Site 1227 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 1227 (427 m water depth) is in the immediate vicinity of Leg 112 Site 684, in a small fault-bounded sediment pond in the Trujillo Basin on the Peru continental shelf. The Trujillo Basin lies within the Peru upwelling zone, and its sediments are correspondingly rich in organic carbon. The total organic carbon (TOC) content of Site 684 sediment samples ranges between 1.2% and 10.6% (Shipboard Scientific Party, 1988). The average TOC concentration of these samples is approximately an order of magnitude higher than the average concentration at open-ocean Site 846 (Leg 201 Site 1226) (Shipboard Scientific Party, 1988, 1992a) and is about two orders of magnitude higher than the TOC content of open-ocean Site 851 (Leg 201 Site 1225) (Shipboard Scientific Party, 1988, 1992b).
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). The composition of the brine differs from site to site, perhaps because of differences in its degree of dilution and the nature of its interaction with the surrounding sediments (Suess, von Huene, et al., 1988). Detailed chemical analyses indicate that this brine is of marine origin and is early Miocene in age (Kastner et al., 1990). The Leg 112 Initial Reports volume suggested that it enters the younger sediment column by diffusion from interstitial brine in underlying Miocene sediments (Suess, von Huene et al., 1988). Kastner and colleagues (1990) inferred that it is emplaced by stratigraphically bounded advection from north to south. The sulfate depletion of the brine at Site 1227 presumably results from bacterial sulfate reduction closer to the brine's source (e.g., deeper in the sediment column). Whatever the brine's mode of emplacement, Site 1227 provides an opportunity to study how the presence of sulfate-depleted brine affects subseafloor life in organic-rich sediments. Consequently, it provides an excellent standard of comparison for Sites 1228 and 1229, which are affected by the intrusion of sulfate-rich brine into, respectively, sulfate-bearing and sulfate-depleted sediments.
Leg 112 shipboard chemistry suggests that concentrations of methane at Site 684 increase by at least three orders of magnitude (from 102 to 105 µL/L) over the first 50 to 60 meters below seafloor (mbsf) and remain between 104 and 105 µL/L to at least 100 mbsf. Ethane and butane concentrations also increase downhole to maximum concentrations at ~60 mbsf (Shipboard Scientific Party, 1988). In contrast, concentrations of dissolved sulfate decline from a near-seawater value to zero over the uppermost 30 or 40 mbsf (Shipboard Scientific Party, 1988). These profiles of dissolved hydrocarbons and sulfate indicate that the hydrocarbons and the sulfate are simultaneously destroyed by sulfate-reducing prokaryotic communities at ~40 mbsf.
Concentrations of several dissolved chemical species increase steadily to the base of the hole (ammonium, chloride, calcium, and magnesium). The increases in dissolved chloride, calcium, and magnesium provide evidence of the brine diffusing upward into the sediment column. Alkalinity exhibits a maximum value at ~40 mbsf, where the rate of anaerobic oxidation of methane appears to be greatest. The magnesium/calcium ratio peaks at 12 mbsf and steadily declines to the base of the hole, presumably as a result of dolomitization throughout the methane-rich sedimentary interval (Shipboard Scientific Party, 1988).
All of these patterns of sedimentary interstitial water concentration are inferred to result from relatively high levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with the brine introduced at depth. 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 684.
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-108