, silica values decrease to 170 µM. Between 100 and 150 mbsf, the silica concentration varies repeatedly from ~500 to 100 µM. From 150 to 300 mbsf, the silica concentration remains fairly constant (77-126 µM).

Similar trends were observed downcore for both alkalinity and potassium. Alkalinity values range between 3 and 4.5 mM near the seafloor and then decrease to 1.23 mM at 75 mbsf. Below 150 mbsf, alkalinity remains relatively constant (2.2-2.7 mM) to 300 mbsf. Similarly, the maximum concentration of potassium (12.72 mM) occurs near the seafloor (1.5 mbsf) and decreases to ~2 mM between 75 and 85 mbsf. Below 150 mbsf, the potassium concentration remains fairly constant at 5 mM. Phosphate concentrations peak at 17 µM near the seafloor. Below 40 mbsf, the phosphate concentration remains below detection limits, except for a single analysis at 116.2 mbsf that reaches 3 µM.

It is likely that clay mineral reactions are responsible for the downhole silica, potassium, alkalinity, and phosphate profiles.

Concentrations of sodium and chloride (Fig. F30) approximate modern seawater values near the seafloor in Hole 1166A. By 2.74 mbsf, sodium and chloride values have risen sharply to 493 and 566 mM, respectively. Concentrations remain consistently high to ~75 mbsf. Between 95 and 137 mbsf, sodium increases slightly and chloride shows a small decrease in concentration. Salinity at Site 1166 steadily decreases from 35.5 to 32.5 at the base of the hole.