INTERSTITIAL WATER GEOCHEMISTRY

The main objectives of the IW program at this site were to provide geochemical proxies for the presence and abundance of gas hydrate and to establish constraints on the processes responsible for a circular "bright spot" observed on the seafloor reflectivity data (see "Introduction"). Two holes were cored at Site 1248, and we recovered 33 IW samples at a frequency of approximately two whole-round samples per core in both Holes 1248B (4 samples) and 1248C (29 samples). The IW geochemistry data are tabulated in Table T4 and are illustrated in Figure F12.

Site 1248 was cored to a TD of 149 mbsf (Hole 1248B), just below the depth of the seismic reflector denoted as Horizon A (see "Introduction"). This horizon lies just below the BSR; thus, the bulk of the IW data obtained at this site lies within the gas hydrate stability zone (GHSZ). The composition of the IW is influenced by gas hydrate geochemistry and by the possible migration of fluids that leads to the accumulation of these hydrate deposits in near-surface sediments. The distribution of dissolved chloride and sulfate provides important clues that can help us unravel gas hydrate dynamics and methane fluxes at this site. The downcore distribution of dissolved constituents in pore fluids can potentially be used to establish the presence and magnitude of upward fluid advection in accretionary margins (e.g., Torres et al., 2002). However, the data collected at Site 1248 cannot be used to quantify vertical flow rates because of the poor recovery of near-surface sediments and consequent low sample resolution. Further uncertainties are introduced by dilution of the pore water by gas hydrate dissociation during core recovery. Although in theory, the chloride data could be used to constrain the amount of freshening during gas hydrate recovery, these estimates are based on assumptions pertaining to the in situ chloride concentration, which in the near-surface sediments at Site 1248 is highly uncertain (see below).

The chloride profile at Site 1248 (Fig. F12) shows a pronounced anomaly in the upper 20 mbsf, which corresponds to a zone of gas hydrate presence near the seafloor. From 20 mbsf to the BSR (115 mbsf), the chloride distribution is characterized by excursions to lower concentrations superimposed on a linear trend assumed to reflect in situ chloride values. These chloride anomalies reflect dissociation of gas hydrates during recovery and can be used to estimate the amount of gas hydrate present in the sediments when used in conjunction with background in situ chloride data (see "Interstitial Water Geochemistry" in the "Explanatory Notes" chapter and "Interstitial Water Geochemistry" in the "Site 1244" chapter). We assumed a background concentration defined by the data points that appear to fit a smooth profile at this site, as indicated by the shaded portion of the chloride distribution (Fig. F13A). These background values correspond fairly well to the chloride concentrations obtained for Site 888, a site with no evidence of gas hydrate drilled west of the accretionary margin during Leg 146 (Kastner et al., 1995). Gas hydrate was indeed recovered from Site 1248 at depths corresponding to the observed chloride anomalies (Cores 204-1248C-1X, 3X, and 4X) (see "Lithostratigraphy"). In addition, thermal anomalies seen in IR data (see "Infrared Scanner" in "Physical Properties") and variations in the resistivity data (see "Downhole Tools and Pressure Coring") also suggest the presence of gas hydrate from the seafloor to the BSR. The percentage of hydrate in the pore space of sediments below 20 mbsf, calculated from the Cl^- anomalies, ranges from 0% to 5%.

Even though we cannot quantify the advective rate at this site based on IW data (because of the poor recovery in near-surface sediments), a qualitative comparison between Sites 1248 and 1244 is consistent with a much faster rate of methane advection at Site 1248 (Fig. F13). The sediments below 40 mbsf at both sites have discrete chloride anomalies that indicate the presence of gas hydrate. However, at Site 1244 the upper sediments (above 40 mbsf) do not contain enough methane to allow for hydrate formation, as revealed by the absence of chloride anomalies (Fig. F13B) and the presence of dissolved sulfate in the upper 10 mbsf (Fig. F13C). In contrast, at Site 1248 there is abundant hydrate in the near surface and there is no sulfate in the shallowest sample collected at this site (Sample 204-1248B-1H-1, 0-20 cm). We believe that this difference reflects active advection of methane-bearing pore fluids or free gas at this site.

Site 1248 is a locus for carbon cycling through the process of anaerobic methane oxidation (AMO), which occurs in the shallow subsurface. Sulfate concentrations are near zero in the shallowest IW sample (2.2 mM) (Sample 204-1248B-1H-1, 0-20 cm) and remain near zero downhole (Table T4; Fig. F12). Seawater sulfate and upward-moving methane are co-consumed near the seafloor by a consortium of microbes carrying out AMO:

CH₄ + SO₄^2- HCO₃^- + HS^- + H₂O

(Reeburgh, 1976). AMO typically occurs under diffusive conditions at the sulfate/methane interface (SMI) (for a discussion see "Interstitial Water Geochemistry" in the "Site 1244" chapter). However, because at Site 1248 methane is delivered by advection of water and/or free gas, typical biogeochemical zones (e.g., Froelich et al., 1979) are not present and sulfate reduction of sedimentary organic matter may decrease in importance. AMO occurring at methane seeps typically creates millimolar quantities of dissolved sulfide in the pore water (e.g., Paull et al., 1995; Sahling et al., 2002; Torres et al., in press). The sulfide concentration in the Leg 204 IW samples will be measured postcruise.

Alkalinity is anomalously high in the upper tens of meters of Site 1248 as compared to nonseep locales such as Site 1244 (Table T4; Fig. F12). Alkalinity increases rapidly with depth as methane carbon, delivered to the shallow subsurface by advection, is converted to dissolved carbon dioxide (CO₂) by AMO. High interstitial alkalinity also promotes carbonate deposition and likely explains authigenic calcium carbonate observed in the sediment (see "Lithostratigraphy").

Consistent with the depletion of sulfate throughout the core, the pore waters at this site have large dissolved barium concentrations. The dotted line in the barium distribution (Fig. F14) reflects the estimated in situ content, which reaches levels as high as 43 µM in Sample 204-1248B-1H-1, 0-20 cm, after correcting for dilution resulting from gas hydrate dissociation. These levels are much higher than the bottom seawater concentration, which at this site is ~9 x 10^-2 µM (M. Torres, unpubl. data). The presence of dissolved sulfate in the pore fluids constitutes a barrier to dissolved barium transport because barium precipitates as solid barite (BaSO₄) in these horizons. Figure F14 illustrates the difference in the barium distributions between Sites 1244, where there is sulfate in the upper 10 mbsf, and Site 1248, where fluid transport has depleted the sulfate even in the uppermost sample recovered at this site. At locations where advective flow results in transport of sulfate-depleted barium-rich fluids to the seafloor, barium is released to the bottom water (Torres et al., 1996, 2002). At Hydrate Ridge, large barium fluxes have been measured with benthic instrumentation (M. Torres, unpubl. data; Tryon et al., 2002), which is consistent with the observations obtained by drilling at this site.

The advection of methane likely influences the carbonate geochemistry at this site. The sediments in lithostratigraphic Unit I (0-39 mbsf) contain small carbonate nodules, an authigenic phase that was not observed at the other sites drilled during this leg (see "Lithostratigraphy"). The dissolved boron distribution at this site shows an increase in the upper 50 mbsf (Fig. F15). The excursions to lower boron values superimposed on this trend reflect dilution of the boron signal by hydrate dissociation. It is possible that the boron increase reflects an aragonite-to-calcite transformation (see "Interstitial Water Geochemistry" in the "Site 1244" chapter). Although the magnitude of the boron maximum at Site 1248 is similar to that observed at Site 1244, the dissolved strontium profiles are different between these two sites. Whereas no significant increase in Sr²⁺ was observed at Site 1244, at Site 1248, the dissolved strontium distribution parallels the boron profile, suggesting a stronger involvement of this element in carbonate diagenetic processes. Postcruise analyses of the carbonate fraction will enhance our understanding of the diagenetic processes associated with the carbon cycle on Hydrate Ridge. This is particularly significant at Site 1248, where advection of methane-rich fluids is probably responsible for the presence of scattered carbonates on the seafloor and within the sediments.