Fifty-two interstitial water samples were squeezed from selected 10- to 50-cm-long whole-round samples for chemical and isotopic analyses at Site 1175. Sample depths ranged from 1.4 to 400.3 mbsf. Samples were collected from every section in Cores 190-1175A-1H and 2H, from four sections in Core 3H, from three sections in Core 4H, and from two sections in Core 5H. One sample per section was collected from the remainder of the site, except for Cores 190-1175A-24X, 34X, 35X, 36X, and 40X, because of poor recovery and/or the poor condition of these cores.
Elemental concentrations are reported in Table T12 and plotted as a function of depth in Fig. F20. Eight major and minor dissolved anions and cations that sensitively reflect microbially mediated or inorganic water-rock (sediment and oceanic basement) reactions were determined for each sample. The former includes alkalinity and sulfate, and the latter are Cl, Ca, Mg, Na, K, and Si. Every second sample in the top 50 mbsf and every third sample in the remainder of the section were analyzed for ammonium. Salinity and pH were also determined.
The outstanding characteristics of the interstitial water concentration-depth profiles at Site 1175 are the intense microbially mediated reactions in the top ~200 m of the section. Only minor or no changes in chemical gradients occur throughout the section in the abiogenic components. Additionally, Cl concentration profiles can be potentially used to constrain the timing of sediment slumping events.
Cl concentrations were determined with a relative analytical uncertainty of 0.1% based on duplicate or triplicate titrations of all samples. Overall, Cl concentrations slightly decrease with depth from 562 to 551 mM and remain close to seawater concentrations. This is in contrast to most oceanic sites and Sites 1173 and 1174, where Cl decreases more rapidly with depth in the shallower part of the section (by ~3.2%) as a result of the diffusion of low-chlorinity interglacial seawater downward into the section. The lack of a shallow gradient at Site 1175 is interpreted to imply that sediment slumping has actively transported interglacial seawater into the section. It also implies that this slumping has occurred in the time interval since seawater has freshened as a result of glacial melting, approximately the last 10 k.y.
Additionally, in the top 100 mbsf and between 270 and 300 mbsf, there are small-scale fluctuations, on the order of 1-3 mM, over 10 m. Again, these fluctuations are most likely caused by the slumping of sediments that have pore fluids with Cl concentrations that reflect glacial and interglacial ocean chlorinities.
Na concentrations are relatively constant, having small-scale fluctuations similar to chloride. Concentrations increase by ~2% between the shallowest samples and ~100 mbsf. This increase is mostly caused by Na release from clay as a result of ion exchange with ammonium. Concentrations then decrease with depth to 3% lower at 400 mbsf. This decrease is partially due to the decreasing ammonium concentrations and the net uptake of sodium into clay ion exchange sites, as well as to diffusion to greater depths where temperatures are higher and silicate reactions more intense.
K concentrations decrease smoothly from 11.2 mM, which is slightly greater than the seawater concentrations (10.4 mM), through a small maximum of 11.5 mM at ~5 mbsf to 8.69 mM at the bottom of the section. The decrease is most pronounced in Unit III. The profile suggests diffusion into a silicate reaction zone at greater depth, located below the drilled section. The higher than seawater concentrations in the upper part of the section are mostly caused by the expulsion of K from clay minerals as a result of ion exchange by ammonium.
Dissolved Si concentrations increase from ~550 uM close to the sediment-seawater interface to ~1,000 uM at 300 mbsf. At the base of Unit II, the gradient reverses, and similar to K, Si concentrations decrease nearly linearly with depth to ~600 uM, suggesting a silicate reaction zone at greater depth, most likely the same phase(s) that control the uptake of K from the pore fluids. Diatom dissolution is the most likely candidate for the increase in Si concentrations with depth; the solubility of opal-A increases with temperature and thus with depth. The solubility of opal-A at 20°C is ~1000 µM. Diatoms are present throughout the section and diminish in abundance with depth (see "Biostratigraphy"). The small-scale fluctuations in Si concentrations at all depths at this site reflect the great sensitivity of dissolved Si to local variations in lithology.
Both Mg and Ca concentrations decrease rapidly with depth in the top 15-20 m of the section. Ca reaches its lowest concentration of 3.15 mM and Mg has a local minimum of 45.7 mM. These two minima coincide with the depth of an alkalinity minimum. Below the depth of this shallow Mg minimum, concentrations decrease to ~250 mbsf. These data suggest that in the upper 20 mbsf, authigenic dolomite is precipitating and both Mg and Ca are consumed from solution. Below this zone, dolomite formation proceeds as a result of dolomitization of preexisting calcite. This is evidenced by the inverse gradient of Mg and Ca; Mg is removed from the pore fluids, whereas Ca is added. In this section, abundant biogenic calcite is available for dolomitization. Calcite ranges from ~3 to >30 wt% (see "Organic Geochemistry"). Deeper in Unit II, almost no change in Mg and Ca concentrations is observed. In Unit III, however, Mg concentrations decrease with depth but Ca concentrations remain constant, indicating that these cations are no longer involved in carbonate diagenesis and that Mg is involved in silicate reactions at greater depth. The sink for Mg seems to be the same as for K and Si and probably Na, implying that a K-Mg silicate (±Na) is forming at greater depth. The alkalinity profile discussed below indicates that alkalinity is also consumed by the deeper reaction.
Sulfate concentrations rapidly decrease with depth and reach zero at ~15 mbsf. This sulfate reduction zone is present at 2.5 times greater depth than at Site 1173 and 3 times deeper than at Site 1174. Because the sulfate reduction zone was sampled at high resolution, it is possible to quantify the rate of sulfate reduction as a function of depth. Preliminary analysis, based on the curvature of the sulfate vs. depth profile, indicates that sulfate reduction rates decrease linearly with depth.
A broad ammonium maximum of ~5 mM is found between ~75 and 200 mbsf. The magnitude of the concentration maximum is similar to Site 1173 but spans an interval that is approximately five times larger. In contrast, maximum concentrations are about one-half of those at Site 1174 and, again, the interval is broader. Ammonium is produced during the microbially mediated decomposition of organic matter. At such high concentrations, ammonium occupies clay ion exchange sites, expelling mostly K and Mg and some Na into the pore fluid. As at Sites 1173 and 1174, the existence of a midsection maximum implies a deep sink for ammonium. However, no chemical or biochemical mechanism has been identified yet.
Alkalinity increases rapidly and reaches a maximum of 33.1 mM at ~15 mbsf, where sulfate concentrations reach zero. This is expected because alkalinity is produced during sulfate reduction. The depth of the maximum coincides with the depth of Mg and Ca minima, driving authigenic carbonate formation. At greater depths, alkalinity decreases monotonically but remains high (~20 mM). The almost-constant concentration between ~100 and ~300 mbsf may be the equilibrium concentration for dolomitization of calcite at the low temperatures (<~22°C) at this site. Shore-based calculations will test this hypothesis. The reversal and decrease in alkalinity below ~300 mbsf (see "Magnesium and Calcium") suggests a hydrous K-Mg silicate forming at greater depth.
In summary, pore fluid chemical compositions at this site are less altered from seawater than at Sites 1173 and 1174. Four processes are principally responsible for the observed concentration variations: (1) opal-A and diatom dissolution, (2) carbonate diagenesis, primarily two dolomitization reactions, (3) silicate reaction at depths greater than the section drilled, and (4) microbially mediated reactions. In particular, carbonate diagenesis is expected to influence the physical properties, such as porosity and grain density. Unlike at Sites 1173 and 1174, where ash alteration dominated the chemistry of the pore fluids, the volcanic ash layers at this site are unaltered and do not affect the chemistry of the pore fluids because of the much lower geothermal gradient of 54°C/km (see "Physical Properties"). Even the prominent ash layer at ~100 mbsf, clearly recognized in all physical properties data, does not affect the pore fluid chemistry. Lastly, a unique feature of this site compared to Sites 1173 and 1174 is the lack of a shallow chloride gradient, which implies that slumping has occurred in approximately the last 10 k.y.