The main objectives of the IW program at this site were to provide geochemical proxies for the presence and abundance of gas hydrate and to establish constraints on the nature of the seismic reflectors, B and B´, observed in the seismic data. At Site 1246, these reflectors are present at ~60 and ~100 mbsf, which are above the depth of the BSR. In contrast, at Site 1244, these seismic reflectors are present below the BSR (see "Introduction"). Only one hole was cored at Site 1246, and we recovered 33 IW samples at a frequency of approximately two whole rounds per core. The IW geochemistry data are tabulated in Table T3 and are illustrated in Figure F8.
Site 1246 was cored to a total depth of 132 mbsf (Hole 1248B) just below the BSR, which in the seismic data lies at 114 mbsf (see "Introduction"). Thus, the bulk of the IW data obtained at this site lies within the GHSZ. The composition of pore water in this zone is influenced by gas hydrate geochemistry and by the possible effect of the presence of Horizon B above the BSR.
The downhole chloride content exhibits similar overall features to those previously observed in pore water recovered from Sites 1244 and 1245 in that the excursions to low chloride values are present in a well-defined zone ranging from the FO of gas hydrate at ~40 mbsf to the inferred depth of the BSR at ~114 mbsf. These chloride anomalies reflect dissociation of gas hydrate during recovery and can be used to estimate the amount of gas hydrate present in the sediments when used in conjunction with background in situ chloride data (see "Interstitial Water Geochemistry" in the "Explanatory Notes" chapter and "Interstitial Water Geochemistry" in the "Site 1244" chapter). We assumed anomalous low chloride values relative to the background concentration defined by data points that appear to fit a smooth profile at this site, as indicated by the shaded portion of the chloride distribution shown in Figure F9A. These background values correspond fairly well to the chloride concentrations obtained for Site 888, located west of the deformation front, which showed no evidence for the presence of gas hydrate (Kastner et al., 1995). At Site 1246, gas hydrate was indeed recovered from sediments where low chloride anomalies were observed (see "Lithostratigraphy"). In addition, thermal anomalies seen in infrared (IR) data (see "Physical Properties") and variations in LWD resistivity data (see "Downhole Tools and Pressure Coring") also suggest the presence of gas hydrate from ~40 mbsf to the BSR. Estimates on amount of gas hydrate based on the Cl- anomalies range from 0% to a single-point maximum of 20% of the pore space.
As a result of the IW sampling frequency of only two whole rounds per core, the shape of the sulfate profile cannot be constrained, nor can an estimate of methane flux be given for Site 1246. Headspace methane concentrations (Table T4) (also see "Organic Geochemistry") locate the sulfate/methane interface (SMI) at ~7 mbsf.
Lithium is a highly mobile alkali ion, and its chemical mobility is strongly dependent on temperature (Edmond et al., 1979; Seyfried et al., 1984). At Site 1246, a release of lithium from aluminosilicates at depths below 1 km, where the formation temperature exceeds 80°C, is documented by an overall increase in its concentration with depth. Superimposed on this trend we observe a minimum in the lithium concentration at the depth of seismic Horizon B (Fig. F10), which reflects incorporation of this element into clay minerals during alteration of volcanic material at low temperatures. At both Sites 1244 and 1246, the depth of the interval associated with Horizon B corresponds to a decrease in the dissolved lithium concentration, as shown in Figure F10.
The distribution of dissolved iron at Site 1246 shows a pattern similar to that observed at Site 1244, with an increase to a maximum of 30 µM at 40 mbsf (Fig. F11). This distribution likely reflects cycling of iron minerals, which are precipitated as iron sulfides below the SMI and are remobilized at depth. The dissolved iron remains higher than the background levels (<3 µM) throughout the GHSZ. Postcruise analyses of the distribution and isotopic composition of dissolved sulfide and of sulfide minerals will likely provide constraints on the nature of the iron biogeochemical cycling at this site.