The main objectives of the interstitial water (IW) program at this site were to measure geochemical proxies for the presence and abundance of gas hydrate, which had been previously observed in near-surface sediments at this location (Suess et al., 2001) and resulted in poor recovery in the upper 40 mbsf. Twelve holes were drilled at this site, and we collected a total of 49 whole rounds for pore water analyses. Seven samples were taken from Hole 1249B, which was drilled with the new RAB-8 instrument (see "Operations" in "Logging While Drilling" in "Downhole Logging"). These data were collected to compare the quality of samples recovered using this tool with that of the conventional ODP coring devices. Hole 1249C was cored in its entirety with the APC and sampled for IWs at a resolution of approximately two samples per core, for a total of 20 samples spanning the entire depth of the borehole. In Holes 1249D and 1249E, pressure coring devices were deployed to recover gas hydrate from the upper 30 mbsf, where core recovery was generally poor. We collected two samples from Hole 1249D and one sample from Hole 1249E, in an effort to better constrain the chloride distribution in these upper sediments. Hole 1249F was heavily sampled for microbiological studies. We collected nine samples from this hole in a coordinated program with the shipboard microbiologists. The IW geochemistry data are tabulated in Table T4 and are illustrated in Figure F13.
Site 1249 was cored to a TD of 90 mbsf (Holes 1249B and 1249F), thus the bulk of the IW data obtained at this site lies within the gas hydrate stability zone (GHSZ). The composition of IWs in this zone is influenced by gas hydrate geochemistry and by the effects of fluid advection. Fluid flow in this area has been shown to reach rates of 100-300 cm2/yr (Torres et al., 2002; Tryon et al., 2002).
The pore fluids recovered at Site 1249 show a pronounced enrichment in the dissolved chloride values, which in Hole 1249B reach a maximum value of 1008 mM at 6.96 mbsf (Sample 204-1249B-3H2, 41-51 cm). During gas hydrate formation, ions are excluded from the hydrate structure and will accumulate in the interstitial fluids. This process causes the pore water to become saltier. If the rate of hydrate formation is slower than that of ion diffusion and/or that of advective transport, the excluded ions will be removed from the zones of hydrate formation by advection or diffusion. When this is the case, gas hydrate dissociation during core recovery results in freshening of the pore fluids by addition of water formerly sequestered by gas hydrate (Hesse and Harrison, 1981) (see "Interstitial Water Geochemistry" in the "Explanatory Notes" chapter). In these situations, negative chloride anomalies relative to in situ concentrations are proportional to the amount of gas hydrate. These negative anomalies, relative to background in situ values, have been observed at other sites drilled during Leg 204 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter; "Interstitial Water Geochemistry" in the "Site 1245" chapter; "Interstitial Water Geochemistry" in the "Site 1246" chapter; and "Interstitial Water Geochemistry" in the "Site 1248" chapter).
However, at Site 1249, we infer that methane hydrate is forming at a faster rate than ions can be removed from the surrounding fluid, leading to the observed positive chloride anomalies (i.e., chloride values that are saltier than baseline values) (Fig. F14). Suess et al. (2001) reported such chloride enrichments in fluids recovered at 1.2 mbsf from the southern summit of Hydrate Ridge. The chloride anomalies observed by drilling at Site 1249 confirm the presence of a gas hydrate-generated brine at this location and extend the minimum depth of high chloride values to 10 mbsf.
The chloride anomalies observed in the whole-round IW samples reflect not only the in situ enrichment but include a freshening component resulting from gas hydrate decomposition during core retrieval and processing. In an effort to recover the most pristine sample that would reflect the in situ chloride concentration, we inspected and collected 10 samples from the working half of Cores 204-1249F-3H and 7H, as listed in Table T5, within 90 min of core retrieval. The dissociation of gas hydrate resulted in heterogeneous textures within the core. Some sediment retained its original structure ("dry appearance"), whereas nearby material had liquefied during gas hydrate dissociation leading to very unconsolidated sediment ("wet appearance"). Sediments with unconsolidated mousselike textures have been used by the sedimentologists to infer locations where gas hydrate was present in the core (see "Lithostratigraphy"). An example of such a heterogeneous texture, induced by very localized dissociation of gas hydrate, is shown in Figure F15. Paired samples (~10 cm3) were collected from both consolidated and mousselike sediments for pore water analyses of chloride content (shipboard) and isotopic characterization of the water (shore based). Companion samples were taken for measurement of moisture and density (MAD) (see "Physical Properties" in the "Explanatory Notes" chapter). The shipboard results of this experiment are listed in Table T5. In all cases, samples collected from the "wet-looking" sediment indeed have a significantly higher water content and a chloride concentration that was significantly lower than a nearby sample, which had retained its coherent structure. These differences are illustrated in Figure F14B. Pore fluids in the "dry-looking" samples had the highest chloride concentrations, with a maximum of 1368 mM (Sample 204-1249F-3H-1,76-78 cm [9.76 mbsf]).
The high chloride content in the pore fluid reveals that gas hydrate at Site 1249 is forming in a system where the rate of gas hydrate formation exceeds the rate at which excess salts can be removed by diffusion and/or advection. In a perfectly closed system, complete dissociation of gas hydrate during core recovery will, theoretically, result in a chloride concentration equivalent to that of the pore fluid before gas hydrate formation. The fact that whole-round core samples from the upper 40 mbsf show a wide range of chloride values reflects the heterogeneity of the gas hydrate distribution and sampling artifacts. The sampling artifacts are induced in part by the selective removal of gas hydrate samples from the core for a variety of studies on the composition (see "Organic Geochemistry") and distribution (see "Lithostratigraphy") of these deposits.
Zero to near-zero concentrations of sulfate in the shallow subsurface (1.7 mM) (Section 204-1249C-1H-1 [1.10 mbsf]) identify Site 1249 as a locality where methane is delivered directly to the seafloor. Advection promotes the co-consumption of methane and bottom water sulfate by the microbially mediated process of anaerobic methane oxidation (AMO) (see "Interstitial Water Geochemistry" in the "Site 1248" chapter). This results in elevated pore water alkalinity as methane carbon is oxidized to form dissolved carbon dioxide (Table T4; Fig. F13).
Underwater vehicle surveys over the southern Hydrate Ridge summit revealed extensive bacterial mat coverage, clearly indicating communities that actively utilize methane. Indeed, dense aggregates of a structured Bacteria-Archaea consortium exist within Beggiatoa mats collected from active seeps on Hydrate Ridge (Boetius et al., 2000). This consortium apparently carries out the AMO at the sediment surface (e.g., Boetius et al., 2000; Reeburgh, 1976). Near-surface pore water samples (0-2 cm below seafloor) and benthic instrumentation deployed at these sites have documented fluxes of methane and H2S out of the sediments on the summit of Hydrate Ridge. Sahling et al. (2002) calculated H2S fluxes as high as 63 mM/m2/day, and Torres et al. (2002) document methane fluxes of 10-100 mM/m2/day out of mat-covered sites at this location.
The presence of a brine in the upper 20 mbsf affects the concentration of all dissolved species. In the same way as Cl- ions are excluded from the hydrate structure, so are other dissolved ions. This results in enrichments that are clearly noticeable in the Na+, K+, Ba2+, Sr2+, and Mg2+ distributions (Fig. F12). Superimposed on the enrichment resulting from brine formation is the effect of rapid advection of fluids that have been modified at depth and in near-surface sediments. For example, sulfate is so readily utilized by methane oxidizers that its concentration is below detection limits even within the brine. Sulfate in mat-covered sediments on Hydrate Ridge is consumed within the upper 5 cm below seafloor (Torres et al., 2002). The advection of calcium-depleted fluids, plus in situ consumption by carbonate formation, results in a profile showing very low calcium concentration within the entire depth sampled. Similar depletion in dissolved calcium was reported by Torres et al. (2002) in sediments covered by bacterial mats at the summit of southern Hydrate Ridge. At these sites calcium reaches the background levels of 2 mM at 2-5 cm below seafloor, consistent with advective transport at rates ranging from 50 to 100 cm/yr. The calcium and sulfate data obtained by drilling at Site 1249 is consistent with these observations.
Fluid advection and sulfate depletion also result in very large dissolved barium concentrations throughout the sediments. Even the shallowest sample analyzed (Sample 204-1249C-1H-1, 110-125 cm [1.10 mbsf]) has a concentration of 90 µM, which is three orders of magnitude higher than the barium content of the bottom water at this site (~9 x 10-2 µM) (M. Torres, unpubl. data). 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). At Hydrate Ridge, high barium fluxes have been measured with benthic instrumentation (M. Torres, unpubl. data; Tryon et al., 2002) that are consistent with the observations obtained by drilling at this site.