Thirteen in situ temperature runs were made at this site: nine APCT tool and four DVTPP runs (Fig. F30). APCT data were modeled using the software program TFIT (as described in "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter) using measured thermal conductivities (see "Physical Properties"). Temperatures for the DVTPP runs were measured directly from the data because the longer time taken for this measurement results in temperatures that appear to have reached equilibrium. Only the deepest of the DVTPP time series shows the frictional pulse that is expected when the temperature probe is extracted. The two shallower DVTPP measurements yield temperature estimates that are slightly higher than, but generally consistent with, the APCT measurements. The DVTPP at 138.5 mbsf did not yield a reliable measurement.
The temperature estimates are given in Table T15 and shown in Figure F31. We note that at this site, no dedicated mudline temperature measurement was taken. Mudline temperatures taken for ~5 min prior to recovery are very variable and range from 4.36° to 5.51°C. The average is given in the table, but we did not include this data point in the determination of temperature gradient. The APCT data alone yield a temperature gradient of 0.053°C/m. If the DVTPP data are included, the apparent temperature gradient increases to 0.058°C/m. The apparent mismatch between the observed BSR depth of ~112 mbsf and the depth of 128-137 mbsf calculated from the temperature data, assuming a pure methane/seawater system, implies a temperature anomaly of 0.5°-1.0°C at the BSR, similar to that observed at several other sites. Although additional analysis of the data is needed to resolve questions about instrument calibration, we can conclude that the rate of vertical advection of acqueous fluid must be slow (see "Downhole Tools and Pressure Coring" in the "Site 1249" chapter).
Three in situ pressure measurements were made at this site using the DVTPP. They do not show the expected decay in pressure with time. Analysis will be conducted postcruise.
The ODP PCS was deployed seven times at Site 1250. Five deployments were successful (i.e., a core under pressure was recovered). The ball valve did not fully close during the other deployments. 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 (0-140 mbsf) and (2) to identify the presence/absence and concentrations of gas hydrate within the GHSZ.
Specific depth intervals were targeted for deployment of the PCS. Three cores (Cores 204-1250C-9P [71-72 mbsf] and 204-1250D-5P [35-36 mbsf] and 13P [103.5-104.5 mbsf]) were recovered from above the BSR at ~112 mbsf. Two other cores (Cores 204-1250D-18P [135.2-136.2 mbsf] and 204-1250F-4P [119-120 mbsf]) were recovered from below the BSR.
The PCS cores were degassed for 579-1800 min after recovery on board (Table T15). Pressure was recorded during degassing experiments of all cores, except for Core 204-1250F-4P (Fig. F32). 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 was disassembled. The lengths of the cores were measured (Table T16), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 10- to 810-mL increments. The measured incremental and cumulative volumes are plotted vs. time on Figure F32. The cumulative volume of released gas varies from 1385 (Core 204-1250D-18P) to 5390 mL (Core 204-1250F-4P) (Table T16). The volume of the last gas splits varies from 10 (Cores 204-1250C-9P and 204-1250D-13P) to 30 mL (Core 204-1250D-5P). This observation suggests that almost 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 (3.0%-8.3% of gas mixtures) suggests that air was not always 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. F21).
Sediments in cores recovered by the PCS have lithologies similar to sediments recovered by the APC and XCB cores at adjacent depths (see "Physical Properties"). Porosity values measured in APC and XCB cores taken near the PCS were averaged and used to estimate the methane concentrations in situ (Table T16).
The concentrations of methane in situ were 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 65.7 to 278.3 mM of methane in pore water. These concentrations have been compared with the theoretical methane-solubility curve extrapolated from values calculated for higher pressures (greater depths) (Handa, 1990; Duan et al., 1992) (Fig. F33). Preliminary analysis of gas concentrations suggests that gas hydrate may have been present in concentrations varying from 0.6% to 2.2% of pore volume in all three cores (Cores 204-1250C-9P and 204-1250D-5P and 13P) recovered from above the BSR. Interestingly, no evidence of gas hydrate presence was found in the pressure record of core degassing (Fig. F32). Free gas appears to be present in relatively high concentrations (perhaps around 4% of pore space) at a depth of ~7.5 m below the BSR (Core 204-1250F-4P). Only dissolved methane seems to be present in the core (Core 204-1250D-18P) retrieved from ~24 m below the BSR. 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.
The HYACINTH pressure coring tools were deployed twice at Site 1250: one with the FPC (Core 204-1250C-18Y [FPC 4]) and one with the HRC (Core 204-1250D-17E [HRC 3]). After the lessons learned from the procedures and the tool adjustments made at Site 1251, we hoped that a pressurized core might be recovered from this site. A good core was recovered with the FPC, but full retraction into the autoclave was prevented as a result of inadvertent line tension during the coring operation. The HRC, on the other hand, recovered a short core at full in situ pressure, which was considered a significant success. Core 204-1250D-17E (HRC 3) was then transferred under full pressure and logged in the V-MSCL before being depressurized. Unfortunately, the DSA tool was still dogged by technical difficulties and failed to provide any useful downhole data during the coring operations. However, the rig floor data proved valuable in analyzing some aspects of the behavior during each deployment, especially given the fact that there had been a brief tension load on the wireline during the FPC deployment.
After transferring Core 204-1250D-17E (HRC 3) to the logging chamber, the density profile was measured in the V-MSCL, where we discovered how short the core was (20 cm). This short core had sediment densities of ~1.65 g/cm3 and produced some obvious gas layers when depressurized (Table T17).
The HRC was deployed at Site 1250 in a water depth of 796 m (see Table T18). Core 204-1250D-17E (HRC 3) was taken in the bottom of Hole 1250D at 134.2 mbsf. Lithologies at this depth were lightly indurated silty clays that were being cored with full recovery by the APC. These sediments were considered suitable for the FPC, and it was thought that they might be stiff enough to be cored using the HRC. A modified finger catcher was made and fitted for this deployment in the hope that it might help retain the core in this type of formation. The deployment went well, with the procedures being essentially the same as for previous runs (see "Downhole Tools and Pressure Coring" in the "Site 1251" chapter and "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter). The HRC was lowered in the drill string at 72 m/min while circulating and rotating. Pumping was stopped at 890 m, the tool was lowered down slowly to the landing position, and a slack of 4 m was given on the wireline. The drill string was then lowered to TD with the wireline following. The blowout Preventer (BOP) on the wireline was closed, and pumping was started at 82 gallons per minute (gpm). A pressure spike was observed at 540 psi, indicating that coring had begun, and then pumping continued at 95 gpm for 20 min before stopping and opening the BOP. The drill string was lifted to 937 m before the BOP was closed again and pumping continued for 2 min (to ensure full stroke) at 80 gpm. The BOP was again opened, and the tool was lifted on the wireline very slowly for the first 16 m before being raised to the surface at 110 m/min. During the coring operation the weight on bit was set at ~15,000 klb, and both the active and passive heave compensators were activated. The tool was recovered and placed on the trestles in the usual way. It was disassembled and the autoclave moved to the transfer system where an internal pressure of 88 kbar was measured. The core was retracted into the shear transfer chamber under full pressure where it was sheared and successfully moved into the logging chamber. After the core had been logged in the V-MSCL, it was depressurized in stages before being removed and curated as a 28-cm-long core. Note that this short core was the first of its kind to have been recovered and logged in a liner under full in situ pressure.
A single FPC deployment was made at Site 1250 in a water depth of 796 m (see Table T18). Core 204-1250C-18Y (FPC 4) was recovered from the bottom of Hole 1250C at 137.5 mbsf. Lithologies at this depth were lightly indurated silty clays that were suited to the APC and considered suitable for the FPC hammer mechanism. Operationally, the deployment ran smoothly as per previous deployments (see "Downhole Tools and Pressure Coring" in the "Site 1251" chapter) with the downhole procedures being followed and the active heave compensator being used throughout. A good core was recovered (84 cm long) but the inner rod had not stroked out completely and the core had only partially retracted into the autoclave. The pressure was released by drilling small holes in the liner prior to removing the core. Once again during this deployment the FPC data logger worked well but the Lamont DSA tool only collected data from the very beginning part of the test. An analysis of the rig data concluded that at one time a significant tension had been applied on the wireline during the coring operation (which probably prevented a full stroke occurring) and should be avoided by paying out more slack wire on subsequent deployments.