Whole-round cores for interstitial water analysis were taken from Hole 1131A at a rate of one sample per section from the upper 15 cores and every other core thereafter, recovery permitting. Samples were analyzed according to the procedures outlined in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. These data are presented in Table T8 and Figures F14, F15, and F16.
Salinity shows a uniform increase to a value of 72 at 173.7 mbsf. Below this depth, the increase continues but is erratic, perhaps resulting from minor amounts of contamination by surface seawater (Fig. F14). A maximum value of 84 is reached at a depth of 448.4 mbsf. A similar pattern is observed in the concentration of Cl-, which attains a maximum concentration of 1574 mM at 467.4 mbsf. The Na+/Cl- ratio increases from seawater values of 0.83 to a maximum of 1.1 ± 0.1 between 81.0 and 173.7 mbsf and then returns to seawater values near the base of the cored interval (Fig. F14). The concentration of K+ mirrors the increase in Cl-, and the K+/Cl- value does not deviate significantly from that of seawater.
The concentrations of Mg2+ and Ca2+ exhibit significant decreases in the upper 300 mbsf, with Mg2+ falling as low as 9.81 mM and Ca2+ to 2.22 mM at 108.9 mbsf (Fig. F15). Below this depth, concentrations steadily rise toward the base of the cored interval, although normalized ratios are still lower than in normal seawater (Fig. F15). The concentration of Sr2+ shows two principal increases, the first between the sediment/seawater interface and 45.8 mbsf, and the second between 192.6 and 294.8 mbsf (Fig. F15). Both these increases represent the recrystallization of aragonite and high-Mg calcite (HMC) to low-Mg calcite (LMC) and dolomite. The first is associated with the loss of HMC from the core and the appearance of dolomite, whereas the second is associated with increasing cementation (see "Lithostratigraphy") and decreasing aragonite.
The concentration of SO42- shows a slight increase in the upper portion of Site 1131, before decreasing to a concentration of 14.7 mM at a depth of 275.8 mbsf (Fig. F16). When normalized to Cl-, this decrease represents a depletion of SO42- in the pore water by almost 90%. Over the same interval, the NH4+ concentration reaches 26.9 mM (Table T8). Iron was only present at or below detection limits throughout the majority of the cored interval, the exception being in the uppermost 45.8 m and in several samples at the base of the hole.
Alkalinity shows a maximum value of 135.02 mM at a depth of 173.7 mbsf. Toward the bottom of the hole, alkalinity decreases to a value of 2.93 mM. Values for pH and pH determined using the push-in electrode (ppH) are relatively consistent, decreasing to 5.9 at a depth of 486.9 mbsf (Fig. F16).
The sediments at Site 1131 are composed of LMC, HMC, quartz, aragonite, and dolomite (Table T9, also in ASCII format; Fig. F17). At the top of the core, HMC is the dominant mineral; however, the concentration of HMC decreases downward to zero at the boundary between lithostratigraphic Units I and II (see "Lithostratigraphy"). The decrease in HMC is concurrent with the increase in dolomite, which reaches sustained concentrations of ~20% with a maximum concentration of 30%. At the base of Unit II, the concentration of aragonite decreases to ~5%.
Site 1131 is characterized by a significant amount of carbonate recrystallization associated with the oxidation of organic material by sulfate-reducing bacteria and the consequent formation of H2S. This recrystallization is evident from (1) the increase in the concentration of Sr2+ in the pore fluid (Fig. F15), (2) changes in mineralogy noted from the X-ray diffraction analyses (Fig. F17), and (3) observation of increased cementation (see "Lithostratigraphy"). Although the recrystallization occurs throughout the sediments, the nature of the Sr2+ profile indicates that the most significant region is between 200 and 300 mbsf where the concentration of Sr2+ exceeds 1000 µM. The second region of intensive recrystallization is in the upper 40 mbsf, coincident with the disappearance of HMC from the sediments and the appearance of dolomite. However, because the disappearance of HMC coincides with the boundary between lithostratigraphic Unit I and II, at least part of the decrease may be sedimentological in nature, rather than diagenetic.
An important observation is the presence of excess SO42- in the upper portion of the core between 64.8 and 108.9 mbsf. The most probable source for this extra SO42- is oxidation of H2S, which diffuses from lower in the core where high concentrations of this gas are present (see "Organic Geochemistry").
As was the case at Sites 1126, 1127, and 1130, Site 1131 is characterized by the presence of a saline brine within the drilled section. The maximum salinity determined is similar to that measured at Sites 1126 and 1127. An intriguing observation is the presence of a maximum in the Na+/Cl- ratio, which occurs at a depth between 90 and 110 mbsf. Although it is possible that this high ratio could be an analytical artifact, its presence has been replicated using the concentrations of (1) Na+ calculated by charge difference, (2) Na+ measured using the ion chromatograph, and (3) Cl- determined by titration and ion chromatography. Based on the concurrence of the results from these different techniques, we feel confident that these sediments contain a brine that had a Na+/Cl- ratio close to unity and, therefore, had been involved in the dissolution and precipitation of halite and other evaporite minerals. If these observations are correct, then the question arises as to why the region with the highest Na+/Cl- ratio is not located in the region of the highest salinity. One explanation may be that since the brines were emplaced in the sediments, the Na+ and Cl- ions have diffused into sediments of lower Na+ and Cl- content. Because the diffusion rate of these two ions is approximately similar (Li and Gregory, 1974), the original Na+/Cl- ratio of the brine has been maintained at the depth in the sediments at which the brine was first introduced, even though concentrations subsequently declined as a result of diffusion. The Na+/Cl- ratio decreases with depth because the brine was originally emplaced over a pre-existing brine, which had a Na+/Cl- ratio similar to that of seawater. The underlying brine could have entered the sediments during a previous sea-level lowstand. It is possible that the sequence of brine emplacement followed by sea-level rise, sediment deposition, and diffusion could have occurred a number of times coincident with the major sea-level changes during the Pleistocene.
A comparison of the analyses of anions and cations from Site 1131 samples revealed the presence of excess negative charge. This phenomenon was also noticed on Leg 166 for samples from Site 1007 (Shipboard Scientific Party, 1997). As a result of the positive association between the charge-balance anomaly and the normalized concentration of Mg2+ at Site 1007, the anomaly was postulated to result from complexing between SO42- and Mg2+. The same association was recognized at Site 1131 (Fig. F18). Although we have no definitive explanation for this association, we note that the anomaly was not seen at other sites drilled during Leg 182. These other sites also did not exhibit a combination of large sulfate and Mg2+ depletions. Hence, we concur with the idea that the imbalance must be linked to the systematic presence of an ion pair in the pore waters.