Site 1229 was one of three Leg 201 sites selected for drilling on the continental shelf of Peru. These shelf sites were collectively selected to provide records of microbial activities, communities, and geochemical consequences in organic-rich ocean-margin sediments.
The principal objectives at this site were
Site 1229 is located on the Peru shelf in 150.5 m water depth. It is in the immediate vicinity of Leg 112 Site 681. As described in "Principal Results" in the "Site 1227" chapter, geochemical studies of Leg 112 sites show that brine is present below the seafloor in the Trujillo and Salaverry Basins (Suess, von Huene, et al., 1988). Site 1229 provides an opportunity to study how the presence of sulfate-bearing brine affects subseafloor life in organic-rich, sulfate-depleted, methane-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1227 and 1228, which are, respectively, affected by the intrusion of sulfate-free brine into organic-rich, sulfate-depleted sediments and the intrusion of sulfate-rich brine into sediments with sulfate-bearing interstitial waters.
Shipboard chemical analyses from Leg 112 indicate that the concentrations of methane at Site 681 increase from 102 to 105 µL/L in the first 40 m of the sediment column and decline from 105 to 102 µL/L between 73 and 100 meters below seafloor (mbsf) (Shipboard Scientific Party, 1988). In contrast, the concentrations of dissolved sulfate decline to 0 mM over the first ~30 mbsf, remain at or near 0 mM until 75 mbsf, and then increase steadily with greater depths (Shipboard Scientific Party, 1988). This downhole pattern of sulfate concentrations indicates active sulfate reduction at depths above 30 mbsf and below ~75-100 mbsf. The downhole pattern of methane concentrations indicates that methane is created at depths of 60-70 mbsf and diffuses to the overlying and underlying zones of active sulfate reduction, where both sulfate and methane are destroyed.
Chloride concentrations increase steadily to the base of the hole. Ammonium concentrations decline slightly from the sediment/water interface to 12 mbsf, increase from 12 to 80 mbsf, and then begin to decline again. Alkalinity also declines from the sediment/water interface to 12 mbsf, increases to a subsurface maximum at 32 mbsf, and then declines again with depth. Calcium and magnesium concentrations exhibit minimum values at ~30 mbsf and then increase steadily to the base of the hole. The magnesium/calcium ratio exhibits a broad peak from ~0 to 40 mbsf and then steadily declines to the base of the hole (Shipboard Scientific Party, 1988).
These downhole profiles of dissolved chemical concentrations are collectively inferred to result from high levels of biological activity and biologically driven solid-phase alteration throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with a sulfate-bearing brine introduced at depth. Acridine orange direct counts show that prokaryotic cells are present in samples taken from as deep as 80 mbsf at Site 681 (Cragg et al., 1990). Viable prokaryotes were found and potential activity rates were identified in the same samples (Cragg et al., 1990). The subsurface extent of key electron donors (hydrogen, acetate, and formate) and electron acceptors with standard free-energy yields greater than that of sulfate (oxygen, nitrate, manganese oxide, and iron oxides) was not determined for Site 681.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 201IR-110