Forty-five pore fluid samples were squeezed from selected 10- to 60-cm-long whole-round samples for chemical and isotopic analyses. Sample depths ranged from 1.4 to 394.8 mbsf. One 5-cm-long core-catcher sample was also selected from the bottom of the hole at 440 mbsf. Because only 0.5 mL of pore fluid was recovered from this sample, it was sealed for shore-based analyses and is not included in Table T12 or Figure F16. Samples were collected from every section in Cores 190-1176A-1H and 2H, from three sections in Core 3H, and from two sections in Cores 4H and 5H. One sample per section was collected per core from the remainder of the site where core quality and recovery were adequate for a pore fluid sample. Because of poor core recovery in Cores 190-1176A-28X through 35X, 37X, and 38X, pore fluid chemical data are not available for 239-325 and 326-354 mbsf.
Elemental concentrations are reported in Table T12 and plotted in Figure F16. As at the former sites, eight major and minor dissolved anions and cations that sensitively reflect inorganic or microbially mediated water-rock reactions were determined for each sample. The anions are Cl, Ca, Mg, Na, K, and Si, and the cations are alkalinity and sulfate. Salinity and pH were also determined. Every third sample was analyzed for ammonium. Between 300 and 400 mbsf, only three samples were analyzed for alkalinity because of the small volume of pore fluid obtained in this depth interval.
The outstanding characteristics of the pore fluids concentration-depth profiles at Site 1176 are the sharp discontinuities, particularly in the Cl gradient, that occur between ~230 and 320 mbsf, the return of most abiogenic components (except for K) to near-seawater concentrations below this discontinuity, and the intense microbially mediated reactions that occur at rather shallow depth, in the top 100 m of the section, and dominate the inorganic diagenetic reactions at this depth interval. Carbonate and sulfide formation and ion exchange reactions are the important diagenetic reactions at this site.
Cl concentrations were determined with a relative analytical uncertainty of 0.1% based on duplicate or triplicate titrations of all samples. Cl concentrations are constant in the top 50-60 mbsf and then monotonically decrease with depth (Fig F16). The profile between ~60 and 240 mbsf is a diffusional profile driven by a deeper nonrecovered low-Cl fluid source. Lower than background core temperatures of 11°-12°C (instead of 14°-16°C) measured on the catwalk suggest that a small percent of the freshening observed in the pore fluids of Core 190-1176A-25X and particularly Core 26X may be caused by the dissociation of disseminated gas hydrate. Evidence of diffusion of low-chlorinity interglacial seawater into the sediment section was not observed at this site. One explanation for this is that repeated slumping has reworked these upper sediments, mixing zones with slightly different Cl concentrations (see "Inorganic Geochemistry" in the "Site 1175" chapter and "Lithostratigraphy").
Below ~320 mbsf, the pore fluids have slightly higher than seawater Cl concentrations. The transition from lower than seawater Cl to close to seawater Cl concentrations occurs between ~230 and 320 mbsf. The upper boundary of this zone, in which core recovery was marginal, corresponds to the boundary between lithostratigraphic Units II and III (see "Lithostratigraphy"). This residual pore fluid composition reflects only slight modification by diagenesis. Presently the temperature at this depth interval is low (~20°C) (see "Physical Properties"); thus, reactions are slow.
The Na concentration-depth profile, although similar overall to that of Cl, shows two distinct and interesting features: a small minimum at ~23 mbsf, which is the depth of the alkalinity maximum, and a broad maximum centered at ~100 mbsf that corresponds to the ammonium maximum. The minimum suggests intimate involvement in authigenic carbonate formation, whereas the maximum is caused by expulsion into the pore fluid from clay ion exchange sites by ammonium. Consequently, in Units I and II the Na/Cl ratios, 0.885 and 0.878, are slightly but significantly higher than the seawater ratio of 0.859.
K concentrations generally decrease with depth from slightly higher than seawater values to 67% of seawater concentrations at the base of the section. A small minimum is observed at the depth of the alkalinity maximum. The broad maximum that corresponds to the depth of the ammonium maximum has been observed at each of the former sites drilled in the vicinity (i.e., Sites 1173-1175); it reflects expulsion into the pore fluids from clay ion exchange sites. The minimum associated with the alkalinity maximum was not observed at the other sites. This minimum is observed at this site because of the high carbonate content, ranging from ~8 to 27 wt%, in Units I and II (see "Organic Geochemistry"). Unlike Cl or Na, at the base of the section K does not return to seawater concentrations but continues to be consumed by a diagenetic reaction or by mixing with a deep-seated K-depleted fluid.
Dissolved Si concentrations reach high values of ~750 µM close to the sediment-water interface and increase monotonically with depth to a maximum of 914 µM at ~215 mbsf, close to the boundary between lithostratigraphic Units I and II. Below this, the gradient reverses and the concentrations decrease with depth. The maximum concentration value is close to the solubility of opal-A at the prevailing temperature of ~12°C at ~200 mbsf. As at all other sites, diatom dissolution controls Si concentrations. Diatoms are present throughout the section but are greatly diminished in abundance and preservation at the depth interval that corresponds to the Unit II/III boundary (see "Biostratigraphy"). Indeed, at ~230 mbsf Si concentrations drop to ~650 µM.
Similar to those at adjacent Site 1175, Ca and Mg concentration-depth profiles indicate intense carbonate diagenesis in lithostratigraphic Units I and II. The Ca profile is a mirror image of the alkalinity profile, whereas the Mg profile is distinct. The Mg distribution mimics that of Ca above the Ca minimum; below the Ca minimum they exhibit inverse gradients—Ca concentrations increase and Mg concentrations decrease with depth. Both Ca and Mg concentrations are lower or equal to seawater concentrations throughout the section. The lack of elevated Ca concentrations indicates that Ca and Mg are principally involved in carbonate rather than silicate reactions. This is presumably due to the prevailing low temperatures at this site
Ca concentrations drop sharply by ~7 mM, to a minimum of 3 mM (28% of seawater value) in the top 20 m of the section, with a gradient of 0.38 mM/m. The Ca and Mg minima coincide with the alkalinity maximum, indicating authigenic dolomite precipitation. However, the drop in Mg concentrations in the upper 20 m of the section is ~13 mM (from a seawater value of ~54 mM to ~41 mM), a gradient of 0.65 mM/m, indicating that Mg uptake is almost double Ca uptake. This suggests that in the top section, Mg is simultaneously consumed by both authigenic dolomite formation and dolomitization of the precursor biogenic calcite. Below the Ca minimum to the base of Unit II, the inverse profiles of Ca and Mg indicate that dolomitization of calcite is the dominant reaction in most of the section.
At the base of the section, like Cl and Na, Ca and Mg return to almost seawater concentrations. The only reactions that can increase Mg concentrations in pore fluids are ion exchange and dedolomitization. However, net Mg ion exchange is limited to the upper 100 mbsf because insufficient ammonium is present below 100 mbsf and the Ca concentrations necessary for dedolomitization are much too low. Hence, the observed Mg increase at the base of Unit III unequivocally indicates that these pore fluids are only slightly modified seawater.
Sulfate concentrations rapidly decrease with depth and reach zero at ~20 mbsf, where alkalinity has its maximum value. At Site 1175, zero sulfate occurred at a shallower depth (~15 mbsf). Preliminary analysis of the shallow high-resolution data reveals that sulfate reduction rates decrease with depth in the top 10 m of the sulfate gradient zone, whereas in the lower half of the zone, the sulfate reduction rate is at maximum at the very base of the zone. From 20 mbsf to the top of Unit III, the pore fluids contain no sulfate. At the base of the section, sulfate concentrations increase rapidly, and at the deepest sample recovered the concentration is ~17 mM, almost 60% of the seawater value. The lower-than-seawater sulfate concentrations in a section with pore fluid compositions near that of seawater suggest that bacterial sulfate reduction has lowered the concentrations to present levels. This lowering may have occurred either before this subsection was buried, or sulfate reduction is ongoing at a very low rate. In this depth interval, alkalinity has a constant value of 15 mM, suggesting either a steady state between production as a result of sulfate reduction and reaction or no production.
Alkalinity increases rapidly with depth to a maximum value of ~29 mM at ~20 mbsf, the depth at which sulfate concentrations reach zero value and Ca is at a minimum. At greater depths, alkalinity decreases monotonically to ~11 mM at the top of Unit III, suggesting a sink at greater depths. The gap in the profile at the crucial depth between ~240 and 320 mbsf does not allow full interpretation of the decreasing trend. The constant concentration of ~15 mM at the base of the section is discussed in "Sulfate".
Ammonium produced by bacterially mediated decomposition of organic matter increases in concentration with depth, having a broad maximum of ~5 mM between ~20 and 200 mbsf. The maximum concentration and depth span are almost identical to those observed at the adjacent Site 1175. At such a high concentration, it preferentially occupies clay ion exchange sites, thus expelling Mg, K, and Na into the pore fluid, as discussed above. The decrease with depth implies an ammonium sink at greater depth; however, the specific reaction(s) involved is as yet unknown.
At this site, the pore fluid chemistry is less modified from seawater than at the deeper-water, warmer Sites 1173 and 1174 and is modified to a similar extent as at the adjacent slope-basin Site 1175. The two most important reaction types that control the pore fluid chemistry are (1) carbonate diagenesis, mostly dolomitization, and (2) microbially mediated reactions with products intimately involved in the carbonate diagenesis. Other important reactions are opal-A (diatom) dissolution and clay ion exchange reactions with ammonium. At the base of the section, pore fluids maintain a composition close to seawater, as indicated by the return to seawater concentrations of all abiogenic components except for K. There is no evidence for important silicate reactions except diatom dissolution, as expected for the low geothermal gradient at this site.
The Cl concentration profile indicates diffusion between a low-Cl fluid zone at depth and the seafloor. Unfortunately, this interesting diffusion profile has a large data gap at a crucial depth interval because of low and no core recovery between ~240 and ~320 mbsf. Assuming that the Cl gradient is indeed driven by a low-Cl fluid possibly associated with the OOST fault, the Mg, Ca, Na, and K values suggest that this fluid has elevated Ca concentrations and is depleted in Na, K, and Mg. This chemical composition is similar to that of geochemically distinct fluid identified in structural and stratigraphic horizons at Site 1174.
Evidence of diffusion of low-chlorinity interglacial seawater into the uppermost sediment section is not observed at this site. This may be due to either rapid deposition of the upper 50-60 m of section or reworking of sediments by slumping.