Site 1230 was the single hydrate-bearing site selected for drilling during Leg 201. The principal objectives at this site were
Site 1230 is located on the lower slope of the Peru Trench in 5086 m water depth. Sediments of this area are part of the accretionary wedge just landward of the Peru Trench (Suess, von Huene, et al., 1988). The lithologies, sediment age, and many geochemical and geophysical characteristics of the target site were well characterized by Leg 112 studies of nearby Site 685 (Shipboard Scientific Party, 1988). The upper 200 m of Pleistocene to Holocene sediment is a clay-rich diatomaceous mud, partly accreted by downslope transport from the shelf. At ~200 meters below seafloor (mbsf), a stratigraphic hiatus of ~4.5 m.y. separates the slope deposit from upper Miocene diatom ooze (Shipboard Scientific Party, 1988). Authigenic carbonates and phosphates are sparse, whereas pyrite framboids are abundant throughout the section (Shipboard Scientific Party, 1988). Calculated sedimentation rates are high; they average 250 m/m.y. for the Miocene sequence and 100 m/m.y. for the Pleistocene section (Shipboard Scientific Party, 1988). These high rates are consistent with sedimentation in a lower-slope basin or trench axis.
The surface waters over Site 1230 are part of the Peru upwelling system and are biologically highly productive. The organic carbon content of the sediment is high at Site 685 (Shipboard Science Party, 1988). Methane concentrations were observed to rise above 1 bar by 11.6 mbsf and remain in the range of 104-105 µL/L throughout the cored sediment column down to 432 mbsf (Kvenvolden et al., 1990). Concentrations of ethane and butane generally increase downhole from 1 to 100 µL/L, and the methane/ethane ratio decreases from 105 to 103. The Leg 112 Scientific Party found visual evidence of methane hydrate at 99 and 164 mbsf in the form of small pieces of dark gray hydrate (Shipboard Scientific Party, 1988; Kvenvolden and Kastner, 1990). The samples looked like rounded pieces of mudstone but felt cold and showed bubbling foam. Based on this information, Site 1230 provides an excellent opportunity for assessing the nature of microbial communities and their activities in hydrate-bearing sediments rich in organic material and under high hydrostatic pressure.
Concentrations of dissolved sulfate decline to 0 mM between the first and second core analyzed at Site 685 (between 3 and 18.1 mbsf) (Shipboard Scientific Party, 1988). Chloride concentrations range between 525 and 555 mM. The maximum concentration is associated with the most shallow sulfate-free sample (18.1 mbsf) and was suggested by the Leg 112 Shipboard Scientific Party to lie just above hydrate at the top of the hydrate stability field. Salinity, alkalinity, dissolved ammonium, phosphate, and magnesium concentrations rise to maximum values in the interval of 107-134 mbsf, decline sharply between 165 and 235 mbsf, and then decrease gradually to the base of the hole at ~450 mbsf. The maxima in alkalinity (156 mM), ammonium (31,760 µM), and phosphate (826 µM) were the highest then known from deep-ocean drilling (Shipboard Scientific Party, 1988). Downhole variation in chloride and calcium concentrations is generally opposite to the variation in these other chemical species. The pH drops below 7 at 133 mbsf and remains below 7 to the base of the hole (Shipboard Scientific Party, 1988).
These patterns of interstitial water (IW) chemistry are inferred to result from high levels of biological activity throughout the sediment column, coupled with hydrate formation and diffusive exchange with the overlying ocean. 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 685.
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-111