Fifty-two pore fluid samples were squeezed from selected 10- to 47-cm-long whole-round samples for chemical and isotopic analyses. Sample depths ranged from 306.5 to 828.8 mbsf. One sample per core was collected except from Core 190-1177A-11X, from which three samples were collected, and Cores 190-1177A-51X, 52X, and 54X, from which two samples were collected. Because of poor core recovery, no pore fluids were recovered from Cores 190-1177A-21X, 22X, 28X, and 32X.
Elemental concentrations are reported in Table T13 and plotted in Fig. F18. As at other Leg 190 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. Thirty-seven samples were analyzed for ammonium and eighteen for Si.
The outstanding characteristics of the pore fluids' concentration-depth profiles at Site 1177 are the sharp discontinuities that occur between and within lithostratigraphic units and a high sulfate interval that ranges across the turbidite unit, which spans over approximately the middle half of the sediment section, from 420 to 735 mbsf. The most prominent discontinuities correlate with changes in measured physical properties and suggest that either specific intervals of sediment effectively remain as closed systems with respect to vertical diffusion or that localized diagenetic reactions have begun recently. The most conspicuous interval is situated in Unit IV, between 775 and 805 mbsf. This interval has a porosity of ~60% (see "Physical Properties") and is sandwiched between sediments with ~40%-45% porosity, the prevailing porosity being from ~450 mbsf to the bottom of the section.
Cl concentrations were determined with a relative analytical uncertainty of 0.1% based on duplicate or triplicate titrations of all samples. Chloride concentrations are similar to modern bottom water in this region and smoothly vary within a range of ~1% between 307 and 406 mbsf. Between 406 and 421 mbsf, there is a sharp discontinuity with concentrations dropping by 2.7%. This discontinuity is just below the designated boundary of lithostratigraphic Units I and II and is coincident with a ~10% reduction in average porosity. Within the remainder of Unit II, concentrations are nearly constant.
Pore fluid Cl concentrations in lithostratigraphic Units III and IV are characterized by a large range with relatively large changes occurring over short intervals. Some of the variance may be partially due to contamination of samples by surface seawater that is used as a drilling fluid. However, in Unit IV all of the samples have sulfate concentrations (see "Sulfate") that constrain the magnitude of drilling fluid contamination to <6%. This demonstrates that most of the Cl variance is not due to drilling fluid contamination. In Unit III, sulfate is not a highly sensitive indicator of contamination because the pore fluid sulfate concentrations are similar to surface seawater concentrations. Unit III contains numerous intervals with abundant sand, which is probably more susceptible to drilling fluid contamination. Hence, as a precaution, we have not plotted data with sulfate concentrations >25 mM in this unit, although these data are presented in T13. Further shore-based chemical and isotopic analyses should resolve this question.
Within Unit III, there is a general decrease in Cl concentrations; the lowest concentrations are ~7% lower than those of seawater. This freshening is most likely due to the dehydration of smectite and possibly is enhanced by Cl uptake into an authigenic hydrous phase. In Unit IV, Cl concentrations range from ~2% greater to 6% less than modern average seawater concentrations, and one sample has Cl concentration >15% below modern average seawater concentrations. Hydration of volcanic ash layers and the underlying basement, as well as smectite dehydration, most likely contributes to the Cl variability.
Sulfate concentrations are below detection limits through Unit I and the first 10 m of Unit II. They then smoothly increase through Unit II to a concentration plateau of ~25 mM at ~520 mbsf in Unit III. Concentrations drop back to zero in the 50-m interval between 700 mbsf and the Unit III/IV boundary. The sulfate depletions in Units I and IV have been driven by microbial sulfate reduction. In contrast, the near-seawater concentrations in Unit III indicate that since the time of burial, there has been relatively little microbial activity in this unit. This is probably the result of the low average sedimentation rate in Unit III (see "Biostratigraphy"), which allowed microbial oxidation of the sediment organic matter and simultaneous sulfate diffusion into the turbidite section prior to burial. Indeed, the organic carbon content is very low (generally <0.5 wt%), except for woody fragments in some horizons, which are not easily utilized for microbial activity.
Along with the sharp gradients at the Unit III boundaries, the high sulfate concentrations imply that there is limited diffusive transport across these boundaries. Assuming one-dimensional vertical diffusion, diffusive flux and scale-length calculations indicate that the effective bulk diffusion across these boundaries is on the order of 1 × 10-7 cm2/s or less, more than an order of magnitude lower than diffusion coefficients calculated from porosity and formation factor data (see "Physical Properties"). Based on the diffusion coefficients calculated from porosity and formation factor data, sulfate should have been depleted by diffusive transport, both upward and downward, out of the unit in <1 m.y. after burial by the Shikoku Basin sediments (~6 Ma). Alternatively, discontinuities may indicate the recent onset of a sulfate-consuming reaction in Unit IV or a sulfate-producing reaction in Unit III.
Overall Na concentrations are similar to Cl concentrations. In Unit I, there is a limited concentration range with a smooth increase with depth; no adjacent samples vary by >0.6%. In contrast, Unit III is characterized by an average concentration ~4% lower than in Unit I with a range of nearly 10%. The Na/Cl ratios in Unit I are greater than those of seawater, consistent with the addition of Na to the solution by ion exchange or ash alteration. The ratio varies smoothly over a range of <1% in Units II and III. In the high porosity zone in Unit IV, Na concentrations are higher than those of seawater, most likely because of Na release and hydration reactions related to ash alteration; ion exchange with clays may have played a minor role.
Between 406 and 421 mbsf, K has a sharp discontinuity; concentrations drop by ~50%. The K profile overall resembles that of Cl, suggesting that they are coupled by the same or related reactions. In Unit I, concentrations decrease smoothly from ~8 to ~6 mM at 409 mbsf. Similar to Cl, Unit II K concentrations, at 4 mM, are effectively invariant. In Unit III, there is a broad minimum of ~2 mM, centered at ~575 mbsf. Similar to Cl, there is a wide dispersion of concentrations in Unit IV. However, the relative concentration range is much greater; concentrations vary by approximately a factor of two with K and Cl correlated. The low concentrations and the variability of K concentrations in Unit II imply an in situ pore fluid-sediment silicate reaction with a sink for K superimposed on the dilution reflected in the Cl profile. In Unit IV, K concentrations sharply increase to approximately average seawater concentration within several horizons. This may reflect in situ dissolution of volcanic ash or sealing of pore fluids within this unusual high-porosity unit.
Mg profiles are similar to sulfate profiles in that concentrations are depleted in Units I and IV and there is a broad zone with near-seawater concentrations in Unit III. In Unit I, concentrations decrease smoothly from ~33 mM at 306 mbsf to 30 mM at 406 mbsf (39%-44% depletion relative to modern seawater concentration). Between 406 and 421 mbsf, there is a concentration discontinuity as there is with Cl. Similar to sulfate, Mg increases in Unit II to ~51 mM (~94% of modern seawater concentration). In Unit IV, Mg concentrations decrease to a minimum of ~17 mM (32% of seawater concentration). Except for a few carbonate-rich layers in Unit III, carbonate contents are low in the sediments at this site. Therefore, the formation of dolomite is most likely not responsible for the Unit I and Unit IV depletions in Mg. Instead, reactions with ash, as well as with basement, in Unit IV to form a Mg silicate are the most likely sink for Mg.
Ca concentrations are depleted by ~25% relative to seawater at 306 mbsf (Section 190-1177A-1R-4). This depletion may be due to the precipitation of carbonates, which is driven by the production of alkalinity that is associated with sulfate reduction at shallower depths. Concentrations then increase through Units I and II, reaching a maximum in Unit III of ~20 mM at 666 mbsf. In Unit IV, concentrations fall into a limited range of 15.5-17.7 mM, except in the same sample that has the lowest Cl concentration (Section 190-1177A-49R-3). The Ca increase is most likely due to the alteration of ash and the underlying basement.
Alkalinity is elevated in Unit I by 15 to 20 mM as a result of the sulfate reduction that has occurred in this unit. In the limited number of samples analyzed in the remainder of the core, it is less than ~8 mM and appears to be controlled by carbonate solubility as it varies inversely with Ca.
Silica concentrations are in equilibrium with opal-A solubility at 20°-25°C in Units I and IV; both units have about the same porosity. In Units II and III, concentrations decrease to ~200 µm, which indicates control by a mature terrigenous clay assemblage.
Ammonium decreases sharply through Unit I from 2250 µM at 306 mbsf to 750 mM at the Unit I/II boundary, indicating an ammonium sink which is not yet determined. Throughout the remainder of the core, ammonium concentrations vary within a limited range of 380-950 µM.