INTERSTITIAL WATER GEOCHEMISTRY

A total of 115 whole-round samples were collected for pore water analyses at Site 1251 (70 samples from Hole 1251B, 4 samples from Hole 1251C, 32 samples from Hole 1251D, and 8 samples from Hole 1251E). Routine samples were collected at a frequency of approximately two whole-round samples per core in the upper 200-225 mbsf, followed by a sampling rate of one whole-round sample per core below these depths. A higher-resolution sampling protocol was used within the anaerobic methane oxidation (AMO) zone in Hole 1251E, where sampling occurred at a frequency of two whole-round samples per section in a coordinated program with the shipboard microbiologists. Interstitial water (IW) geochemistry data are tabulated in Table T4 and are illustrated in Figure F12.

The distribution of dissolved chloride at Site 1251, shown in Figure F12, displays an approximately linear decrease with depth at a rate of ~0.46 mM/m to ~300 mbsf with deviations that might be related to gas hydrate dynamics. Similar decreases were observed at other sites drilled on the Cascadia margin (see "Interstitial Water Geochemistry" in the "Site 1244" chapter) (also see Kastner et al., 1995) and probably reflect diffusion of chloride ions from present-day seawater values to the low-chlorinity fluids that characterize the accreted sediments below 300 mbsf. The onset of low-chloride fluids observed between 300 and 310 mbsf corresponds to a seismic reflector that might image the top of the accreted sedimentary wedge (see "Introduction"). This horizon also corresponds to the top of lithostratigraphic Unit III (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy") and the onset of borehole breakouts in the LWD data (see "Logging While Drilling" in "Downhole Logging"). As discussed for Site 1244, the chloride decrease in the IW at Site 1251 corresponds to an increase in dissolved lithium (Fig. F13), suggesting a fluid source deeper than 1 km. This inference is similar to the interpretation based on IW data from Sites 889 and 890 on the Cascadia margin off Vancouver Island (Kastner et al., 1995). At Site 1251, the deep-seated fluids have chloride concentrations of 435 mM. Similarly, the dissolved chloride in the lower sections of Hole 1244C averaged 472 mM (see "Interstitial Water Geochemistry" in the "Site 1244" chapter). In contrast, the deep fluids at Sites 889 and 890 have an average concentration of 370 mM (Fig. F14). The difference might reflect variations in the composition of the sediments and/or variations in the depth to which accreted sediments have been buried.

The dissolved chloride distribution in the sediments above the BSR is different from all the other sites drilled during Leg 204. The other sites all show repeated excursions to low chloride values throughout much of the GHSZ. In contrast, at Site 1251, we only observe a significant chloride anomaly just above the BSR, indicating that significant amounts of gas hydrate are present only near the base of the GHSZ, with very little gas hydrate present in the sediments above. Drilling in Hole 1251B had no recovery of gas hydrate near the BSR (see "Operations"), and no hydrates were identified by the chloride data above or below the BSR in Hole 1251B (Fig. F14). However, the second attempt to drill through the BSR at this site recovered Core 204-1251D-24X, whose infrared (IR) temperature data indicated the presence of gas hydrate throughout the core (see "Physical Properties"). The four IW samples taken from Core 204-1251-24X (Table T4) all show chloride anomalies indicating the presence of gas hydrate (Fig. F14). Larger anomalies were observed in Sections 204-1251D-24X-3 and 24X-4 and correspond to gas hydrate occupying ~20% of the pore space.

The clearest record of the sulfate gradient and the position of the sulfate/methane interface (SMI) comes from Hole 1251E (for a general discussion on the SMI see "Interstitial Water Geochemistry" in the "Site 1244" chapter). Here, the SMI is located at ~4.5 mbsf (Fig. F15), where interstitial sulfate first reaches a minimum concentration concomitant with increasing methane concentration as documented by headspace gas data from Hole 1251C (see "Organic Geochemistry"). The zone where AMO occurs was sampled extensively by the microbiology team at this site (see "Microbiology"). The sulfate profile is approximately linear between 1.5 and 4.5, with curvature at the top and bottom of the profile that probably reflects oxidation of sedimentary organic matter by sulfate and/or fluid advection processes.

Following the method developed by Borowski et al. (1996) and outlined at Site 1244 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter), the estimated methane flux at Site 1251 is 5.5 x 10^-3 mM/cm²/yr, based on a sulfate gradient of 8.8 mM/m (estimated from the linear portion of the curve) (see Fig. F15), a sulfate diffusion coefficient of 5.8 x 10^-6 cm²/s at 5°C, and an average porosity of 0.70%. For comparison, the estimated methane flux at Site 1251 is about twice that calculated for Site 1244 and about three times larger than that estimated at the Blake Ridge, a large passive-margin gas hydrate terrain. These estimates assume methane delivery through diffusion only and that the linear portion of the sulfate curve is mainly created by sulfate demand at the SMI. If significant advection of water or free gas occurs or if sulfate depletion through AMO is of minor importance, then this estimate is invalid.

The early diagenesis of marine sediments is often dominated by organic matter decomposition (e.g., Berner, 1980). Interstitial alkalinity (a proxy for total dissolved CO₂) (CO₂), ammonium (NH₄⁺), and phosphate (PO₄^3-) concentrations increase with increasing depth, reaching maximum concentrations at ~40, ~80, and ~100 mbsf, respectively (Table T4; Fig. F12). From the shape of the profiles, it is likely that the microbial decomposition of sedimentary organic matter is most active in the upper 100 mbsf of the sedimentary section. During postcruise shore-based research, we will have the opportunity to correlate microbial abundance and activity to key interstitial constituents that identify organic matter decomposition.

The alkalinity profile at Site 1251 is curious in that an anomalous deflection toward lower values occurs between 50 and 90 mbsf (Table T4; Fig. F12). This decrease in alkalinity is not an artifact because data from Holes 1251B and 1251D show the same pattern. Localized decreases in interstitial concentrations of CO₂ are often a result of authigenic carbonate formation. However, there is no macroscopic evidence for authigenic carbonate precipitation in this portion of the stratigraphic sequence, and sediment carbonate content is not elevated in the zone of lowered alkalinity (see "Lithostratigraphy"). Detailed shore-based sampling and microscopic and chemical analyses may resolve the question.

The distribution of the dissolved ions in pore fluids often (Fig. F12; Table T4) provides clues on the nature of the fluid sources, diagenetic reactions, and microbiological processes within these sediments. We will discuss the pore water distributions in lithostratigraphic Units I and II, followed by changes observed in lithostratigraphic Unit III (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy").

The increase in dissolved barium to 75 µM following depletion of dissolved sulfate at 4.5 mbsf follows the general trend described at other Leg 204 sites (Fig. F16). The overall increase in dissolved boron in the upper 40 mbsf (Fig. F12), similar to that observed at Site 1244, may be related to carbonate diagenesis (e.g., Deyhle et al., 2001), as evidenced by the similarity between alkalinity and boron distributions. As described for Site 1244, the transformation of aragonite to calcite may release boron and magnesium into the IW. Such a process has been documented by laboratory studies (Kitano et al., 1978) and by boron distribution in carbonate phases (Deyhle et al., 2001). Although the upper 23 mbsf show low dissolved iron content, the sediments between 23 and 130 mbsf show tremendous variability in the dissolved iron values in both Holes 1251A and 1251B. This section of high iron concentrations corresponds to lithostratigraphic Subunits IA, IB, and IIA, which are all characterized by the presence of abundant sulfide minerals (Fig. F17). The onset of high iron in the pore fluids corresponds to the top of a debris flow deposit (see "Lithostratigraphy") and a large excursion in the MS data (see "Magnetic Susceptibility" in "Physical Properties"). Recycling of iron below the depth of sulfide depletion is likely to be responsible for the observed iron distribution.

The increase in lithium and strontium within the deep accreted sediments of lithostratigraphic Unit III reflects release of these ions from aluminosilicates at depth, as discussed for Site 1244 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter). This zone is similar in composition to that reported for Sites 889 and 890 in the northern section of the accretionary prism (Kastner et al., 1995). In addition to high lithium and strontium, the sediments corresponding to lithostratigraphic Unit III (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy") show a marked change in the dissolved barium and boron content. Particularly striking at Site 1251 is the sharp increase in dissolved boron in the pore fluids of accreted sediments below 315 mbsf. These high levels (~1.3 mM) result from the loss of boron from aluminosilicate minerals during the accretion of sediments. Tectonically expelled waters in subduction zones have been shown to have elevated boron contents (up to 2 mM), and their distribution and isotopic composition have proven useful in elucidating fluid processes in these environments (You et al., 1996; Spivack and You, 1997). The drastic increase in dissolved boron at Site 1251 likely demarks the contact with the accreted sediments at this site.