A total of 241 pore water samples were analyzed. These range from samples collected from a depositional basin, where fluid advection rate is very low (Site 1251) (Fig. F1) to the ridge summit (Sites 1248, 1249, and 1250), where rapid upward methane transport occurs (Torres et al., 2004). The isotopic data are listed in Tables T1, T2, T3, T4, T5, T6, T7, T8, and T9. The precision of the 13C measurements based on replicate analyses of a NaHCO3 stock solution over a 7-week period was ±0.08.
Pore water profiles for sites at the ridge flanks and adjacent basin (Sites 1244, 1245, and 1251) (Fig. F2A, F2B) reveal the classic decrease in 13C values in the shallow subsurface resulting from oxidation of organic matter by sulfate, with minimum 13C DIC values of –22.46 (Site 1244) and –24.86 (Site 1245) coincident with the depth of sulfate depletion (Fig. F2D). These results show that although Hydrate Ridge is an area of extensive methane advection and gas hydrate formation, rapid burial of the sediments at the flanks of the ridge apparently limits the amount of methane that diffuses to the sulfate reduction zone, thus precluding a major contribution to the total DIC by anaerobic methane oxidation. This inference, based on stable isotope distributions, is supported by geochemical models based on stoichiometric sulfate and alkalinity gradients (Claypool et al., this volume), as well as by analyses of microbial activities, which at these sites show very low anaerobic methane oxidation rates (A. Boetius, pers. comm., 2004).
Upward fluid flow at the ridge summit leads to the formation of massive gas hydrate deposits in the near-surface sediments of Sites 1249 and 1250 (Tréhu, Bohrmann, Rack, Torres, et al., 2003; Tréhu et al., 2004). Pore water data are consistent with this inference, showing sulfate concentrations near zero even in the shallowest pore water samples. The 13C DIC in all fluids sampled at the summit sites is +15 (Fig. F2E, F2F), as with the fluids sampled below the sulfate reduction zone at all the other sites drilled during Leg 204. These high values reflect preferential removal of the 13C-depleted DIC during methane formation (e.g., Claypool and Kaplan, 1974) such that the residual interstitial water bicarbonate shows a marked enrichment in 13C. Advection of these fluids to near-surface sediments at the ridge summit results in the observed 13C DIC distribution.
In addition to characterizing the DIC sources (organic matter degradation vs. methane oxidation) and transport mechanisms (advection of deep fluids at the ridge summit), the 13C DIC in pore fluids has been incorporated into studies of carbonate phases collected during the leg to provide the framework needed to unravel the history of gas hydrate formation and destabilization recorded in benthic foraminifers and authigenic carbonate phases on Hydrate Ridge and elsewhere (Bohrmann et al., 1998; Teichert et al., 2003a, 2003b).