INORGANIC GEOCHEMISTRY

Inorganic chemical analyses were conducted on 23 interstitial water samples squeezed from whole-round samples at a frequency of one per core in the first six cores and one every third core thereafter from Holes 1143A and 1143C. Analytical methods are detailed in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. The concentrations of dissolved interstitial constituents are presented in Table T13, and the profiles with depth are shown in Figure F20. Interstitial water profiles at Site 1143 are characteristic of sediments in which sulfate reduction and volcanic ash alteration are the primary reactions controlling the concentrations of dissolved constituents.

Chloride (Cl^-) concentrations in interstitial waters at Site 1143 are relatively constant, ranging from 549 to 563 mM (Fig. F20A; Table T13). A small increase in the Cl^- concentration occurs from 549 mM near the surface to 561 mM at 35.9 mbsf. Below this depth, the Cl^- concentration does not significantly change. Interstitial water salinities fluctuate between 33 and 35 (Fig. F20B; Table T13), with higher values in the uppermost section. The decrease in salinity observed from the top 0 to ~120 mbsf is probably a result of the removal of dissolved sulfate from interstitial waters during sulfate reduction.

The major changes reflected in the sulfate (SO₄^2-), ammonium (NH₄⁺), phosphate (HPO₄^2-), and alkalinity profiles from 0 to ~200 mbsf are interpreted to be caused by the diagenesis of organic matter via sulfate reduction. This process has led to the depletion of dissolved sulfate in interstitial waters by the sulfate reduction reaction (Emerson et al., 1980):

In this process, SO₄^2- is consumed, and alkalinity (i.e., HCO₃^-), NH₄⁺, and HPO₄^2- are byproducts.

Dissolved SO₄^2- concentrations decrease from seawater values at the top of the core to 7.3 mM at 200 mbsf but never reach zero for the remainder of hole, indicating that sulfate reduction is incomplete (Fig. F20C; Table T13). Incomplete sulfate reduction suggests that methanogenesis is not an important process in these sediments, which is in agreement with the low methane values measured in sediment (<10 ppmv) (see "Organic Geochemistry"). A deep zone of sulfate reduction also jibes with the relatively low organic matter contents at this site (see "Organic Geochemistry"). In addition, the HS^- produced during organic matter diagenesis is likely to react with iron to form "iron sulfide" minerals (e.g., "FeS" and "FeS₂"), a conclusion supported by the fact that no H₂S was detected at Site 1143 (see "Organic Geochemistry"). Disseminated pyrite as well as pyritized burrows were observed throughout much of the core (see "Lithostratigraphy").

As a result of sulfate reduction, dissolved HPO₄^2- concentrations show a distinct peak of 33.7 mM at 7.35 mbsf and then decrease rapidly to ~1.6 mM at 73.9 mbsf (Fig. F20D; Table T13). Sulfate reduction is also reflected in the NH₄⁺ profile (Fig. F20E; Table T13), which has an inverse relationship with sulfate. The NH₄⁺ concentration increases continuously in the sulfate-reduction zone to 1.4 mM and then is constant to the base of the hole.

Alkalinity increases from 4.3 mM at 1.45 mbsf (slightly higher than the 2.3 mM seawater value) to a maximum of 8.0 mM around 35.8 mbsf (Fig. F20F; Table T13), then declines slowly to a minimum of 7.3 mM around 102.3 mbsf before increasing again to reach 13.9 mM at the base of the Hole 1143A (398.3 mbsf). Increasing alkalinity in the uppermost interval may be attributed to HCO₃^- production accompanying sulfate reduction. The alkalinity increase below the zone of major sulfate reduction indicates some other source of alkalinity. However, pH remains in a narrow range (7.23-7.96) throughout the entire sedimentary column (Table T13).

Silica (H₄SiO₄) climbs sharply from surficial values of 476 mM to a maximum of 693 mM at 16.9 mbsf before steadily declining to a minimum of 220 mM at 102.3 mbsf (Fig. F20G; Table T13). Between 75 and 160 mbsf, H₄SiO₄ concentration values are low (<300 mM). Below this interval, H₄SiO₄ concentrations gradually increase to a maximum value of 1467 mM at the bottom of Hole 1143A. The two high H₄SiO₄ concentration intervals (0-75 mbsf; 160-400 mbsf) are associated with intervals of higher biogenic silica content (see "Lithostratigraphy") and may result from the high solubility of amorphous, biogenic silica (opal-A).

Dissolved potassium concentrations (K⁺) decrease downhole from ~12.4 mM near the surface to 9.5 mM at 102.4 mbsf, and the profile then has a small positive inflection in the interval at 100-200 mbsf (Fig. F20H; Table T13). Below this interval, the K⁺ increases to 13.3 mM at 321.7 mbsf and decreases to 11.7 mM at the bottom of Hole 1143A. The interval of low values in the K⁺ concentration is also characterized by low values of the H₄SiO₄ and corresponds to the interval in which large numbers of green clay layers are observed in the sediments (see "Lithostratigraphy"). The decrease of K⁺ in this interval may reflect uptake of K⁺ during clay mineral formation or alteration.

Magnesium concentrations (Mg²⁺) decrease linearly with depth from near-seawater values at the top (51.5 mM) to a minimum of ~23.7 mM at the bottom of Hole 1143A (Fig. F20I; Table T13). The profile of dissolved calcium concentration (Ca²⁺) decreases from near-seawater values at the surface (10.3 mM) to 5.5 mM at ~102.4 mbsf (Fig. F20J; Table T13). Below this level, Ca²⁺ increases continuously, reaching maximum values of 19.6 mM near the bottom of Hole 1143A. The Ca²⁺ minimum suggests that sulfate reduction and alkalinity production are promoting inorganic calcite precipitation in the upper 100 mbsf (Fig. F20I; Table T13).

The slope of the Ca²⁺ vs. Mg²⁺ relationship is positive and close to unity below 150 mbsf (Fig. F20K; Table T13), suggesting the loss of 1 mM of Mg²⁺ for every gain of 1 mM Ca²⁺. This exchange of cations suggests that alteration of volcanic materials may be driving the concentrations of these elements in the interstitial waters. Calcium increases and magnesium decreases downhole can reflect upward diffusion of the signal of chemical alteration in underlying oceanic crust, or in situ alteration of volcanic material to smectite in the sediment. Although we do not know the composition of the basement rock, a number of volcanic ashes are observed throughout the sediments at this site (see "Lithostratigraphy").

The upper part of the lithium (Li⁺) profile exhibits a minor increase in concentration from the top to 88 mM at ~100 mbsf (Fig. F20L; Table T13). Below this depth, the Li⁺ strongly increases, reaching the unusually high values of 3940 mM in interstitial waters near the bottom of Hole 1143A. The low values in Li⁺ concentrations at the top of Hole 1143A are probably a result of Li⁺ uptake during authigenic calcite precipitation. Dissolved strontium concentrations (Sr²⁺) increase from 93 mM near the top of Hole 1143A to 1367 mM at the bottom (Fig. F20M; Table T13). The increases in both Li⁺ and Sr²⁺ concentrations below 100 mbsf most likely reflect dissolution of biogenic silica and/or reactions involving dissolution of volcanic glass.

The alkalinity, K⁺, H₄SiO₄, Li⁺, and Ca²⁺ profiles all increase substantially below 200 mbsf (Fig. F20; Table T13). This interval corresponds to lithologic Subunit IB, which was recognized primarily based on its higher carbonate content (see "Lithostratigraphy"). This higher carbonate interval also corresponds to the major increase in density and decrease in porosity in the downhole logs (see "Wireline Logging") and in the physical properties measurements (see "Physical Properties"). These changes in lithology and physical properties may contribute to the interstitial water profiles by increasing the availability of carbonate for dissolution and recrystallization processes and by reducing the diffusional capacity of the sediments so that signals of sediment/water interactions are more localized within lithologic Subunit IB.