Leg 201 was the first ODP leg devoted exclusively to the characterization of microbial communities in the deep biosphere of marine sediments. Seven drilling sites in the eastern equatorial Pacific Ocean and along the Peru continental margin (Fig. F1) were targeted to cover a broad spectrum of electron acceptor, electron donor, organic substrate, and sedimentary conditions, including sites with and without methane hydrate. Previously drilled sites (Deep Sea Drilling Project and ODP Sites 321, 846, and 851) located near the targeted sites provided baseline data for interstitial water chemistry, lithology, and sediment properties. Data reported here are from one open-ocean site (1231) and two equatorial upwelling sites (1225 and 1226) that are characterized by relatively low total organic carbon content (0%–0.9%) and moderate to low rates of sediment accumulation (D'Hondt, Jørgensen, Miller, et al., 2003; D'Hondt et al., 2004). Previous geochemical studies indicate that modern seawater flows through the basaltic basement underlying all three of the sites (Baker et al., 1991; Oyun et al., 1995). Sediments at the sites comprise variable mixtures of primarily nannofossil and diatom oozes with volcanic glass and hydrothermal input at Site 1231.
Sediments at Site 1225 (Site 851) were deposited on ~11-Ma basaltic crust in a relatively high productivity region between the South Equatorial Current and the North Equatorial Counter Current. This site is located at 3760 m water depth and is representative of the eastern equatorial region. Site 1226 (Site 846) is ~2300 km east-southeast of Site 1225 and 300 km south of the Galapagos Islands. This site is influenced by the South Equatorial and Peru currents and overlies ~16.5-Ma basement at 3297 m water depth. Open-ocean Site 1231 (Site 321) is located at 4827 m water depth in the Peru Basin. The Pleistocene- to Eocene-aged organic-poor sediments at Site 1231 are representative of most of the world ocean (D'Hondt, Jørgensen, Miller, et al., 2003; D'Hondt et al., 2004).