MATERIALS AND METHODS

Southern Hydrate Ridge was drilled and cored during Leg 204 in 2002 (Tréhu, Bohrmann, Rack, Torres, et al., 2003) (Fig. F1B). Three sites (Sites 1248, 1249, and 1250) were drilled at the ridge summit. Of these sites, Site 1250 has the deepest penetration and recovered ~180 m of sediments of Holocene to early Pleistocene age. The sediment is mainly composed of clay to silty clay with biogenic components, with turbidite layers intercalated as minor lithologies of the core. Three sites (Sites 1245, 1246, and 1247) were drilled at the northern and western flanks of the ridge. Site 1245 was cored deepest to a depth of ~500 mbsf. The upper sediments (~30 mbsf) at Site 1245 are mainly composed of clay with authigenic carbonate and foraminifer-rich interlayers and are devoid of gas hydrates. These sediments are underlain by diatom- or nannofossil-rich clay to silty clay with frequent sand-rich turbidites and some volcanic ash layers. At the eastern slope basin of the ridge (Sites 1244, 1251, and 1252), Site 1251 was cored deepest to a depth of ~440 mbsf in Holocene to upper Pliocene sediments. The sediment sequence is mostly composed of clay to silty clay with several turbidite and debris flow layers. The sequence between 130 and 300 mbsf at Site 1251 is enriched in siliceous and calcareous biogenic components; the deeper stratigraphic section is composed of partly lithified clay to claystone with glauconite grains and authigenic carbonate of various morphologies. Site 1252 is the only one of the nine drilled sites where no BSR is observed.

Whole-round sediment intervals (10–20 cm in length) were sampled soon after recovery according to standard ODP protocol. These were carefully skinned to remove potential seawater contamination during recovery and squeezed to extract interstitial water samples using a Manheim-type squeezing system. In addition, other water samples were collected from dissociated massive gas hydrate samples, gas hydrate–bearing sediments (sometimes identified by soupy or moussy texture), and dry-looking sediments near gas hydrate occurrences.

Dissolved chloride concentration was determined onboard by titration with silver nitrate with a precision better than 0.4% (Torres et al., 2004a). Aliquots of water samples were sealed in glass vials for postcruise stable isotope analyses at the University of Tokyo (Japan). The D values were measured by the chromium-reduction method of Gehre et al. (1996) using the automated hydrogen preparation system, H/Device of Finnigan, equipped with MAT 252 Finnigan mass spectrometer. Results are reported in permil () delta notation relative to Standard Mean Ocean Water (SMOW). Analytical precision is better than ±0.5. The ¹⁸O values were measured by the CO₂-equilibrium method of Epstein and Mayeda (1953) with a MAT 252 mass spectrometer. Results are reported in delta values relative to SMOW with an analytical precision better than ±0.15.