Site 1250 was drilled on the summit of Hydrate Ridge, where we expect fluid advection to be recorded by geochemical tracers as previously observed at Site 1249. At Site 1250, we collected 73 samples for interstitial water (IW) analyses from four holes. Hole 1250C was sampled at a resolution of approximately two samples per core, for a total of 31 samples. Hole 1250D was dedicated to microbiological studies, and we collected 15 samples in a coordinated program with the shipboard microbiologists. Three samples were collected from the near-surface sediments in Hole 1250E. From Hole 1250F, we collected 24 samples, 15 of which were collected at a resolution of one sample per section in the 100- to 130-mbsf interval to resolve chemical signals that could allow us to test the possibility of gas hydrate presence in the second BSR. In addition, sampling in Hole 1250F was aimed at constraining fluid flow through sediments imaged as Horizon A in the seismic data (see "Introduction"). Additional samples extended into deeper sediments that were not penetrated by prior boreholes. The IW geochemistry data are tabulated in Table T3 and are illustrated in Figure F16.
Site 1250 was cored to a TD of 180 mbsf (Hole 1250F), which is below the depth of the seismic reflector known as Horizon A. Thus, the composition of the IW at this site is influenced by gas hydrate geochemistry and by the possible migration of fluids, which might ultimately lead to the formation of authigenic carbonate deposits at and below the seafloor. The most obvious expression of authigenic carbonate formation is a massive buildup known as the Pinnacle, which is located immediately west of Site 1250.
The pore fluids recovered from the upper 50 mbsf at Site 1250 show an enrichment in dissolved chloride relative to background seawater values. In Hole 1250C, Cl- reaches a maximum value of 598 mM at 5.07 mbsf (Sample 204-1250B-2H-CC), and in Hole 1250E, it reaches 613 mM at 13.9 mbsf (Sample 204-1250C-2H-5, 140-150 cm). These enrichments are significantly lower than those measured at Site 1249, where chloride concentration in whole-round IW samples reaches 1008 mM. As discussed for Site 1249, the high chloride content in the pore fluid reveals that at Site 1250 the rate of gas hydrate formation exceeds that at which excess salts can be removed by diffusion and/or advection. (see "Interstitial Water Geochemistry" in the "Site 1249" chapter).
The chloride anomalies observed in the whole-round IW samples reflect not only the chloride enrichment in situ, but include a component of freshening resulting from gas hydrate decomposition during core retrieval and processing. We have not attempted to estimate the amount of hydrate in the upper 50 mbsf because we do not know the chloride composition of the in situ pore fluid before it is freshened by gas hydrate dissociation.
Below 50 mbsf, the chloride profile shows a gently sloping baseline toward slightly fresher values (Fig. F17A). The background chloride values correspond fairly well to the concentrations obtained for Site 1245, where there is no indication of chloride enrichment (see "Interstitial Water Geochemistry" in the "Site 1245" chapter). Thus, we have estimated the amount of gas hydrate in this portion of the sedimentary sequence to reach 15% of pore space, with average values ranging from 0% to 6% (Fig. F17B).
There is no change in the chloride concentration below the depth of the BSR at ~114 mbsf. Chloride remains at a constant value of 544 ± 3 mM from the base of the gas hydrate stability zone (GHSZ) to the bottom of the borehole (Fig. F17). Thus, there is no indication in the chloride data of gas hydrate presence below the BSR, as might be suggested by the second BSR observed in the seismic data (see "Introduction").
The presence of authigenic carbonate deposition at and near the seafloor in the proximity of Site 1250 has been previously associated with methane seepage (Teichert et al., in press; Johnson et al., in press). Accordingly, sulfate concentrations are near zero in the shallowest sample (Sample 204-1250C-1H-1, 0-10 cm) and remain near zero downhole (Table T3; Fig. F16). This is because seawater sulfate and upward-moving methane are co-consumed by a consortium of microbes carrying out the net process of anaerobic methane oxidation (AMO) (e.g., Reeburgh, 1976; Boetius et al., 2000). This process has been discussed for Sites 1248 and 1249 (see "Interstitial Water Geochemistry" in the "Site 1248" chapter and "Interstitial Water Geochemistry" in the "Site 1249" chapter). Alkalinity is anomalously high in the upper tens of meters (Table T3; Fig. F16) as compared to nonseep locales like Site 1244, also reflecting advection of fluids. Alkalinity increases rapidly with depth as methane carbon, delivered to the shallow subsurface by advection, is converted to dissolved carbon dioxide by AMO.
The advection of calcium-depleted fluids, plus the local depletion of calcium resulting from authigenic carbonate formation near the seafloor, results in a calcium profile showing very low calcium concentrations within the entire borehole. Similar depletion in dissolved calcium was observed at Site 1249 (see "Interstitial Water Geochemistry in the "Site 1249" chapter) and was reported by Torres et al. (2002) in sediments covered by bacterial mats on the summit of southern Hydrate Ridge.
Consistent with observations at Sites 1248 and 1249, we observe very high barium concentrations in the near-surface fluids, which result from upward flow of sulfate-depleted, barium-rich fluids. This barium transport results in large benthic fluxes of barium at cold-seepage sites (e.g., Torres et al., 1996).
The lithium profile (Fig. F18) shows a trend to increasing values with increasing depth, which has been observed at all sites drilled during Leg 204. This increase is believed to result from remobilization of lithium from aluminosilicates at depth, a process that has been observed at the Cascadia and other accretionary margins (e.g., Kastner et al., 1995; Chan and Kastner, 2000). Lithium shows positive deviations from this linear trend at the depths associated with the seismic reflector known as Horizon A at Sites 1245 and 1248 (see "Introduction"; and "Interstitial Water Geochemistry" in the "Site 1245" chapter; "Interstitial Water Geochemistry" in the "Site 1247" chapter; and "Interstitial Water Geochemistry" in the "Site 1248" chapter). At Site 1250, a similar lithium increase is observed at the depth of the Horizon A reflector, possibly indicating transport of fluids from a depth deeper than 1 km, where the temperature has reached that exceeding the threshold needed for lithium release (>80°-100°C) (e.g., Edmond et al., 1979; Seyfried et al., 1984).