INORGANIC GEOCHEMISTRY

Interstitial water samples were squeezed from 93 whole-round samples at a frequency of two per core in Cores 184-1144A-1H to 20H and 26X to 48X. Additional samples were taken at a frequency of one per core from Cores 184-1144B-22X through 26X to fill a sampling gap in Hole 1144A. Inorganic chemical analyses were conducted on a subset of these interstitial water samples. Salinity was measured in every sample. Alkalinity and pH were measured once per core. The concentrations of Ca²⁺, Mg²⁺, K⁺, Na⁺, and SO₄^2- were measured at a frequency of one per core, except in the sampling gap at Hole 1144A. All other analyses were made one per core for the first nine cores and every third core thereafter. In addition, chloride was measured two per core in Cores 184-1144A-39X through 47X to look for evidence of gas hydrate decomposition. 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 Fig F23. Interstitial water profiles at Site 1144 are characteristic of sediments in which sulfate reduction, volcanic ash alteration, and dissolution-recrystallization of biogenic minerals are the primary reactions controlling the concentrations of dissolved constituents.

Chloride (Cl^-) concentrations in interstitial waters at Site 1144 are relatively constant, ranging from 549 to 561 mM (Fig. F23A; Table T14). Interstitial water salinities decrease downhole from 35 near the top to 31 near the base of the hole (Fig. F23B; Table T14). The decrease in salinity from 0 to 75 mcd is probably related to the removal of dissolved sulfate from interstitial waters during sulfate reduction. The major goal of the high-density interstitial water sampling at this site was to look for low salinity and chlorinity values, as well as dilution of other interstitial water signals, as evidence of dissociating gas hydrates. At sites where gas hydrates have been observed (e.g., Ocean Drilling Program Site 994), interstitial waters showed a distinct downhole trend toward lower chlorinity values, as well as very low chlorinity values (<500 mM) in the zone where gas hydrates were observed (Leg 164; Shipboard Scientific Party, 1996). Whereas high-resolution measurements of chlorinity are left to shore-based investigations, the small variation in shipboard chlorinity and salinity measurements suggest that gas hydrates are not present at this site.

Sulfate was detected only in interstitial waters from Core 184-1144A-1H (Fig. F23C; Table T14), and the concentration of dissolved sulfate was essentially zero below this depth. Methane levels increase at this depth, indicating that methanogenesis takes over as the dominant reduction process within the sediments (see "Organic Geochemistry"). Such a shallow switch from sulfate reduction to methanogenesis suggests that the supply of organic material to this site is high. As a product of both sulfate reduction (see "Inorganic Geochemistry" in the "Site 1143" chapter) and methanogenesis (Gieskes, 1983), ammonium concentrations increase rapidly in the top 22 mcd and then remain quite high until 394 mcd, where there is a decrease toward the base of the hole (Fig. F23D; Table T14). Also in response to sulfate reduction and methanogenesis, dissolved HPO₄^2- concentrations peak at 260 mM at 22 mcd and then decrease toward a minimum of 6 mM at the base of the hole (Fig. F23E; Table T14). Within this overall downhole decrease is an excursion toward lower HPO₄^2- values between 39.9 and 117 mcd. Alkalinity increases rapidly between the first and second core in response to sulfate reduction, from 3.9 mM at 3 mcd to a maximum of 34.9 mM at 13 mcd (Fig. F23F; Table T14). Below this depth, alkalinity generally decreases downhole to a minimum of 7.1 mM at the base of the hole. Superimposed on this downhole trend toward lower alkalinity is a shift toward lower values with a local minimum at 75 mcd, followed by a shift toward higher values with a local maximum at 117 mcd. Throughout Hole 1144A, pH is relatively constant ranging from 7.4-8.2 (Table T14).

Dissolved potassium (K⁺) concentrations decrease downhole from ~12.5 mM near the surface to 6.8 mM at 486 mcd (Fig. F23G; Table T14). This shift is probably related to uptake of potassium during clay and volcanic ash alteration. Magnesium concentrations (Mg²⁺) decrease with depth from 51.5 mM at the top to a minimum of ~33 mM at the bottom of the hole (Fig. F23H; Table T14), with an excursion toward first lower, then higher Mg²⁺ values between 56 and 126 mcd. The profile of dissolved calcium concentration (Ca²⁺) decreases from 9.2 mM to 4.8 mM between the first and second core (Fig. F23I; Table T14), and then Ca²⁺ increases slowly downhole to a maximum 9.1 mM at 429 mcd. Below this depth, Ca²⁺ begins to decrease toward the base of the hole. Overall, the alteration of basaltic volcanic ash, which is documented throughout the sediment at this site (see "Lithostratigraphy"), is likely the dominant process altering the profiles of these two elements.

In the profiles of alkalinity, Mg²⁺, and HPO₄^2-, an excursion between 53 and 188 mcd overprints the overall downhole trends of these constituents. Sharp events like this are unexpected in interstitial water profiles below the zone of organic matter reduction. One possible source of this signal at Site 1144 is the dissolution of pteropod aragonite within the sediments. Within the upper 160 mcd at Site 1144, there are peaks in the ratio of pteropods to foraminifers (see "Biostratigraphy") near the surface and at 108 mcd that correspond to the variations in the dissolved Mg²⁺ and alkalinity in interstitial waters. Thus, the interstitial water peaks within these intervals could reflect the dissolution of relatively unstable, Mg-rich aragonite. How this process would lead to lowered phosphate values is less clear.

Dissolved silica (H₄SiO₄) increases in the first three cores and then is high and relatively constant downhole at 795 mM ± 31 until it decreases abruptly to 400 mM between 404 and 440 mcd (Fig. F23J; Table T14). This shift coincides with a severalfold increase in magnetic susceptibility (see "Physical Properties") and a large decrease in the abundance of siliceous microfossils (see "Biostratigraphy"). Thus, the decrease in dissolved silica is related to the decrease in the siliceous microfossils available to be dissolved. Lithium (Li⁺) decreases from 24 mM to 11 mM between the first and second cores and then increases linearly with depth to a maximum of 30 mM (Fig. F23K; Table T14). Dissolved strontium concentrations (Sr²⁺) increase from a minimum of 68 mM at 22 mcd to a maximum of 202 mM at the base of the hole (Fig. F23L; Table T14). The increases in both Li⁺ and Sr²⁺ concentrations downcore most likely reflect exclusion of these elements during dissolution of biogenic calcite (Sr²⁺ and Li⁺) or dissolution of biogenic silica (Li⁺). The gradients with depth in Li⁺ and Sr²⁺ at Site 1144 are much lower than those at Site 1143 (see "Inorganic Geochemistry" in the "Site 1143" chapter). This is probably a reflection of the much lower percentage of carbonate at this site (see "Organic Geochemistry") and the much shorter interval in which the sediments could be chemically altered.