Site 1250 is located near the summit of Hydrate Ridge, a few hundred meters from Site 1249. As at Site 1249, methane is present all the way up to the seafloor at Site 1250 (see "Hydrocarbon Gases" in "Organic Geochemistry"). In the upper 10-20 mbsf, sulfate is essentially absent through the entire cored section (see "Carbon Cycling" in "Interstitial Water Geochemistry"). In the upper 10-20 m, hydrates are abundant (see "Lithostratigraphy"), and authigenic carbonates precipitate (see "Lithostratigraphy"), presumably from oxidation of advected methane (see "Carbon Cycling" in "Interstitial Water Geochemistry" and "Hydrocarbon Gases" in "Organic Geochemistry"). Site 1250 was drilled deeper than Site 1249; coring penetrated the base of the GHSZ at ~110 mbsf and Horizon A at ~150 mbsf and reached a maximum depth of 180 mbsf.
Samples were taken for microbiological studies from Holes 1250D and 1250E (Table T10). Attempts were made to preserve near-surface sediments for AMO studies. The liner containing the first core from Hole 1250D was badly damaged during recovery, and the associated fluid intrusion made the core unsuitable for microbiology. Sampling began with Core 204-1250D-3H and proceeded to the TD of Hole 1250C (Core 19X). Because sulfate was not detected in any Site 1250 sediments, it seems likely that AMO takes place at the seafloor or in the very upper sediment layers. Because of interest in this zone, a total of 16 m of sediment in two cores was recovered from another hole (Hole 1250E) shot specifically for microbiology.
Thermal anomalies indicating hydrate were seen throughout the GHSZ in Hole 1250D (see "Infrared Scanner" in "Physical Properties"). As at Site 1249, infrared (IR) thermal anomalies that indicated diffuse hydrate did not influence where microbiological samples were taken. Postcruise examination of IR images will help unravel the hydrate story. No samples were taken next to massive hydrates at this site. Sampling of shallow cores taken from Hole 1250E was complicated by their gassy nature. Microbiological samples from top layers were in very close proximity to readily observable, cross-cutting gas hydrate veins in multiple orientations that rapidly disappeared upon exposure.
As at other sites that intersect Horizon A, Site 1250 was influenced by heavy hydrocarbons in deeper sediment layers (see "Hydrocarbon Gases" in "Organic Geochemistry"). Drilling in Hole 1250D was stopped at 145 mbsf and before Horizon A was penetrated, but deep sediments that contained thermogenic hydrocarbons were sampled for microbiological studies. Thermogenic sources of methane at this site may make biological production of methane unfavorable.
No samples for PFT analyses were taken at Site 1250.
Fluorescent microsphere tracer data are summarized in Table T11. Abundant microspheres in both outer and inner layers of samples from shallow cores taken from Hole 1250E reiterate the difficulty of sampling soft, gassy sediments. With the possible exception of Hole 1244F, microsphere quality assurance evaluations from Leg 204 suggest that all APC mudline cores should be regarded as questionable for microbiological analysis. No mudline XCB cores were taken for microbiology during this leg. In nearly all shallow cores collected during this leg, either high levels of contamination were indicated or microsphere deployment was unsuccessful and cores have an unknown amount of contamination. All cores taken from Hole 1250D were treated with microspheres, and in most cores, no interior spheres were detected.
In view of the difficulties encountered with recovering suitable near-seafloor material under the conditions at the crest of Hydrate Ridge and considering the microsphere tracer observations, a different method of sample collection, such as box coring or multicoring, should be used.