Site 1245 is located on the northern slope of the southern summit of Hydrate Ridge. This site provided the opportunity to sample an environment influenced by wet gas hydrocarbon input (see "Organic Geochemistry") as well as high methane fluxes. As at Site 1244, the overall goals were to quantify and describe methanogenesis, methanotrophy, and general microbial community structure in the presence of methane and methane hydrate. Chloride gradients at this site are markedly different than those on the eastern side of Hydrate Ridge and may reflect a difference in tectonic and depositional environments. Although subtle, these differences may noticeably influence the structure and activity of associated microbial communities.
AMO appears to dominate the sulfate and methane signals in the upper sediments at Site 1245. The SMI is at ~7 mbsf (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry" and "Hydrocarbon Gases" in "Organic Geochemistry"). The top three cores from Hole 1245D were sampled intensively (one to two samples per section) in coordination with the IW investigations (Table T13).
To examine methanogenesis at Site 1245, we sampled regularly downhole past the base of the GHSZ and well into the zone where free methane gas may be present (Table T13). Deeper sampling was spread over Holes 1245B and 1245C (the deepest sample was from 435 mbsf). Below 130 mbsf, ethane and longer-chain hydrocarbons became more prominent (see "Hydrocarbon Gases" in "Organic Geochemistry"), indicating a thermogenic source of methane combining with a biological source of undetermined magnitude.
Sediments associated with hydrates were not deliberately sampled at Site 1245; however, IR temperature anomalies, indicating small amounts of disseminated or veined hydrates, were present in the GHSZ in Hole 1245C.
A series of ash layers at ~180 mbsf (see "Lithostratigraphy") likely correspond to seismic Horizon A (see "Introduction"). Ash layers also have longer-chain hydrocarbons (see "Hydrocarbon Gases" in "Organic Geochemistry") and increased lithium concentration (see "Major and Minor Element Distributions" in "Interstitial Water Geochemistry"), which are indications that these ash layers may act as a conduit for migration of deeper gases and pore water. Because these sections of ash were precious, we sampled with particular care. Before the core sections were split, visible ash layers were identified and the section to be sampled was cut in two, allowing the microbiologists to take plugs of sediment directly from the cut ends of the core. The two pieces of the core section were then rejoined and passed through the multisensor track (MST) and the regular core flow procedure. Samples were taken from inside and outside of the ash layers.
Most core sections sampled for microbiology had 5-g subsamples taken from outer and inner layers for gas chromatography analysis as described in "Perfluorocarbon Tracer" in "Shipboard Microbiological Procedures and Protocols" in "Microbiology" in the "Explanatory Notes" chapter. Samples were analyzed as described, and raw data are presented in Table T14. Differences between outside and inside samples generally indicate minimal penetration of tracer. Notable exceptions may reflect penetration along fractures in sediments or discrepancies in handling and sampling, particularly when dealing with XCB cores.
A comparison of fluorescent microsphere penetration in core interiors and exteriors is summarized in Table T14. Except on the few occasions that deployment of microspheres failed (the Whirl-Pak bag did not break), microscopic analysis of outer core layers showed numbers of microspheres ranging up to 105/g of sediment, whereas microsphere numbers in inner core layers were generally below the detection limit of 10/g of sediment. Microsphere deployment was unsuccessful in the Hole 1245D mudline core. Future legs employing microsphere quality assurance methods may wish to consider experimenting with different bag types and core catcher attachment when coring mudlines or soft sediment.