From Holes 1174A and 1174BA, 127 interstitial water samples were squeezed from selected 10- to 50-cm-long whole-round samples for chemical and isotopic analyses. Sample depths ranged from 1.4 to 66.5 mbsf at Hole 1174A and from 150 to 1110.25 mbsf at Hole 1174B. Samples were collected from every section in Cores 190-1174A-1H and 2H, from three sections per core in Cores 190-1174A-3H and 4H, from two sections in Core 190-1174A-5H, and from every section in the remaining cores from Hole 1174A. From Hole 1174B, one section per core was sampled, except for Cores 190-1174B-71R and 72R, from which two sections were sampled, and Cores 30R, 48R, 58R, 63R, and 65R, which were not sampled because of either recovery problems or the need to squeeze a previously collected sample for a longer time in order to recover adequate volumes of pore fluid. Pore fluids from the four largest 50-cm whole rounds collected from Hole 1174B (Cores 190-1174B-23R, 34R, 43R, and 78R) were subsampled for interstitial water and He isotope analyses.
Elemental concentrations are reported in Table T14 and plotted in Figure F33. For large samples, nine major and minor dissolved anions and cations that sensitively reflect microbially mediated or inorganic water-rock (sediment and oceanic basement) reactions were determined. The former include alkalinity, sulfate, and ammonium, and the latter are Cl, Ca, Mg, Na, K, and Si. On smaller samples, a subset of these elements was determined, depending on the quantity of fluid recovered. Salinity and pH were also measured.
The main characteristics of the interstitial water concentration-depth profiles at Site 1174 are similar to those at Site 808 (Leg 131). Compared to Site 1173, microbially mediated reactions at Site 1174 are more intense and are significant to greater depths. The sulfate reduction is complete by 4 mbsf at Site 1174, and peak ammonium concentrations at Site 1174 are approximately twice those at Site 1173, centered at 200 mbsf as compared to 50 mbsf. There are major changes in chemical gradients associated with both lithostratigraphic boundaries (Units II and III, the trench wedge, and Shikoku Basin facies) and subtle changes associated with tectonic features (protothrust and décollement). At the depth that corresponds to the protothrust intersection and Unit II/III boundary, there are significant excursions, most distinctly exhibited in the Cl and Si concentrations, that may indicate hydrologic activity. A high-resolution record of pore fluid chemistry was recovered across and within the Nankai Trough décollement for the first time. The chemical changes across the décollement are subtle despite significant changes in physical properties there. Steep concentration gradients, particularly in Cl, Na, and Ca, are observed in the deepest ~50 m of the site, between 1060 and 1110 mbsf, caused by either diagenetic reactions involving hydration in the basal volcaniclastic section or by diffusional communication with an upper basement fluid flow system.
Cl concentrations were all determined in duplicate resulting in a relative analytical uncertainty of 0.1%. Concentrations increase from slightly greater than bottom water concentrations (558.5 mM) to 577 mM (a 3.3% increase) at a depth of 21 mbsf. This trend is most likely due to the diffusion of lower chlorinity interglacial water into the sediments and the hydration of volcanic ash. Concentrations then decrease to a minimum value, 566 mM, at the Subunit IIA/IIB lithostratigraphic boundary (~337 mbsf) (see "Lithostratigraphy"). Between 337 and 468 mbsf, concentrations increase to 597 mM, where there is both a sharp reversal in the overall gradient and localized concentration spikes. It is unclear from the data whether the reversal corresponds to the Unit II/III lithostratigraphic boundary (483.2 mbsf) or to the protothrust intersection (~470 ± 5 mbsf). This gradient reversal and the localized spikes could be maintained by any combination of (1) ongoing dehydration reactions in compositionally distinct layers, (2) sustained flow of freshened fluid from depth, or (3) episodic flow of freshened fluids from depth. Below ~500 mbsf, concentrations smoothly decrease for ~230 m, consistent with this interval being dominated by diffusive transport. Assuming one-dimensional vertical diffusion, an average porosity of ~45%, a formation factor of 10 (see "Physical Properties"), and molecular diffusion of 5.74 × 10-5 cm2/s corrected for an in situ temperature of ~90şC, it would take on the order of 650 k.y. to establish this diffusion profile.
Within the décollement (808-840 mbsf) (see "Structural Geology"), there is a local Cl maximum (496 mM). Cl concentrations smoothly increase to the maximum value for ~50 m above the upper boundary of the décollement. In contrast, at the lower boundary of the décollement, there is a very sharp decrease of ~10 mM over 10 m. The cause of the Cl maximum in the décollement is as yet unclear.
At greater depth, there is a low-Cl zone in the 200 m interval below the décollement, with minimum concentrations that are ~17% diluted relative to seawater. The Cl minimum is found at an almost identical distance below the décollement at adjacent Site 808. Note, however, that the dilution is ~21% at Site 808, whereas at the reference Site 1173A it is considerably less, close to 9%. Below the Cl minimum, in a 100-m interval (~950 to 1050 mbsf), concentrations slightly increase with depth, with minor discontinuities superimposed on the general gradient. In the lowermost ~100 m of the underthrust section, Cl concentrations increase monotonically, approaching seawater concentrations at 1110 mbsf. Hydration reactions in the lower volcaniclastic or an underlying upper basement fluid flow system may be responsible for the increase in the Cl concentrations.
Na concentrations increase from a near-seawater value to a broad maximum, an 8.5% increase, between ~30 and 300 mbsf. This increase is due to a combination of processes: diffusion of low-salinity interglacial water, ion exchange driven by ammonium production, and ash alteration. Except for the discontinuity at the depth of the protothrust (between ~450 mbsf and basement), concentrations decrease, generally following the Cl trend, except in the deepest samples, where Na does not follow the Cl increase, indicating the uptake of Na into the basement.
K concentrations decrease smoothly from slightly greater than seawater concentrations to approximately half seawater concentrations at 414 mbsf, where there is a sharp discontinuity associated with the Subunit IIB/IIC boundary. Concentrations decrease by ~30% in 16 m. This is a large decrease, especially in the depth interval of high ammonium concentrations, where K is being expelled by ammonium into the pore fluid from clay ion exchange sites. This decrease suggests that K is being incorporated into authigenic zeolite. Below this depth, concentrations asymptotically approach a nearly constant value of ~1.5 mM.
The higher than seawater concentrations are caused by K expulsion from clay mineral ion exchange sites, partial dissolution of volcanic ash, and slightly elevated concentrations in the glacial ocean. The nearly constant concentrations at depth are consistent with equilibrium control by potassium-containing silicate phase, primarily K zeolites.
Dissolved Si concentrations increase from ~600 to 1250 mM between 275 and 315 mbsf, corresponding to the Subunit IIA/IIB boundary. There is a small decrease to ~1000 µM at the Subunit IIB/IIC boundary, across which concentrations drop to ~200 µM. Below this depth, concentrations generally increase, approximately doubling by 1050 mbsf. This general trend is interrupted by a few discrete maxima that correspond to velocity maxima (see "Physical Properties") that are most likely caused by diagenetic reactions associated with ash layers, leading to some cementation. Thus throughout the section, the Si concentration-depth profile reflects the varying dominant silicate diagenetic reactions. The key reaction in Subunits IIB and IIC is the dissolution of siliceous biogenic material that rapidly diminishes in abundance and eventually disappears at the base of Subunit IIC (see "Biostratigraphy"). This is consistent with a shift in grain density at the Unit II/III boundary (see "Physical Properties"). In Unit III and through the décollement in Unit IV, the control seems to be quartz solubility, and the few maxima are most likely related to ash alteration. Diagenesis of the basal silicoclastics is responsible for the doubling of the silica concentrations below the décollement.
Mg concentrations generally decrease with depth, whereas calcium concentrations generally increase. There are, however, significant changes in their concentration gradients that indicate the occurrence of a variety of distinct reactions. Mg and Ca concentrations decrease sharply in the uppermost 50 m. This decrease may be due to the formation of dolomite. Mg concentrations then decrease to a minimum at the Subunit IIA/IIB boundary depth. At greater depth, Mg concentrations only vary within a narrow range. There appears to be an increase in Mg in the two deepest samples, but this may be an artifact of the high Ca concentrations on the Mg determination.
Ca concentrations increase smoothly below the shallow minimum until ~50 m above the Subunit IIA/IIB boundary. At this depth, concentrations increase up to the boundary. There is another sharp increase at the Subunit IIB/IIC boundary. Below this they are relatively constant through the décollement. Below the décollement, concentrations increase, indicating a possible source of Ca from the volcaniclastic section and/or oceanic basement.
Sulfate concentrations decrease extremely rapidly with depth and reach zero in Core 190-1174A-1H between 2.9 and 4.4 mbsf. Because of the very few data points available for this narrow sulfate reduction zone, it is difficult to distinguish between reduction resulting from local organic matter oxidation and reduction resulting from methane oxidation at the bottom of this zone. From 4 to ~640 mbsf, close to the base of Unit III, the sulfate concentrations vary between 0 and 1.5 mM. Most of these very low concentrations are probably related to small amounts of postsampling sulfide oxidation. Dissolved sulfate concentrations increase with depth in the lower Shikoku Basin section, reaching 8-10 mM below the depth interval of the Cl minimum and remaining constant to the bottom of the section. The same pattern of sulfate reappearance below the sulfate reduction zone has been documented at the reference Site 1173 and at Site 808. At Site 1173, the reappearance is found at ~240 mbsf, whereas at this site it is found at ~660 mbsf, 400 m deeper. Interestingly, the décollement is also ~400 m deeper than the proto-décollement at the reference site. At Site 808, as at this site, sulfate reappears at ~140 m above the décollement. The presence of dissolved sulfate at such great burial depths and at temperatures that range from being within the limits of biological activity to those that are above this limit may have important consequences for microbiology. The systematics of the sulfate distributions at these sites may reflect a dynamic relationship between sedimentation rates, temperature, and microbial sulfate reduction rates.
An ammonium concentration maximum of ~7 mM is found at ~13.6 mbsf; at Site 1173 it is at twice this depth. In Hole 1174B, ammonium concentrations continue to increase to 12.4 mM at ~170 mbsf. It is produced by the microbially mediated decomposition of organic matter. At such high concentrations, ammonium occupies clay ion exchange sites, expelling K and Mg into the pore fluid. Concentrations then decrease to <1 mM at ~600 mbsf. The deeper samples at Site 1174 were not analyzed for ammonium because of the very small recovery of pore fluids from the whole-round core samples. The deep sink for ammonium is as yet unidentified.
Alkalinity has a concentration maximum of 50 mM at ~6 mbsf in Hole 1174A. Its production is primarily driven by the microbial decomposition of organic matter. Alkalinity decreases monotonically to ~7 mM at 353 mbsf. At the depths of Ca and Mg concentration maxima, alkalinity concentrations sharply decrease, indicating authigenic carbonate (most likely dolomite) precipitation. At the alkalinity minimum, however, Mg decreases but Ca increases, suggesting either dolomitization of a precursor Ca carbonate or the flow of a low-alkalinity, low-Mg, high-Ca fluid. The latter is supported by the minimum Cl concentration at this same depth. The second and strongest minimum in alkalinity, 2.3 mM, is at the approximate depth of the protothrust and the Unit II/III boundary. The increase in alkalinity in Unit III toward the décollement is unusual and may be related to the increase in sulfate at this depth interval. The decrease in alkalinity below the décollement is caused by carbonate precipitation in this volcaniclastic high-Ca zone at the base of the section and/or in the upper basement.
In summary, the highly modified seawater pore fluid chemistry indicates that at this site fluid flow may occur or may have occurred in the recent past at three horizons: at the boundary between Subunits IIA and IIB, along the protothrust, and below the décollement. The basic shipboard geochemical analyses indicate similar pore water characteristics at all three depths: mostly CaCl2-type fluid, depleted in Cl, Na, Mg, alkalinity, and, possibly, K and enriched in Ca and Si relative to seawater. Additional shore-based geochemical analyses, mineralogical analyses, and hydrologic studies will be important in distinguishing between in situ pore-water modification, advection of modified fluids from greater depth, and some combination of the two. Furthermore, both carbonate and silicate diagenesis are widespread in a hierarchy of reactions, which may affect some of the measured physical properties. Microbially mediated reactions are most intense in the top 200 m of the section. The products are involved in some of the diagenetic reactions mentioned above.
Because this site is situated along a transect between the arcward Site 808 and the reference Site 1173, it plays an important role in understanding the evolution of the geodynamics, hydrogeology, and geochemistry of this subduction zone.