A total of 120 IW samples were collected from Site 1245. We collected 70 whole-round samples in Hole 1245B, with a sample spacing of approximately 2 per core in the upper 125 mbsf, followed by a sampling resolution of 1 whole-round sample per core below this depth. Hole 1245C was dedicated to microbiological studies (see "Microbiology"). Thirteen whole-round samples were collected from this hole; these samples were taken adjacent to core sections sampled for postcruise studies of microbiological processes below the sulfate/methane interface (SMI). In addition, nine samples were taken from 53.12 to 53.38 mbsf from the working half of the core to evaluate Cl- distribution around a 2-cm-thick hydrate layer. In Hole 1245D, we used a high-resolution whole-round sampling protocol within the anaerobic methane oxidation (AMO) zone (approximately 2 whole-round samples per section for a total of 19 samples) in a coordinated program with the shipboard microbiologists. The deep sedimentary sequence at this site was sampled by rotary coring in Hole 1245E, from which we collected seven samples. The IW geochemistry data are tabulated in Table T5 and are illustrated in Figure F23.
The chloride distribution in pore fluids above the BSR at Site 1245 shows similar features to that observed at Site 1244, namely, the presence of excursions with anomalous low chloride values above the BSR that are thought to represent gas hydrate dissociation during core retrieval. These chloride anomalies can be used to infer the presence and amount of gas hydrate in the sediments (detailed in "Interstitial Water Geochemistry" in the "Explanatory Notes" chapter and "Interstitial Water Geochemistry" in the "Site 1244" chapter). Based on the chloride distribution, we predict that the onset of gas hydrate at Site 1245 occurs at ~55 mbsf, a depth that is controlled by interactions among methane concentration, temperature, and pressure conditions. Indeed, the methane content of the sediments at this site approaches that needed to form methane hydrate at ~57 mbsf (see "Hydrocarbon Gases" in "Organic Geochemistry"), which is in excellent agreement with the chloride estimate. Similarly, this depth corresponds to the onset of variability in the LWD resistivity data (see "Downhole Logging") and the first occurrence of temperature anomalies measured with the IR camera (see "Physical Properties"). The percent hydrate in the pore space of sediments calculated from the Cl- anomalies ranges from 0% to 25% and is limited to the zone between 55 and 130 mbsf (Fig. F24).
To further characterize the relationship between the observed chloride anomalies and the presence of gas hydrate, we conducted an experiment from the working half of Core 204-1245C-7H, from which samples were collected ~90 min after the core arrived on deck. Temperature anomalies observed with the IR camera while the core was on the catwalk indicate the presence of gas hydrate in Section 204-1245C-7H-5. This section was transferred immediately to the core laboratory, where it was split in half. Hydrate was, indeed, observed as a 2-cm-thick layer between 35 and 45 cm and oriented at a high angle to bedding. Approximately 10 cm3 of sediment was collected from the hydrate layer and at various distances from it. Pore waters were extracted from these samples to measure interstitial chloride concentration (shipboard) and isotopic composition of the water (shore based). Shipboard results from this experiment are listed in Table T6 and illustrated in Figure F25. As shown in this figure, samples collected within 5 cm of the hydrate layer show significant anomalies in the chloride content, whereas samples collected at distances >10 cm from the hydrate layer do not show any deviation from the background chloride values.
High-resolution sampling in Hole 1245D allows firm characterization of sulfate and methane profiles as well as the identification of the position of SMI (Fig. F26). Sulfate generally decreases downcore and is consumed most rapidly between 4 and 6 mbsf. Minimal sulfate values combined with rapidly increasing methane headspace concentration (see "Hydrocarbon Gases" in "Organic Geochemistry") locate the SMI at ~7 mbsf.
The shape of the sulfate profile is nonlinear. Strong curvature is present at the top of the profile, and there is no definitive linear portion of the sulfate concentration data. The strong curvature may be due to the oxidation of sedimentary organic matter through the process of sulfate reduction, as well as to the effect of fluid advection. These processes can be better quantified with knowledge of the isotopic composition of the methane gas and of the dissolved inorganic carbon, sulfate, and sulfide. All these analyses will be carried out postcruise. As a consequence, no shipboard assessment of sulfate flux (e.g., see "Interstitial Water Geochemistry" in the "Site 1244" chapter) can be presented for this site.
Pore fluids from the 190- to 300-mbsf zone show a decrease in the concentrations of calcium, magnesium, and strontium, which is accompanied by a decrease in alkalinity (Fig. F27). This observation suggests that there is a CO2 sink within this depth interval, which would most likely be caused by authigenic carbonate formation. Carbonate (CaCO3) is observed to increase within this zone, predominantly resulting from an increase of biogenic CaCO3 in the sediments (see "Lithostratigraphy"). It is possible that this biogenic component provides nucleation sites for authigenic carbonate formation.
The effects of diagenetic reactions within accreted sediments are usually documented by the distribution of chloride, lithium, and strontium as well as by the isotopic composition of these elements. At Site 1244 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter) and at other sites previously drilled on the Cascadia margin (Kastner et al., 1995), reactions at depth are characterized by an increase in dissolved lithium and strontium, accompanied by a decrease in dissolved chloride concentration. Site 1245 sediments do not show the marked chloride gradient with depth below the BSR observed at Site 1244, suggesting that the thick package of young, uplifted, and folded strata lying below the BSR at this site has not undergone significant dehydration of hydrous silicates. However, there is an increase in dissolved lithium (Fig. F28) that indicates removal of this element from aluminosilicates at temperatures >70°C, as predicted from laboratory experiments (e.g., Edmond et al., 1979; Seyfred et al., 1984) and previous field observations in accretionary margins (Kastner et al., 1995; Chan and Kastner, 2002). The increase in dissolved lithium with depth in the upper 350 mbsf at both Sites 1244 and 1245 has a similar gradient, which is consistent with the similar geothermal gradient observed at these two sites (see "Downhole Tools and Pressure Coring").
Superimposed on its linear increase with depth, dissolved lithium shows excursions to higher concentrations at 180 and 420 mbsf, respectively (Fig. F28). The first of these excursions corresponds to the depth of the seismic reflector known as Horizon A (180 mbsf) (see "Introduction"). This increase suggests migration of lithium-enriched fluids from depths below 1 km, where burial temperature reaches the 70°-100°C threshold needed for lithium release from alumnosilicates. Horizon A was identified in the sediments to correspond to a thick interval (~174-181 mbsf) of volcanic ash, with two distinct ashes observed as sandy layers near 180 mbsf (see "Lithostratigraphy"). Presumably, the sand layers represent a migration path for fluids coming from deeper intervals within the accretionary wedge. Gases collected from the sediments between the BSR (131 mbsf) and Horizon A (180 mbsf) also show enrichment of heavy hydrocarbons (see "Hydrocarbon Gases" in "Organic Geochemistry"), which is consistent with the postulated migration of fluids from a deep-seated source.
The second lithium maximum is present from 420 to 440 mbsf (Fig. F28). This interval corresponds to the boundary between lithostratigraphic Units III and IV (see "Lithostratigraphy") and perhaps represents another zone of lateral fluid migration. Analysis of the isotopic composition of lithium in the dissolved and solid phases from this site will enhance our understanding of the fluid sources and pathways and fluid migration patterns in the complex hydrogeological system at Site 1245. These data will help to constrain processes pertaining to the gas composition and fluid flux to the GHSZ.