Whole-round cores for interstitial water analysis from Holes 1126A and 1126B were taken at a rate of two samples per section in the upper five cores, one per core down to Core 182-1126B-8H, and every other core thereafter, recovery permitting. No samples were taken from Hole 1126C. Samples from Hole 1126D were taken at a rate of one sample every other core, 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 F15, F16, F17, F18, and F19.
Salinity and chlorinity show no change in the upper 10 mbsf of Holes 1126A and 1126B. Below this interval salinity increases to a value of 100 by ~100 mbsf (Fig. F15). Chlorinity is linearly related to salinity over the entire core. Increases in salinity can be caused by a variety of factors, the origin of which can be constrained by changes in the ratio of cations to chloride (see "Discussion").
Concentrations of Ca2+, Mg2+, K+, Li+, H4SiO40, and Sr2+ all increase with increasing depth and are correlated with changes in salinity (Fig. F16). The concentration of Ca2+ increases from 11.1 mM at a depth of 2.95 mbsf in Core 182-1126A-1H to a maximum of 49.7 mM in Core 182-1126B-17H. Although the increase in Ca2+ is not unexpected considering the increase in Cl-, the concentration of Ca2+ increases in excess of expected changes in Cl- (Fig. F17), giving a normalized change of between 7 and 8 mM Ca2+ over the length of the core. Increases in the concentration of Ca2+ relative to Cl- could arise as a result of the dissolution of low-Mg calcite (LMC) or the influence of a CaCl2 brine (see "Discussion"). The concentration of Mg2+ decreases slightly from 55.9 mM in Core 182-1126A-1H to 53.9 mM in Core 182-1126B-1H (Fig. F16). From this point the concentration increases to 130.9 mM at a depth of 134.7 mbsf in Core 182-1126B-17H. Despite the increase in the concentration of Mg2+, the normalized concentration of Mg2+ decreases by ~24 mM by the bottom of the cored interval (Fig. F17). Explanations for the decrease in Mg2+ of pore fluids usually involve the formation of dolomite or interactions with clay minerals within the section (Gieskes, 1981). At Site 1126 dolomite is absent below 100 mbsf; therefore, the most plausible explanation is that the relatively low concentration of Mg2+ reflects either the alteration of clay minerals within the sedimentary section or inheritance from the original brine. Although K+ shows relatively little change relative to Cl- throughout the core, there is a slight decrease of between 1 and 2 mM in the concentration of K+ normalized to Cl- between ~20 and 80 mbsf. The relative decreases in K+/Cl- are probably caused by clay mineral diagenesis. For example, the conversion of kaolinite (Al2Si2O5(OH)4) to montmorillonite ([Na0.2 K0.1Mg0.1Mg0.8]Mg0.66 Al3.34Si0.8O2(OH)4) will remove varying amounts of Mg2+, K+, and Na+ from pore fluids (Loughnan, 1969).
Concentrations of Na+ were determined using two different methods (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter). Although the concentration of Na+ measured using the ion chromatograph was within 5% (the error quoted for the ion chromatography method) of that calculated by difference in charge balance, the Na+ calculated by difference gave much more consistent trends. In both cases the Na+/Cl- ratio throughout the core is similar to that of surface seawater (Fig. F18).
The concentration of Sr2+ relative to Cl- increases significantly between 20 and 60 mbsf before falling below seawater values throughout the remainder of the core (Table T8). The increase in the Sr2+/Cl- ratio in the upper 40 mbsf arises from the recrystallization of biogenic aragonite and high-Mg calcite (HMC) (Swart and Guzikowski, 1988). The concentration of Sr2+ is limited by the solubility of celestite in the pore waters, which is approximately at saturation in the interval of maximum Sr2+ concentration. The Sr2+/Ca2+ ratio decreases substantially in the lower portion of the core (Fig. F19).
The Li+/Cl- and H4SiO40/Cl- ratios show a steady decrease throughout the core. In contrast to previous ODP sites (e.g., Site 1003, Leg 166), the dissolved Li+ concentration profile is dissimilar to that of Sr2+, with the Li+/Cl- ratio decreasing from seawater ratios to 20 at the base of the core. Dissolved Li+ concentrations have been suggested to be influenced by the early diagenesis of opal-A, the recrystallization of biogenic carbonate, and reactions involving clay minerals (Gieskes, 1983).
The concentration of SO42- normalized to Cl- shows a small decrease from a seawater value of 54 to 47.3 in Core 182-1126B-2H at a depth of 12.40 mbsf. Below this depth the ratio increases again to approximately seawater values before decreasing substantially to ~40 below 70 mbsf (Fig. F17). The decrease in the lower portion of the core is not associated with any increase in alkalinity or noticeable smell of hydrogen sulfide (H2S). One possible explanation is that the brines had become depleted in SO42- before their emplacement in the sediments. The alkalinity/Cl- ratio reaches a maximum at 5.95 mbsf in Core 182-1126A-1H. The maximum reduction in the SO42-/Cl- ratio in the upper portion of the core occurs 6 m above the alkalinity maximum. A slight odor of H2S was noted in this interval. A pH minimum occurs at a depth of 58.9 mbsf in Section 182-1126B-7H-4. Trends in pH, determined from initial millivolt readings taken during alkalinity titrations and using the punch-in electrode, were similar.
The geochemistry of pore fluids at Site 1126 is dominated by the dramatic increase in salinity, which is manifested as shallow as 9.4 mbsf. The salinity reaches a value of ~106 by a depth of 134.7 mbsf in Section 182-1126B-17H-4. Although there is some variation, this value is maintained throughout the remainder of the core, and it is probable that the small decreases seen in the lower portion of the core result from contamination by seawater introduced during drilling.
The occurrence of high-salinity pore waters in continental slope sediments is not unusual and has been documented at numerous other locations (Sotelo and Gieskes, 1978; Couture et al., 1978; Suess, von Huene, et al., 1988; Kastner et al., 1990). In sediments drilled off the Peruvian coast during Leg 112, high salinity values were measured at several sites and were inferred to result from multiple origins, including (1) dense brines arising from Holocene evaporitic systems on the adjacent land masses, (2) dissolution of evaporitic minerals at depth, and (3) fossil brines (Suess, von Huene, et al., 1988). As in the case of the sites drilled during Leg 112, the formation of gas hydrates at Site 1126 can be ruled out as the source of brines because not only is the temperature-pressure regime at Site 1126 not conducive for hydrate formation, but concentrations of methane at Site 1126 are too low for gas hydrate formation. Although the precise origin of the brines at Site 1126 cannot be ascertained at this stage, some constraint can be placed by an examination of the ratios of the major cations relative to Cl-. In particular, if the brine formed from the dissolution of NaCl (halite), then the pore water would tend to have a Na+/Cl- ratio near unity. In the case of the data from Site 1126, the Na+/Cl- ratio, determined using the Na+ calculated by difference, is essentially the same as seawater throughout the core. If the value for Na+ measured using ion chromatography is used (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter), then the Na+/Cl- ratio is slightly elevated with respect to seawater, although still below the value expected from the dissolution of halite (Fig. F18). Dissolution of other types of evaporitic facies can also be ruled out because in spite of the increase in the Ca2+/Cl- ratio, which might reflect the dissolution of gypsum or anhydrite, there is a decrease in the SO42-/Cl- ratio, which suggests consumption rather than addition of SO42-. Consequently, the most plausible explanation for the increased salinity is the presence of a partially evaporated fossil brine. This brine could have been forced into the sediments during a previous sea-level lowstand when a significant portion of the adjacent continental shelf would have been exposed and possibly acted as an evaporitic lagoon.
The sediments at Site 1126 are composed of aragonite, HMC, and LMC, with smaller amounts of quartz, dolomite, and clay minerals in the upper 60 mbsf (Table T9, also in ASCII format; Fig. F20). This interval corresponds to lithostratigraphic Unit I (see "Lithostratigraphy"). The origin of dolomite within this unit is unknown; the dolomite could either be authigenic, related to the small amount of oxidation of organic material observed in the pore-water profiles, or detrital in origin. In lower portions of the hole, aragonite and HMC disappear, probably as a result of diagenesis, and dolomite is also absent. Unit IV (see "Lithostratigraphy") is composed mainly of LMC and quartz.