A total of 44 interstitial water samples were collected from Hole 1075A and analyzed (Table 10). The sampling protocol called for gathering one 10-cm-long whole-round interval from each section of core for the uppermost 60 mbsf, one 10-cm-long whole-round interval from each core from 60 to 100 mbsf, and one 10-cm-long whole-round interval from approximately every third core thereafter. The shallowest sample was taken from 0.88 mbsf and the deepest from 190.9 mbsf. Headspace samples (see "Organic Geochemistry" section, this chapter) were taken immediately adjacent to each interstitial water whole-round sample, thereby providing a comparable high-resolution data set for volatile hydrocarbons.
Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 26) indicate the extent and timing of the degradation of organic matter. As described in the "Organic Geochemistry" section for this site, the sediments from Site 1075 contain elevated levels of TOC, with concentrations averaging 2.6 wt%. This high TOC level is responsible for the high levels of alkalinity within interstitial waters, with a maximum of ~40 mM at 42.4 mbsf. These high alkalinity values are maintained to ~90 mbsf, below which they decrease, perhaps reflecting uptake during diagenetic clay mineral formation or authigenic (Mg-rich) carbonate formation (note that calcium concentrations increase slightly through these depths, as described below). The rate of increase in ammonium is also greatest through the upper 40 mbsf, with continuing increases to the bottom of the hole recording continued degradation of organic matter.
Dissolved sulfate becomes entirely depleted within the uppermost two sections of Core 175-1075A-5H at a depth of ~31 mbsf (Fig. 26). The sharp decrease from seawater values (~29 mM; Millero and Sohn, 1992) at the sediment/water interface to null values at 31 mbsf indicates that the degradation of organic matter has progressed beyond the consumption of oxygen, nitrate, iron, and manganese oxides (e.g., Froelich et al., 1979). Shore-based analyses of dissolved Fe and Mn will lend further insight into the rate of this process.
The processes of organic degradation described above also drive carbonate dissolution and precipitation, as monitored by concentration profiles of dissolved Ca2+, Mg2+, and Sr2+ (Fig. 27). The strong decreases in Ca2+ and Mg2+ through the upper 30 mbsf records the presence of a sink for these elements. Likely phases responsible include Mg-rich calcite, dolomite, clays, and authigenic apatite. The increase in dissolved Sr2+ over the uppermost 30 mbsf is most likely caused by the dissolution of biogenic calcite, which appears to be less important deeper in the section because Sr2+ concentrations increase only minimally downhole. The degradation of the organic matter provides the alkalinity necessary for the precipitation of diagenetic carbonate phases. Considering the strong decreases in dissolved Ca2+ and Mg2+, it appears that dolomite may be forming throughout the uppermost portions of the section. The molar consumption of Ca2+ and Mg2+ during dolomite formation should be the same. The potential involvement of dual mechanisms causing the drawdown of dissolved Mg2+ (described below) makes it unclear at this point whether additional authigenic phases are precipitating. The most likely potentially additional precipitating phase is authigenic apatite.
To separate the potential Mg2+ sinks of dolomitization and clay formation from each other we have extended the deeper trend (at depths >100 mbsf) of Mg2+ depletion stratigraphically up to the bottom-water value of 53 mM (Fig. 27). We suggest that this trend reflects the Mg2+ sink from clay mineral uptake; if so, the extent of Mg2+ drawdown in excess of this (represented in Fig. 27 by the shaded region sandwiched between the "clay uptake line" and the observed data) would be caused by dolomite formation. Postcruise mass balance calculations of the Ca2+, Mg2+, and Sr2+ budgets will provide constraints on authigenic carbonate formation.
Diagenetic changes involving biogenic and organic matter are also recorded by the downcore profiles of dissolved silica and phosphate (Fig. 28). The concentration of dissolved silica increases dramatically through the uppermost 10 m of sediment, recording the dissolution of opal-A, which is predominantly supplied to these sediments by diatoms (see "Lithostratigraphy" and "Biostratigraphy and Sedimentation Rates" sections, this chapter). The concentration of dissolved silica continues to gradually increase downcore, recording the continued dissolution of opal, which remains present into the deepest sections of the sequence (see "Biostratigraphy and Sedimentation Rates" section, this chapter).
Dissolved phosphate increases to maximum values between 50 and 60 mbsf and decreases to the bottom of the hole. Although more sharply defined than the broad alkalinity maximum (Fig. 26), the general correspondence between the phosphate and alkalinity profiles indicates that the phosphate is being released from organic matter during degradation. If authigenic apatite is forming within the upper 60 mbsf, it must do so at a rate that removes dissolved phosphate more slowly than it is being released by organic degradation. The decrease in concentration from 60 to 190 mbsf records uptake either into authigenic phosphate phases or by adsorption onto other mineral surfaces. Very fine-grained authigenic sedimentary phases (see "Lithostratigraphy" section, this chapter) could not be uniquely identified as phosphatic minerals; additional shore-based analyses will specifically target identification of the precise sink for the dissolved phosphate at this site.
Concentrations of both dissolved Na+ and K+ increase with depth downcore (Fig. 29). The concentration of Na+ shows a rapid increase near the surface. It is important to recall, however, that Na+ is determined via charge balance, and this increase may merely reflect the paired increase in dissolved Cl-, as discussed below.
The downcore profiles of salinity and dissolved Cl- show subtle variations (Fig. 30). The decrease in salinity is slight, from values of 35.5‰ near the sediment/water interface to 34‰ at depth. (The resolution of the hand-held refractometer is 0.5‰; thus, a smooth decrease appears as a stepped change.) The stepwise decrease in data shown probably represents a smooth decrease from values of 35.5‰ near the sediment/water interface to 34‰ at depth. The decrease in salinity appears to be caused by the decrease in dissolved sulfate, Ca2+, and Mg2+; note that chlorinity does not follow the trend. Despite the high methane concentrations at this site (see "Organic Geochemistry" section, this chapter), the decrease in salinity is in all probability not related to decomposition of solid methane hydrate. Decomposition of hydrate typically causes a far greater decrease in salinity (to values <28‰) than is observed.
Chloride concentrations increase sharply to maximum values at ~40 mbsf before becoming essentially constant for the rest of the profile (Fig. 30). Chloride concentrations are derived from the highly precise titration method; measurements using the less precise but similarly accurate Dionex instrument (Table 10) confirm this pattern. This increase is not caused by brine formation or other simple subseafloor dilution effects, nor is it monitoring substantial diagenetic release(s) because Cl- is known to behave largely conservatively. Instead, the increase most likely reflects a stacked diffusional signal of seawater Cl- from glacial periods where seawater had higher salinity because of the formation of the ice caps. The increase in dissolved Cl- is ~2%-3% of the measured value, which compares well with the volume of water removed from the oceans and transferred to the ice caps. This effect does not show in the salinity measurements because the pore-water salinity is a summation of all dissolved constituents. Previous ODP legs have also documented a marked increase in dissolved Cl- through the uppermost sections, with maximum values approximately comparable to those observed here. Shore-based analyses with greater analytical precision will further quantify the extent of salinity changes during glaciations.