Site 1247 was cored to a TD of 220 mbsf (Hole 1247B). The interstitial water (IW) program at this site was aimed at providing geochemical proxies for the presence and abundance of gas hydrate and to establish constraints on the updip fluid flow along a sedimentary sequence imaged by the seismic reflector known as Horizon A. This horizon is present at 160 mbsf at this site (see "Introduction") and was previously sampled from 176 to 183 mbsf at Site 1245 (see "Lithostratigraphy" in the "Site 1245" chapter). We recovered 48 IW samples at a frequency of approximately two whole-round samples per core in the upper 140 mbsf, followed by a sampling resolution of one whole-round sample per core below this depth. The IW geochemistry data are tabulated in Table T3 and are illustrated in Figure F12.
The presence of excursions with low-chloride values above the BSR can be used to infer the presence and amount of gas hydrate in the sediments (see "Interstitial Water Geochemistry" in the "Explanatory Notes" chapter and "Interstitial Water Geochemistry" in the "Site 1244" chapter). At Site 1247, freshening chloride anomalies predict the presence of gas hydrate between ~51 and 116 mbsf. Thermal image data obtained with the IR camera suggest that hydrate is present in two major zones: the intervals between 40 and 50 mbsf and between 90 and 120 mbsf. The anomalies in the chloride content of the pore fluids (Fig. F13), however, only show the large effects of gas hydrate dissociation in the interval ranging from 110 to 120 mbsf. This discrepancy reflects the resolution limitations of the chloride anomaly technique in identifying zones of gas hydrate when it is present as distinct thin layers within the core. Any chloride anomaly associated with layered gas hydrates can only be detected if the pore water sample is collected within ~10 cm of the hydrate layer, as illustrated with a high-resolution experiment at Site 1245 (see "Interstitial Water Geochemistry" in the "Site 1245" chapter). Thus, a sampling resolution of two samples per core (i.e., a sample approximately every 4.5 m) can easily fail to identify discrete hydrate horizons.
The chloride anomalies measured in two samples recovered from near the base of the gas hydrate stability zone (GHSZ) indicate that gas hydrate occupies ~10% of the pore space, as illustrated in Figure F13. More commonly, the chloride data suggest that gas hydrates occupy between 0% and 2% of the pore space within the GHSZ.
High-resolution sampling in Hole 1247B allows firm characterization of sulfate and methane profiles as well as identification of the sulfate/methane interface (SMI) (Fig. F14). Sulfate generally decreases downcore within the sulfate-reduction zone as sulfate is removed from the sediments by sulfate reducers. The sulfate profile is linear between 6 and 9 mbsf, with strong curvature above 6 mbsf and below 9 mbsf. Minimal sulfate values combined with a rapid increase in methane headspace concentrations (see "Organic Geochemistry") locate the SMI at ~11 mbsf.
Although the sulfate profile shows curvature, the distinct linear portion of the curve may be caused by anaerobic methane oxidation (AMO) at the SMI. Following the method outlined at Site 1244 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter) (Borowski et al., 1996), we can estimate methane flux of 2.5 × 10-3 mM/cm2/yr, based on a sulfate gradient of 5.5 mM/m (Fig. F14), a sulfate diffusion coefficient of 5.8 × 10-6 cm2/s at 5°C, and average porosity of 63% (see "Physical Properties"). This estimated a methane flux is approximately the same as that calculated for Site 1244, ~30% less than that at Site 1251 and ~1.4 times greater than that estimated at the Blake Ridge.
These estimates assume methane delivery through diffusion only and that the linear portion of the sulfate profile represents sulfate demand at the SMI. If significant water or methane advection occurs or if sulfate depletion through AMO is of minor importance, then this estimate is invalid. The role of AMO in sulfate depletion can be assessed with knowledge of the isotopic composition of the methane gas and that of the dissolved inorganic carbon, sulfate, and sulfide, which will be carried out postcruise.
Diagenetic effects associated with carbonate geochemistry are apparent in the distributions of dissolved strontium and magnesium. These elements show a marked decrease in their concentrations in pore waters from ~8 to 15 mbsf, probably associated with the formation of authigenic carbonates such as those recovered at 19 mbsf (see "Lithostratigraphy"). Another zone marked by low strontium and magnesium concentrations is apparent below 160 mbsf, a zone that also shows a decrease in pore fluid alkalinity (Fig. F12). This zone corresponds to lithostratigraphic Unit III and is characterized by an increase in the nannofossil content of the sediments (see "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy"). It is possible that this biogenic component provides nucleation sites for authigenic carbonate formation, as previously discussed for Site 1245.
Dissolved iron and manganese have similar profiles, showing a coincident increase in their concentrations from the seafloor to ~30 mbsf (Fig. F12). This distribution is likely to reflect remobilization from iron-manganese minerals, which precipitate as sulfides below the SMI. Similar distributions have been observed at Sites 1244-1246 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter; "Interstitial Water Geochemistry" in the "Site 1245" chapter; and "Interstitial Water Geochemistry" in the "Site 1246" chapter). Postcruise analyses of the distribution and isotopic characterization of dissolved sulfide and of solid phases will provide constraints on the nature on the Fe-Mn biogeochemical cycling at sites drilled during Leg 204.
In the near-surface sediments, barium has very low concentrations (Fig. F12), as expected in pore waters where dissolved sulfate is present (see "Interstitial Water Geochemistry" in the "Site 1244" chapter). As sulfate is depleted, barium shows a marked increase to values >100 µM at depths below 25 mbsf. Of particular interest is the increase in dissolved barium observed between 130 and 140 mbsf, where it reaches 200 µM. A similar increase just below the base of the GHSZ was observed at Site 1245, with dissolved barium concentrations as high as 250 µM. This increase might reflect migration of barium-enriched fluids from deep sequences to the depth of the BSR, where gas hydrates serve as a barrier to upward fluid flow.
Dissolved lithium generally increases in concentration with increasing depth. Superimposed on this trend, lithium shows an enrichment in the pore fluids recovered from the depth of the seismic reflector known as Horizon A (160 mbsf) (see "Introduction"). A similar increase associated with Horizon A was observed at Site 1245 (Fig. F15). This increase suggests migration of lithium-enriched fluids from depths >1 km, where burial temperature exceeds the 80°C threshold needed for lithium release from alumnosilicates (e.g., Edmond et al., 1979; Seyfried et al., 1984). Gases collected from the sediments between the BSR (129 mbsf) and Horizon A (160 mbsf) also show enrichment of heavy hydrocarbons (see "Hydrocarbon Gases" in "Organic Geochemistry"), which is consistent with migration of fluids from a deep-seated source. These patterns correspond to observations previously recorded at Site 1245 (see "Interstitial Water Geochemistry" in the "Site 1245" chapter).