Thirty interstitial water samples were gathered from Hole 1085A between 1.4 and 598.75 mbsf. Whole-round samples were taken at a frequency of one per core to 103.1 mbsf and every third core thereafter to total depth (Table 10). The interstitial water chemistry at this site is dominantly controlled by the high carbonate and low organic carbon concentrations in the sediment, which results in very modest variations in chemical gradients of many dissolved species. Also, between 300 and 350 mbsf within lithostratigraphic Subunit IB, several chemical distributions record a marked change in concentration, which corresponds with the transition from APC to XCB coring and, therefore, with a change in the physical nature of the sediment. The species that are most affected by this change appear to be those most involved with clay (re-)mineralization processes.
Downcore profiles of alkalinity, sulfate, and ammonium (Fig. 19) through the upper 50 mbsf reflect the degradation of organic matter that is present in low concentrations. Alkalinity reaches a maximum value of only 27 mM at 46 mbsf, remains at similar values to 84 mbsf, and subsequently decreases to the bottom of the hole. The deepest value of 1.752 mM is by far the lowest alkalinity value (except for near-surface data) observed so far during Leg 175 and may largely reflect the consumption of alkalinity during clay mineral formation ("reverse weathering"). Sulfate is completely consumed by 65 mbsf, which is relatively deep in comparison with previous Leg 175 sites. This shallow gradient reflects both the low levels of organic carbon (see "Organic Geochemistry" section, this chapter) as well as the low sedimentation rate (see "Biostratigraphy and Sedimentation Rates" section, this chapter).
Ammonium reaches a maximum of only ~6000 µM at 188.6 mbsf before decreasing to the bottom of the hole. The increase in dissolved ammonium at ~300 mbsf is also observed in other dissolved species, most notably dissolved Mg2+, and is analytically real. It is important to note that above and below this change, the rates of decrease in ammonium concentration are nearly the same.
The concentration of dissolved Sr2+ increases only slightly within the uppermost 30 mbsf from a value near that of average seawater to 116 µM (Fig. 20). This slight and shallow increase suggests that biogenic calcite dissolution is occurring only to a minor extent in the shallowly buried sediments. Through the remainder of the sequence, dissolved Sr2+ increases to a maximum of 876 µM, with an increase in the rate of increase occurring at ~400 mbsf. The overall continued increase records the dissolution of biogenic calcite, which releases Sr2+ to the interstitial waters. The values of dissolved Sr2+ are the highest observed so far during Leg 175, reflecting the high concentrations of biogenic calcium carbonate in the sediment (see "Organic Geochemistry" section, this chapter).
From the seafloor to 50 mbsf, the concentrations of dissolved Ca2+ and Mg2+ decrease sharply. The decrease in Ca2+ through this depth range (7 mM) is significantly less than the decrease in dissolved Mg2+ (11 mM). As at previous Leg 175 sites, we attribute the decrease in Ca2+ as recording uptake by dolomite or phosphate phases, with the balance of the Mg2+ decrease recording uptake by clay phases.
From 50 mbsf to the bottom of the hole, concentrations of dissolved Ca2+ increase smoothly, with the exception of the interval from ~250 to 450 mbsf, where there is a notable positive excursion above the general trend of the increase (see shaded portion of Fig. 20). The position of the maximum at ~350 mbsf corresponds well with the stratigraphic top of an interval of high CaCO3 (see "Organic Geochemistry" section, this chapter; and the color measurements portion of the "Lithostratigraphy" section, this chapter).
Through this same general interval there is a very dramatic increase (13 mM) in dissolved Mg2+, the depth of which differs from the increase in ammonium by only one core. We thus consider them as occurring essentially at the same depth. Below this sharp increase, Mg2+ concentrations decrease once again smoothly to the bottom of the hole. We are unsure what causes these changes in the ammonium and Mg2+ profiles. There is no large-scale lithostratigraphic variation through this depth (see "Lithostratigraphy" section, this chapter); there is no marked change in porosity recorded in the physical properties data set (see "Physical Properties" section, this chapter), and there is no hiatus in sedimentation or any noticeable change in sedimentation rate (see "Biostratigraphy and Sedimentation Rates" section, this chapter). However, this position is where the coring methodology changed from APC to XCB, thus implying that there is a fundamental change in the character of the lithology that has not yet been observed by the shipboard analytical program. The chemical data are most consistent with a decrease with depth in the porosity, permeability, or formation factor of the sediment; such decreases all act in the direction of slowing down rates of removal of dissolved species, increasing concentration gradients, or both. Given that the alkalinity, dissolved Mg2+, and Na+ (see below) are the predominant species affected by this phenomenon, it appears that the chemical processes responsible for these pore-water changes involve clay phases.
Dissolved silica is present in interstitial waters from Site 1085 at concentrations greater than representative bottom-water values (Fig. 21), indicating the dissolution of biogenic opal. Maximum values of dissolved silica are found at ~100 mbsf, which is the depth range recovered by Cores 175-1085A-10H through 13H, which contain high abundances of diatoms (see "Lithostratigraphy" section, this chapter). From this depth to the bottom of the hole, dissolved silica concentrations decrease, with several short depth intervals of local increases and decreases. These short intervals do not correspond with any large variation in diatom abundance (see "Biostratigraphy and Sedimentation Rates" section, this chapter), nor do they correspond with variations in clay content (see "Lithostratigraphy" section, this chapter). However, at Site 1085 sponge spicules are more abundant than at previous Leg 175 locations (see "Biostratigraphy and Sedimentation Rates" section, this chapter). We hypothesize that continued shore-based research on the distributions of these spicules will provide relevant information about these dissolved silica distributions.
Because of remineralization of organic matter, dissolved phosphate concentrations increase with depth through Unit IA and reach a maximum value of ~50 µM at 50 mbsf (Fig. 21). Through the remainder of the sequence, dissolved phosphate decreases to extremely low concentrations (<5 µM) reflecting the uptake into diagenetic phases.
Concentrations of dissolved K+ are relatively constant through the uppermost 100 mbsf (Fig. 22). Below this depth, dissolved K+ decreases linearly to very low minimum values at the bottom of the hole, most likely caused by uptake by clay phases. There is no change through the 300 mbsf depth interval where ammonium and alkalinity show much change. Dissolved Na+ increases rapidly through litho-stratigraphic Subunit IA and reaches a maximum value at 250 mbsf. There is a large decrease across the 300-mbsf-depth interval, and the scale of this decrease (~35 mM) is far greater than the increase observed in the ammonium and Mg+ profiles. After this sharp decrease, dissolved Na+ concentrations increase toward the bottom of the hole.
Salinity decreases relatively slightly through the sequence (Fig. 23), recording the total decreases in the species described above, particularly alkalinity, Mg2+, Na+, and K+.
Concentrations of dissolved Cl– record an initial increase to a maximum of ~560 mM at 30 to 40 mbsf before decreasing to the bottom of the hole. This initial increase in dissolved Cl– may reflect changes in bottom-water chemistry associated with ice-volume variations through glacial periods.