Six in situ temperature runs were made at this site using the APCT tool, including a dedicated mudline run; two DVTP runs were also made (Table T13; Fig. F28). APCT data were modeled using the software TFIT (as described in "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter) using measured thermal conductivities (see "Physical Properties"). Uncertainty in the extrapolated value of in situ temperature resulting from a subjective analyst picking of tp, ti, and tf is <0.02°C for these high-quality records. Uncertainty resulting from possible errors in measured values of thermal conductivity is estimated to also be ~0.02°C.
Both of the DVTP runs yielded apparent temperatures that are not consistent with the APCT data, and the time series suggest that the probe did not penetrate the sediment properly. These data were not used for the determination of the in situ thermal gradient.
The resulting temperature estimates are shown in Figure F29. The six APCT measurements define a straight line very well, and there is no significant difference in the slope and seafloor intercept if the mudline measurement is excluded. The temperature gradient of ~0.053°C/m results in a predicted depth to the base of the GHSZ of 151-152 mbsf, considerably deeper than the BSR depth of 129-134 mbsf determined from acoustic logging data and VSP measurements. If we add 0.513°C to each measurement, as suggested by the calibration for APCT 11 in an ice-water bath (see "Downhole Tools and Pressure Coring" in the "Site 1246" chapter), the predicted depth to the base of the methane stability zone is 142 mbsf, accounting for half of the apparent mismatch. This is equivalent to the temperature at the BSR being ~0.6°-1.2°C colder than predicted by the methane/seawater stability curve.
No in situ pressure measurements were made at this site.
The ODP PCS was deployed three times at Site 1247. Two of these deployments were successful (i.e., a core under pressure was recovered). The ball valve did not fully close during the other deployment. The main objectives of the deployments were (1) to construct a detailed profile of concentration and composition of natural gases in the upper part of the section (0-125 mbsf) and (2) to identify the presence/absence and concentration of gas hydrate within the GHSZ.
Specific depth intervals were targeted for deployment of the PCS. One core (Core 204-1247B-4P [22.6-23.6 mbsf]) was recovered in shallow sediments, and one core (Core 16P [123.3-124.3 mbsf]) was recovered from above the BSR at ~129 mbsf.
The PCS chambers were degassed for times from 547 to 3454 min after recovery on board (Table T14). Pressure was recorded during degassing experiments (Fig. F30). Gas was collected in a series of sample increments (splits), and most were analyzed for molecular composition (see "Organic Geochemistry"). In addition, gas splits were subsampled for onshore analyses. After degassing, the PCS chambers were disassembled. The lengths of the cores were measured (Table T14), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 15- to 550-mL increments. The measured incremental and cumulative volumes are plotted vs. time (Fig. F30). The cumulative volume of released gas varies from 195 (Core 204-1247-4P) to 6025 mL (Core 16P) (Table T14). No gas was released during the last openings in both degassing experiments, suggesting that all gas present in the cores was collected.
Gases released from the PCS are mixtures of air (N2 and O2), CH4, CO2, and C2+ hydrocarbon gases (see "Gas Hydrate and Pressure Cores" in "Organic Geochemistry"). The abundance of air components in the PCS gas samples (5.6%-52.9% of gas mixtures) suggests that air was not properly displaced from the PCS by seawater during deployments. Methane is the dominant natural gas present in collected gas splits. The molecular composition of gases from the PCS is similar to the composition of gas voids at adjacent depths (Fig. F19).
Sediments in cores recovered with the PCS have lithologies that are similar to sediments recovered with the APC at adjacent depths (see "Physical Properties"). Porosity values measured on samples from APC cores taken near the PCS were used to estimate the methane concentration in situ (Table T14).
The concentration of methane in situ was estimated based on data from the degassing experiment (i.e., total volume of methane) and core examination (i.e., length of recovered core and the porosity of sediments). The calculation yields equivalent concentrations varying from 4.3 to 292.4 mM of methane in pore water. These concentrations have been compared with the theoretical methane-solubility curve extrapolated from values calculated for higher pressures (depths) (Handa, 1990; Duan et al., 1992) and are illustrated in Figure F31.
Preliminary analysis of gas concentrations suggests that gas hydrate may have been present in small concentrations (perhaps <3% of pore volume) in Core 204-1247B-16P, although no evidence of the presence of gas hydrate was found in the pressure record of core degassing (Fig. F30). Methane concentration measured in shallow Core 204-1247B-4P is consistent with the trend of concentrations that can be extrapolated based on the headspace measurements. This confirms that the PCS can be successfully used to study methane generation and flux in shallow sediments. Additional comparison of measured methane concentrations with theoretical methane solubility above and below the BSR will be performed on shore to better estimate if methane was present in situ in solution, in free phase, or as gas hydrate.
No HYACINTH pressure cores were taken at this site.