Site 1225 was selected as a drilling target because its microbial activities and cell counts were expected to be far below those in ocean-margin settings but above those in the lowest activity open-ocean environments.
The principal objectives at Site 1225 were
Site 1225 is located in the eastern equatorial Pacific near the present-day boundary between the South Equatorial Current and the North Equatorial Countercurrent at 3760 m water depth. It lies in the sedimentary bulge created by the rain of biogenic debris from the relatively high productivity equatorial ocean. Geochemical studies of Deep Sea Drilling Project and Ocean Drilling Program (ODP) sites throughout this region have shown that seawater flows through the basaltic basement that underlies the sediments throughout this region (Baker et al., 1991; Oyun et al., 1995). Anomalously low conductive heat flow occurs throughout the region (Von Herzen and Uyeda, 1963; Sclater et al., 1976), possibly because the large-scale flow of relatively cool seawater through the basalts depresses conductive heat flow (Oyun et al., 1995).
The lithologies, sediment age, and many geophysical characteristics of the target site were well characterized by earlier studies of nearby Site 851 (Mayer, Pisias, Janecek, et al., 1992; Pisias, Mayer, Janecek, Palmer-Julson, and van Andel, 1995). Those studies indicated that the site is representative of a large portion of the eastern equatorial Pacific region. The sediments of Site 851 have a continuous biostratigraphy with a minimal age of 11 Ma at the basaltic basement. The gross lithologic and physical properties of the carbonate and siliceous oozes and chalk at Site 851 are characteristic of sediments throughout the region (Mayer, Pisias, Janecek, et al., 1992). The interstitial water chemical profiles at Site 851 exhibit clear evidence of seawater flow through the underlying basalts (and perhaps the lower part of the sediment column) (Oyun et al., 1995; Spivack and You, 1997).
Cragg and Kemp (1995) documented the presence of prokaryotic cells and activity throughout the sediment column at Site 851. For the first few tens of meters below seafloor, counts of both total cells and dividing cells were low relative to counts from similar depths at sites from the Peru shelf and the Japan Sea (Cragg and Kemp, 1995). At greater depths, Site 851 cell counts approached the averaged values from all previously counted sites.
Leg 138 shipboard chemistry showed that concentrations of several dissolved chemical species (ammonium, strontium, and silica) and alkalinity peaked midway down the sediment column. In contrast, dissolved sulfate exhibited maximum values near the sediment/water interface and the basement/sediment interface (Mayer, Pisias, Janecek, et al., 1992). These patterns of sedimentary interstitial water concentration are inferred to result from low levels of biological activity throughout the sediment column, coupled with diffusive exchange with the overlying ocean and with water flowing through the underlying basalts (Spivack and You, 1997). Geochemical modeling suggests that net microbial sulfate reduction in the upper half of the Site 851 sediment column corresponds to a respiration rate of 10-9 to 10-8 mol CO2/cm2/yr (D'Hondt et al., 2002). This rate of respiration is only the barest fraction of the rate of carbon dioxide reduction by photosynthesis in the overlying equatorial ocean (10-3 mol/cm2/yr) (D'Hondt et al., 2002). The subsurface distribution of electron acceptors with higher standard free-energy yields (oxygen, nitrate, oxidized manganese, and oxidized iron) in this region was not determined for Site 851.
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