Site 1251, located in a slope basin to the east of Hydrate Ridge, was a main target site for microbiological study, as it provides a contrast to the ridge summit (Sites 1249 and 1250). Compared to the summit sites, hydrates and thermal anomalies were rare at Site 1251 (see "Infrared Scanner" in "Physical Properties") and the SMI was easily discernible in the sediment column (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry").
Collecting sediments at the SMI, which was estimated at ~4.5 mbsf from methane (see "Hydrocarbon Gases" in "Organic Geochemistry") and sulfate (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry") concentrations, was a prime objective at Site 1251. Site 1251 also has a more rapid sedimentation rate (see "Summary" in "Biostratigraphy") than the other sites sampled for microbiological purposes (Sites 1244 and 1245). This different depositional environment may change the microbial habitat.
Although the methane flux is estimated to be larger than that at Sites 1244-1247 (see "Hydrocarbon Gases" in "Organic Geochemistry"), hydrates were not abundant and the only significant IR thermal anomalies encountered were directly above the BSR (see "Infrared Scanner" in "Physical Properties"). IR images will be correlated with microbiological samples to determine the possible influence of hydrate on sediments sampled.
The depth of penetration makes this an interesting site at which to study methanogenesis. Below the base of the GHSZ at Site 1251, methane is often undersaturated in pore fluids (see "Hydrocarbon Gases" in "Organic Geochemistry"), providing a more favorable environment for methanogenesis.
Intervals sampled for microbiology are summarized in Table T10.
PFT concentrations shown in Table T11 are low both on the inside and outside of the core. Although samples from the inside of the core show a hundred-fold reduction in PFT compared to the outer layers, all concentrations are extremely low and probably reflect loss from sampling vials. Samples were not analyzed until much later in the cruise, and the perfluorocarbon is extremely volatile.
A comparison of fluorescent microsphere penetration in core interiors and exteriors is summarized in Table T11. Most deployments of microspheres were successful (i.e., the Whirl-Pak bag broke). Microscopic analysis of outer core layers showed numbers of microspheres ranging upward from 104/g of sediment, whereas microspheres were generally below the detection limit of 10/g of sediment in the interior of the core.
When Section 204-1251B-1H-1 was split, sedimentologists noticed a peachy orange gelatinous material within a fracture in the sediment at 47-48 cm. Wet-mount fluorescence (Fig. F23) and transmitted-light microscopy revealed an apparent monoculture of coccoid cells, often clumped in tetrads. These cells did not autofluoresce. Samples were preserved for deoxyribonucleic acid extraction.