Site 1226 was selected as a drilling target because its microbial activity was expected to be intermediate between that in ocean-margin settings and that in the lowest-activity open-ocean environments.
The principal objectives at this site were
Site 1226 (3297 m water depth) is located in the eastern equatorial Pacific, 300 km south of the Galapagos Islands, near the present-day boundary between the South Equatorial Current and the Peru Current. Near the sea surface in this region, the advection of water from the Peru Current results in relatively high nutrient levels and biological productivity (Chavez and Barber, 1987). According to its calculated backtrack path, this site has drifted eastward but has remained near its present latitude for most of its history (Pisias et al., 1995; Farrell et al., 1995). Sediment thickness at Site 1226 is 420 m. The oldest sediments immediately overlie basaltic basement and have a biostratigraphic age of 16.5 Ma (Shipboard Scientific Party, 1992a). As described in "Principal Results" in the "Site 1225" chapter geochemical studies of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites throughout this region have shown that seawater flows through the underlying basaltic basement (Baker et al., 1991).
The lithology, sediment age, and many geochemical and geophysical characteristics of the target site were well characterized by earlier studies of Site 846. The gross lithologic and physical properties of the carbonate and siliceous oozes and chalk at Site 846 are characteristic of sediments throughout the region (Shipboard Scientific Party, 1992a; Pisias, Mayer, Janecek, Palmer-Julson, and van Andel, 1995). Leg 138 studies showed that the region has undergone large variations in sediment accumulation over the course of its history. Accumulation of calcium carbonate and opal was unusually low at Site 846 during the Miocene "carbonate crash" of 11-7.5 Ma and was unusually high during the widespread Indo-Pacific "biogenic bloom" that occurred from ~7 to 4.5 Ma (Farrell et al., 1995). The organic carbon accumulation rate is presently high and appears to have gradually increased throughout the Pleistocene (Shipboard Scientific Party, 1992a; Emeis et al., 1995).
Leg 138 shipboard chemical studies of Site 846 show that concentrations of several dissolved chemical species (methane, ammonium, strontium, silica, and alkalinity) peak part way down the sediment column. In contrast, dissolved sulfate, lithium, and calcium exhibit maximum values near the sediment/water interface and the basement/sediment interface (Shipboard Scientific Party, 1992a).
As at Sites 851 and 1225, these patterns of sedimentary interstitial water concentration are inferred to result from modest levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with seawater flowing through the underlying basaltic basement. The sediments of Site 846 have a higher organic carbon content than the sediments of Sites 851 and 1225. Organic carbon content at Site 846 ranges from 0.2% to 1.0% and is highest in the Pleistocene and upper Pliocene deposits. Accordingly, Site 846 exhibits steeper gradients than Sites 851 and 1225 in interstitial water chemical species that respond to prokaryotic mineralization processes, such as sulfate, ammonium, and methane. The distinctly higher concentration of methane at Site 846 than at Site 851 is particularly intriguing because methanogenesis is generally understood to be suppressed by sulfate-reducing bacteria and methane may be oxidized in the presence of sulfate.
The subsurface distribution of key electron donors (hydrogen, acetate, and formate) and of electron acceptors with higher standard free-energy yields (oxygen, nitrate, manganese oxide, and iron oxides) was not determined for Site 846.
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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