Site 1228 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 1228 is in the immediate vicinity of Leg 112 Site 680. 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). Interestingly, at Site 680 the deep brine source of sulfate prevents the interstitial water concentration of sulfate from becoming depleted at any depth. Site 1228 therefore provides an opportunity to study how the introduction of sulfate-bearing brine affects subseafloor life in organic-rich, sulfate-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1227 and 1229, which are, respectively, affected by the intrusion of sulfate-free brine and sulfate-rich brine into organic-rich, sulfate-depleted sediments.
Site 1228 is located in 252 m water depth on the outer shelf edge in the middle of the modern oxygen minimum zone of the Peruvian high-productivity upwelling system. At this depth on the Peru shelf, an oxidized sediment zone is practically absent at the sediment/water interface and sulfate reduction is the predominant mineralization process to the very surface (Rowe and Howarth, 1985; Fossing, 1990; Parkes et al., 1993). The organic content is high at Site 680 (3%-10% total organic carbon [TOC]), and sulfate reduction rates are still detectable with radiolabeled sulfate in samples taken from as deep as 80 meters below seafloor (mbsf) (Parkes et al., 1990).
The lithologic and physical properties at Site 680 change strongly through the 200-m-deep interval drilled during Leg 112 (Shipboard Scientific Party, 1988b). The sediment mainly consists of diatom mud in the upper 50 m of the Pleistocene deposit. Below 50 mbsf, the terrestrial component of the mud is higher but the sediment is primarily biogenic. The lower part of the sediment column consists of a coarse-grained phosphate and feldspar gravel interpreted as drilling artifacts overlying coarse-grained sand cemented by dolomite (Shipboard Scientific Party, 1988b). Dolomite is the primary authigenic phase, but calcite and apatite are also common.
Shipboard chemical analyses from Leg 112 indicate that concentrations of methane at Site 680 are in the range of 10-100 µL/L (0.4-4 µM) in the upper 100 m of the sediment column. Methane was not analyzed at greater depths at Site 680. Concentrations of dissolved sulfate decline from a near-seawater value to a minimum of 6 mM over the uppermost ~50 mbsf and then rise toward higher values in the underlying sediment as a result of diffusion from the underlying sulfate-rich brine (Shipboard Scientific Party, 1988b). A peak sulfide concentration is present between 20 and 40 mbsf (Mossman et al., 1990). Sulfide concentrations were not measured in deeper portions of the underlying brine-affected interval.
Chloride concentrations increase steadily to the base of the hole, and ammonium steadily increases to at least 80 mbsf. Alkalinity exhibits a maximum value at 20 mbsf. Concentrations of calcium and magnesium exhibit minimum values at 5 and 20 mbsf, respectively, and then increase steadily to the base of the hole. The magnesium/calcium ratio peaks at ~5 mbsf and also steadily declines to the base of the hole (Shipboard Scientific Party, 1988b; Kastner et al., 1990).
These downhole profiles of dissolved chemical concentrations are inferred to result from relatively high levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean water and with a sulfate-bearing brine introduced at depth. Prokaryotic cell counts and activities were studied to a depth of 9.1 mbsf at Site 680. Nearly 109 cells/mL were present in all samples analyzed. In most probable number (MPN) cultivation studies, 101 to 105 cells/mL were found to be viable (Cragg et al., 1990; Parkes 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 680.
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-109