INORGANIC GEOCHEMISTRY

Inorganic chemical analyses were conducted on 27 interstitial water samples from Hole 1146A squeezed from whole-round samples at a frequency of one per core in the first nine 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 T15, and the profiles with depth are shown in Figure F18. Interstitial water profiles at Site 1146 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 at Site 1146 are relatively constant above 550 mcd, ranging from 555 to 560 mM (Fig. F18A; Table T15). Below 550 mcd, Cl^- begins to decrease to values <540 mM by 569 mcd. Interstitial water salinities decrease from 34.5 to 32 between 3 and 109 mcd, are relatively constant (~32) between 109 and 476 mcd, and then decrease from 32 to 30 between 476 mcd and the base of the hole (Fig. F18B; Table T15). The decrease in salinity from the top of the core down to 109 mcd is most likely related to the removal of dissolved sulfate from interstitial waters during sulfate reduction. The decreases in Cl^- and salinity below ~476 mcd correspond to a large increase in methane concentration, the appearance of ethane and propane in headspace gas samples (see "Organic Geochemistry" and "Background and Objectives"), a change in sediment color (see "Lithostratigraphy"), and a major decrease in magnetic susceptibility (see "Physical Properties"). A series of seismic reflectors at a depth of ~530 mcd (see discussion, "Site 1146 [SCS-4]" in "Seismic Stratigraphy of Leg 184 South China Sea Sites" in the "Seismic Stratigraphy" chapter; "Organic Geochemistry"), may correspond to the changes in lithology observed at this depth. One possible explanation for these changes is dissociating gas hydrates, which would dilute pore waters and elevate methane levels. At present, little direct evidence exists for gas hydrates at this site beyond the low chlorinity/salinity and the hydrocarbons, no gas hydrates were observed upon core recovery, no disturbance was observed in the split core (see "Lithostratigraphy"), and other interstitial water signals are not diluted (Fig. F18; Table T15), although this last change would be small (~4% dilution). A second possibility is that major changes in clay mineralogy occur at this depth, leading to the release of water and changes in some other dissolved constituents (see pH and lithium below). However, no major changes in clay mineralogy were observed at these depths in the shipboard X-ray diffraction data. Clay mineralogy also would not explain the changes in the hydrocarbons, unless the two were linked through some parameter such as temperature. A third possibility is the migration of low-chlorinity water and hydrocarbons, either laterally or from depth. At present this seems the most viable theory. The depth of the events in the chlorinity and hydrocarbons correlates with seismic Reflector T₂ (see "Site 1146 [SCS-4]" in "Seismic Stratigraphy of Leg 184 South China Sea Sites" in the "Seismic Stratigraphy" chapter), which could be linked to a fault that can be seen extending to basement within 1 nmi of this site. Fluid and hydrocarbons could migrate laterally along this surface, and the changes in lithology could be diagenesis caused by conditions imposed by these fluids.

Sulfate decreases from 24.1 mM near the surface to 0 mM by 68 mcd (Fig. F18C; Table T15) indicating that sulfate reduction is completed by this depth. Below the zone of sulfate reduction, methane levels begin to increase, indicating that methanogenesis takes over at this depth (see "Organic Geochemistry"). Ammonium (NH₄⁺) increases to a maximum of 3.34 mM in the interval of sulfate reduction, decreases to a minimum of 1.14 mM at 262 mcd, and finally increases gradually downhole (Fig. F18D; Table T15). Dissolved phosphate (HPO₄^2-) concentrations increase to a maximum of 103.9 mM in the zone of sulfate reduction and then begin to decrease rapidly to reach near-zero values by 201 mcd (Fig. F18E; Table T15). Alkalinity increases to a maximum of 28.82 mM in the zone of sulfate reduction then decreases downhole to a minimum of 0.51 mM near the base (Fig. F18F; Table T15). The depletion of NH₄⁺, HPO₄², and alkalinity—all products of methanogenesis—with depth supports the idea that methanogenesis is active only in the uppermost sediments and that increases in methane at depth are related to the migration of thermogenic hydrocarbons into the sediments. A pH minimum is centered at the depth of the maximum in NH₄⁺, HPO₄², and alkalinity (Fig. F18G; Table T15). Below this level, pH is relatively constant down to 444 mcd, then increases to a maximum of 8.2 near the base of the hole.

Magnesium (Mg²⁺) concentrations decrease with depth from 51.7 mM at the top to a minimum of ~23.6 mM at the bottom of the hole (Fig. F18H; Table T15). Dissolved calcium concentrations (Ca²⁺) decrease slightly in Cores 184-1146-1H through 4H in response to sulfate reduction (Fig. F18I; Table T15). Then Ca²⁺ increases slowly downhole to a maximum of 21.30 mM at 628 mcd. Dissolved potassium (K⁺) concentrations decrease downhole from ~11.8 mM near the surface to 4.1 mM at 628 mcd (Fig. F18J; Table T15). The most likely cause of changes in the profiles of these three elements is uptake of Mg²⁺ and K⁺ and release of Ca²⁺ during the alteration of clay minerals and, to a secondary extent, alteration of basaltic volcanic ash, which is present in smaller quantities at this site than at other sites during this leg (see "Lithostratigraphy").

Dissolved silica (H₄SiO₄) concentrations are high, 744 ± 60 mM, in the upper part of the hole and then decrease sharply to <300 mM at 170 mcd (Fig. F18K; Table T15). This decrease corresponds to a major decrease in siliceous microfossils in the sediments (see "Sedimentation and Accumulation Rates"). Below this depth, H₄SiO₄ values decrease gradually downhole to a minimum of 71 mM at the base (Fig. F18K; Table T15). Lithium (Li⁺) is very low, <60 mM down to 323 mcd, and then increases sharply, reaching a maximum of 2390 mM at the base of the hole (Fig. F18L; Table T15). This increase correlates with the increase in pH (Fig. F18G; Table T15). Given the correspondence of increasing Li⁺ with lowered Cl^- and increased hydrocarbons, the release of Li⁺ may be occurring because of alteration of clay minerals in response to changing environmental conditions in the sediments. This conclusion, however, is highly tentative and awaits shore-based investigation. Dissolved strontium concentrations (Sr²⁺) are <150 mM above 109 mcd, then increase to a maximum of 1268 mM at 356 mcd and remain high to the base of the hole (Fig. F18M; Table T15). This increase in Sr²⁺ occurs in conjunction with an increase in the percent carbonate from 20 to 60 wt% in Hole 1146A (see "Organic Geochemistry"). Although Sr²⁺ concentrations increase downhole in carbonate sediments in response to calcite recrystallization (Baker, 1986), the simultaneous increase of Sr²⁺ and carbonate percentage at depth at Site 1146 suggests that large changes in carbonate content can also affect the extent of recrystallization.