Site 1231 was selected for drilling in order to study the microbial activities and communities of the organic-poor sediments that characterize much of the world's open ocean. Before drilling at Site 1231, the nature of subseafloor microbial communities in open-ocean clays had never been assessed.
The principal objectives at this site were
Site 1231 is in the Peru Basin at 4827 m water depth. The lithologies, age, and many geochemical characteristics of the targeted sediments were characterized by Leg 34 studies at nearby Site 321 (Shipboard Scientific Party, 1976). The total sediment thickness at Site 321 is 115 m. The sediment is composed of 58 m of late Oligocene to Holocene clay and 57 m of iron-rich late Eocene to early Oligocene nannofossil ooze (Shipboard Scientific Party, 1976). The lower 50 m of sediment at Site 321 is rich in iron and manganese (Dymond et al., 1976). Iron and manganese accumulation rates estimated for the sediments present below 49 meters below seafloor (mbsf) are about an order of magnitude higher than those estimated for sediments above 49 mbsf (Boström et al., 1976). In analyses of six interstitial water samples, dissolved manganese was present at relatively higher concentrations in the upper 45 m of the sediment column (3.5-7.4 ppm) than in the lower 50 m (0-1 ppm) (Brady and Gieskes, 1976). Dissolved sulfate concentrations also appeared to be slightly higher in the upper 45 m (>28 mM) than in the lower 50 m (27 mM) (Brady and Gieskes, 1976). Little or no evidence for other postdepositional reactions was seen among major dissolved ions at Site 321. This finding led Brady and Gieskes (1976) to conclude that any reactions in these sediments occur at such slow rates that their chemical signature is annihilated by exchange with the top and bottom of the sediment column. Consequently, Site 1231 provided a challenging opportunity for assessing the microbial activities and communities of low-activity sediments typical of much of the open ocean.
The subsurface extent of key electron donors (hydrogen, acetate, and formate), electron acceptors with standard free-energy yields greater than that of sulfate (oxygen and nitrate), products of key metabolic reactions (dissolved iron), and other biologically important chemicals was not determined for Site 321.
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.
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