Only one hole was cored at Site 1252, from which we recovered 54 interstitial water (IW) samples. The main objective of the IW program at this site was to provide geochemical proxies that may aid in defining the nature of the accretionary complex sediments, which have been uplifted at this location to a depth of ~120 mbsf (see "Introduction"). In addition, pore water chemistry will be used to provide constraints on biogeochemical processes within the upper sediments and the possibility of gas hydrate presence within the gas hydrate stability zone (GHSZ). IW samples were collected at a resolution of one sample per section in the upper 14 mbsf, to provide adequate sampling within the anaerobic methane oxidation (AMO) zone. In the interval from 14 to 180 mbsf, we collected IW samples at a frequency of approximately two whole-round samples per core, followed by a sampling resolution of one whole-round sample per core to the bottom of the borehole. As a result of time constraints, no shipboard measurement of dissolved cation concentrations was performed at this site, but samples were collected for postcruise analyses. The IW geochemistry data are tabulated in Table T3 and are illustrated in Figure F13.
The distribution of dissolved chloride at Site 1252 displays a linear decrease with depth at a rate of ~0.48 mM/m to ~120 mbsf (Fig. F14). Similarly, a decrease in chloride of ~0.46 mM/m was observed at Site 1251, and it is thought to reflect diffusion of chloride ions from present-day seawater values to the low-chloride fluids that characterize the accreted sediments at these sites (see "Interstitial Water Geochemistry" in the "Site 1251" chapter). At Site 1244, dissolved chloride also shows a linear decrease from ~70 mbsf to the bottom of the hole, where the chloride concentration is 472 mM (see "Interstitial Water Geochemistry" in the "Site 1244" chapter). Consistently, the onset of low-chloride fluids at Sites 1244, 1251, and 1252 all coincide with a seismic reflector that is thought to image the top of the accreted sedimentary wedge, which occurs at ~120 mbsf at Site 1252 (see "Introduction"). Similar pore fluid freshening was observed at other sites on the Cascadia margin and elsewhere and is attributed to clay dehydration deep within the accretionary prism (e.g., Kastner et al., 1995).
In contrast to other sites drilled during this leg, at Site 1252 there are no significant excursions to low chloride values throughout much of the GHSZ. The lack of chloride anomalies does not necessarily mean that no gas hydrate is present. Rather, it indicates that if gas hydrate is present it must be as discrete and narrow intervals, which are not captured with the sampling resolution of two samples per core (i.e., a sample approximately every 4.5 m). This is consistent with the IR temperature data, which show temperature anomalies significantly lower than those measured at other sites, and these anomalies are restricted to very thin intervals (see "Physical Properties"). Furthermore, whereas at Site 1251 there may be a significant accumulation of gas hydrate at the base of the GHSZ (see "Interstitial Water Geochemistry" in the "Site 1251" chapter), such a gas hydrate-bearing interval was not observed at Site 1252.
Three data points define the sulfate gradient within the sulfate reduction zone, where the profile is linear (Fig. F15). The sulfate/methane interface (SMI) is located at ~5 mbsf, defined by both sulfate and methane data. Interstitial sulfate first reaches minimum concentrations between data points at 3.85 and 6.25 mbsf, and methane concentrations, as documented by headspace gas data, rise quickly between 4.00 and 6.40 mbsf (see "Hydrocarbon Gases" in "Organic Geochemistry").
Following the method of Borowski et al. (1996) outlined at Site 1244 (see "Interstitial Water Geochemistry" in the "Site 1244" chapter), the estimated methane flux at Site 1252 is 5.7 x 103 mM/cm2/yr, based on a sulfate gradient of 9.1 mM/m (equivalent to a slope of 0.11 m/mM) (see Fig. F15), a sulfate diffusion coefficient of 5.8 x 10-6 cm2/s at 5°C, and average porosity of 0.70%. For comparison, the estimated methane flux is equivalent to that of Site 1251, about twice that calculated for Site 1244, and about three times larger than that estimated at the Blake Ridge, a large passive-margin gas hydrate terrain (Borowski et al., 1996). These estimates assume methane delivery through diffusion only and assume that the linear portion of the sulfate curve is mainly created by sulfate demand at the SMI. If significant water or methane advection occurs or if sulfate depletion through AMO is of minor importance, then this estimate is invalid.