Leg 201 of the Ocean Drilling Program (ODP), which took place in year 2002, was the first ocean drilling expedition completely dedicated to the study of microbial life and its impact on sediment biogeochemistry deep beneath the seafloor (D'Hondt, Jørgensen, Miller, et al., 2003). In the present study, interstitial waters from six sites (Sites 1225–1229 and Site 1231) from the eastern equatorial Pacific Ocean and the Peru margin (Fig. F1) were retrieved during Leg 201 (Table T1). The positions of the sites were chosen to be close to previously analyzed sites drilled during Deep Sea Drilling Project (DSDP) Leg 34 (Yeats, Hart, et al., 1976) and ODP Leg 112 (Suess, von Huene, et al., 1988) (Table T1). Site 1231 represents a deep-sea setting characteristic of much of the world's open ocean, and organic-poor Neogene deep-sea clays and Paleogene nannofossil ooze were recovered in the Peru Basin. Miocene to Holocene carbonate and siliceous oozes and chalk were recovered at deep-sea Sites 1225 and 1226 in the eastern equatorial Pacific. Both sites are influenced by the relatively high productivity equatorial ocean. The shallow-water Sites 1227, 1228, and 1229 are from the Peru continental margin (Fig. F1) (D'Hondt, Jørgensen, Miller, et al., 2003), and Miocene to Holocene biogenic oozes and terrigenous sediments were cored. The composite section of recovered sediments spans a time interval from the late Eocene to the Holocene (D'Hondt, Jørgensen, Miller, et al., 2003). Organic matter contents differ significantly between sites as a function of water depth and surface water productivity (Table T1). Downcore variation in the sedimentary column of the Peru margin sites (Table T1) demonstrate that sedimentation conditions changed over time. Sedimentary in situ temperatures between 1° and 20°C (Table T1) provide environmental conditions selecting for psychrophilic and mesophilic bacteria. Whereas dissolved methane was observed in the pore waters of a number of settings, none of the investigated sites presented here contained gas hydrates.
Sulfur isotope fractionation in dissolved sulfate of pore waters is an important indicator for the occurrence of microbial sulfate reduction (MSR) in marine sediments (e.g., Hartmann and Nielsen, 1969; Jørgensen, 1979; Jørgensen et al., 2004), and stable sulfur isotope studies were among the first to show by indirect evidence that bacterial sulfate reduction takes place even in deep marine sediments (Zak et al., 1980; Brumsack et al., 1992; Böttcher et al., 1998, 2003, 2004b; Rudnicki et al., 2001). In agreement with the stable isotope evidence, microbial activity in the deep biosphere of marine sediments was found by pore water modeling (Canfield, 1991) and was later directly confirmed by microbiological and radiotracer studies (Bale et al., 1997; Parkes et al., 1994; D'Hondt, Jørgensen, Miller, et al., 2003; D'Hondt et al., 2002, 2004; Parkes et al., 2005).
In the present study, pore waters recovered during Leg 201 were analyzed for stable isotope ratios in dissolved sulfate (34S/32S), together with major and minor ions to identify the activity of sulfate-reducing bacteria in the sediment-pore water system. The covariation of dissolved sulfate concentrations and mass-dependent sulfur isotope discrimination is used to calculate apparent closed-system (Rayleigh) fractionation factors (Hartmann and Nielsen, 1969; Goldhaber and Kaplan, 1974; Claypool, 2004). In most sediments, however, bacterial sulfate reduction takes place under conditions that are at least partly open with respect to dissolved sulfate (e.g., Jørgensen, 1979; Jørgensen et al., 2004). Therefore, the Rayleigh-type fractionation factors are combined with a relationship recently derived by Claypool (2004) for DSDP and ODP cores to estimate for the Leg 201 sites the open system diffusional supply of dissolved sulfate. It is found that stable isotope variations indicate net microbial sulfate reduction in all sites, with the exception of the open-ocean deep-sea Site 1231, and that all sites are at least partially open to diffusional supply of sulfate from the bottom waters and partly from underlying brines. Sulfate supply from underlying brines is additionally found at selected sites. The oxygen isotope geochemistry of dissolved sulfate is discussed in an accompanying publication (Blake et al., this volume).