INORGANIC GEOCHEMISTRY

Inorganic chemical analyses were conducted on 12 interstitial water samples from Hole 1145A, squeezed from whole-round samples at a frequency of one per core in the first six cores and one every third core thereafter. Analytical methods are detailed in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. The concentrations of dissolved interstitial constituents are presented in Table T14, and the profiles with depth are shown in Figure F16. Interstitial water profiles at Site 1145 are characteristic of sediments in which sulfate reduction, alteration of volcanic material, and dissolution-recrystallization of biogenic minerals are the primary reactions controlling the concentrations of dissolved constituents.

Chloride (Cl^-) concentrations in interstitial waters are relatively constant, ranging from 553 to 561 mM (Fig. F16A; Table T14). A small increase in the Cl^- concentration occurs from 553 mM near the surface to 559 mM at 15.38 mbsf. Below this depth, Cl^- concentrations do not change significantly and are close to those of the seawater value (559 mM). Interstitial water salinities have a narrow range (32-34) (Fig. F16B; Table T14) and decrease slightly downhole.

The sulfate (SO₄^2-), alkalinity, ammonium (NH₄⁺), and phosphate (HPO₄^2-) profiles at Site 1145 show significant changes in the upper part of the sediment column (0 to ~100 mbsf), which are interpreted to be caused by the diagenesis of organic matter via sulfate reduction. Dissolved SO₄^2- concentrations decrease from 23.3 mM near the top of the core to 5.4 mM at 53.38 mbsf but never reach zero throughout the remainder of the hole. These SO₄^2- values indicate, as in Site 1143, that sulfate reduction is incomplete (Fig. F16C; Table T14) and that methanogenesis is a minor process in these sediments, which agrees with the low methane values detected in sediment (<14 ppmv) (see "Organic Geochemistry"). The removal of dissolved SO₄^2- ions from interstitial waters also causes the decrease in salinity observed from the top 0 to ~50 mbsf (Fig. F16B).

As expected, the interstitial water concentration of sulfate reduction products NH₄⁺ and HPO₄^2- increases in the sulfate-reduction zone to a broad maximum of 2.5 mM at 43.4 mbsf and 101.4 mM at 15.4 mbsf (Fig. F16D, F16E; Table T14). Below these maxima, both NH₄⁺ and HPO₄^2- concentrations decrease continuously to the base of the hole to the minima of 1.2 and 0.5 mM, respectively.

Alkalinity increases from 9.1 mM at 5 mbsf to a maximum of 21.3 mM at 34.4 mbsf and is nearly balanced stoichiometrically by a depletion of SO₄^2- over this depth range (Fig. F16F; Table T14). Below the sulfate reduction zone (0-50 mbsf), the alkalinity declines to a minimum of 5.1 mM at the base of the hole. The interstitial water pH remains in a narrow range (7.3-7.6) throughout the entire sedimentary column (Table T14).

Dissolved magnesium (Mg²⁺) and potassium (K⁺) concentrations decrease linearly with depth from near-seawater values at the top to a minimum of ~37.6 and 5.7 mM at the bottom of the hole, respectively (Fig. F16G, F16H; Table T14).

Dissolved calcium concentrations (Ca²⁺) decrease downhole from near-seawater values at the surface (10 mM) to 4.6 mM at ~43.9 mbsf (Fig. F16I; Table T14), then remain low and relatively constant between 50 and 100 mbsf. Below this interval, the Ca²⁺ increases to a maximum of 8 mM near the bottom of the hole. The decrease of Ca²⁺ in the upper 100 mbsf of the hole most likely indicates that sulfate reduction and alkalinity production, which has an inverse relationship with Ca²⁺, are promoting inorganic calcite precipitation. Below this level, Ca²⁺ increases, and both the Mg²⁺ and K⁺ decreases downhole are most likely a result of the alteration of basaltic volcanic material, which is observed throughout the sediment at this site (see "Lithostratigraphy").

In the upper part of the hole (0 to ~75 mbsf), dissolved strontium concentrations (Sr²⁺) do not vary significantly (Fig. F16L; Table T14). Below this interval, Sr²⁺ increases continuously, reaching the values of 272.1 mM near the bottom of the hole. This increase below 75 mbsf most likely reflects dissolution of biogenic silica and/or reactions involving alteration of volcanic glass.

Dissolved lithium concentration remains in a narrow range (21-37 mM), similar to that observed in Site 1144 (Fig. F16K; Table T14). In Site 1145, the upper part of the lithium (Li⁺) profile exhibits a significant increase in concentrations from 21.5 mM at the top to 37 mM at ~72.4 mbsf (Fig. F16K; Table T14). Below this level, the Li⁺ decreases to 27 mM at 127.9 mbsf, then increases again to reach 32.8 mM at the bottom of the hole. The low Li⁺ concentrations at the top of the hole are probably a result of an uptake of Li⁺ during authigenic calcite precipitation.

Dissolved silica (H₄SiO₄) increases slightly at the top of the hole to 846 mM at 24.9 mbsf and then is high and constant between 24.9 to 72.4 mbsf. Below this interval, H₄SiO₄ concentrations decrease abruptly and then vary within a lower range of values from 220-276 mM (Fig. F16L; Table T14). This shift was also observed at the higher accumulation rate Site 1144 at 388.5 mbsf (see "Inorganic Geochemistry" in the "Site 1144" chapter) and is synchronous (~1 Ma) in both sites.

In Sites 1144 and 1145, this shift coincides with the major lithologic change at this site, which is expressed best as an increase in the magnetic susceptibility record (see "Physical Properties"). This lithologic change is also characterized by a decrease of the biogenic silica (opal-A) content (see "Biostratigraphy"), which is likely responsible for the decrease of H₄SiO₄ and Li⁺ below 72.4 mbsf.