Ten measurements of in situ temperature, including one mudline temperature taken prior to coring, were made with the APCT tool at this site in Holes 1244B, 1244C, and 1244E. APCT 12 was used in Holes 1244B and 1244C, and APCT 11 was used in Hole 1244E. Four additional temperature measurements were made with the DVTPP, generally at depths greater than those suitable for the APCT tool.
Measurements were taken at ~30-m intervals and span the depth range of 35.1-149.4 mbsf (Table T19). All downhole temperature-tool deployments resulted in temperature histories that showed clear penetration and extraction pulses and smooth temperature decay (see "Downhole Tools and Pressure Coring" in the "Explanatory Notes" chapter). Raw data are shown in Figure F34. Only the portion of the data from the immediate time period before, during, and after tool insertion is shown.
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") and are plotted in Figure F35. Uncertainty in the extrapolated value of in situ temperature resulting from the 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 also estimated to be ~0.02°C.
DVTPP temperatures were picked from the measured temperature recorded late in the time series. For these relatively long deployments, the temperature appears to have approached equilibrium, so no further extrapolation of the DVTPP temperature data was done at this site. Uncertainties in the in situ temperature estimated from DVTPP data are estimated to be ~ 0.1°C because the DVTPP data are noisier than the APCT data. Because mudline temperatures recorded by the DVTPP tip 3 were consistently higher than those recorded by APCT 12, an empirically determined shift of -1.40°C was applied to the DVTPP data measured with tip 3 before plotting the data to determine the subsurface temperature gradient. APCT and DVTPP temperature estimates made at depths of 62.5 and 63.5 mbsf, respectively, yield temperatures that differ by only 0.074°C, verifying this empirical calibration.
During the course of the leg, an unexpectedly important source of uncertainty in APCT measurements became apparent. After APCT 12 showed an apparent temperature jump of >2°C during the course of drilling at Site 1246, we decided to calibrate each APCT tool using an ice-water bath (see "Downhole Tools and Pressure Coring" in the "Site 1246" chapter). This simple experiment revealed considerable variability among the tools. APCT 11, which was used in Hole 1244E, yielded a temperature for ice water of -0.513°C. We do not have a "prejump" calibration for APCT 12, which was used in Holes 1244B and 1244C, as well as at Sites 1245, 1246, and 1248-1251. To estimate the offset between APCT 11 and APCT 12, we calculated the temperature gradient and extrapolated seafloor temperature for each instrument separately (excluding the mudline temperature). Slopes are similar (slope = 0.0615 for APCT 12 and slope = 0.0632 for APCT 11) and their intercepts differ by 0.58°C (intercept = 3.99 for APCT 12 and intercept = 3.41 for APCT 11). This suggests that APCT 12 was calibrated to within ~0.1°C prior to its use at Site 1246. In the remainder of this report, in situ temperatures derived from APCT 11 are corrected by +0.51°C for determination of thermal gradients (but not in the tables); calibration correction is applied to data from APCT 12.
Figure F35 shows the temperature gradient determined at this site and the position of the BSR, as determined from seismic reflection data and confirmed by downhole acoustic logging. The dashed horizontal line shows the predicted depth to the gas hydrate stability field, assuming the temperature gradient determined for Site 1244 and hydrostatic pressure for a pure methane and seawater system (Maekawa et al., 1995). The predicted stability boundary is ~10 m below the BSR, and the temperature at the BSR appears to be ~0.6°C too cold. Possible explanations for this mismatch will be discussed elsewhere.
The DVTPP pore pressure measurement at 62.5 mbsf in Hole 1244C (Fig. F34B) yields a rapid decay to a pressure that is within ~20 psi of the predicted hydrostatic pressure. In contrast, the signal from the DVTPP pressure measurement in Hole 1244C decays gradually (Fig. F34B), suggesting that the pressure probe was inserted into an intact formation at 150.2 mbsf and measured a true formation pore pressure. Pressure had clearly not reached equilibrium after 35 min in the formation, requiring additional modeling to extrapolate from the data to derive an estimate of in situ pressure. This modeling will be part of a shore-based effort. All of the DVTPP runs yield temperature data that are consistent with in situ temperatures measured by the APCT tool but only one of the runs appears to yield an accurate in situ pressure measurement. This suggests that the pressure measurements are more sensitive to minor cracking of the formation around the probe than the temperature measurements.
The first run of the piezoprobe in Hole 1244B was unsuccessful. A second deployment of the piezoprobe in Hole 1244C produced a well-defined pressure-dissipation curve. These results will be used with the data collected from postcruise geotechnical testing of sediment from this site to evaluate the in situ state of stress and permeability of the formation. In addition, the piezoprobe data will be compared with data from the DVTPP as part of a postcruise study.
The PCS was successfully deployed seven times at Site 1244. 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 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. Five cores (Core 204-1244F-4P [23.1-24.1 mbsf]; 204-1244E-6P [39.2-40.2 mbsf], 11P [71.6-72.6 mbsf], and 15P [102-103.1 mbsf]; and 204-1244C-14P [119.5-120.5 mbsf]) were recovered from above the BSR at ~124 mbsf. The other two cores (Cores 204-1244C-16P [130.5-131.5 mbsf] and 18P [141.5-142.5 mbsf]) were recovered from below the BSR.
The PCS cores were degassed for periods of 450-2999 min after recovery on board (Table T20). No pressure was recorded during degassing of Cores 204-1244C-14P, 16P, and 18P because of the lack of equipment. Pressure transducers were not properly calibrated during degassing of Cores 204-1244E-6P and 15P and 204-1244F-4P, and the pressure record is not reported here. Pressure was recorded during degassing of Core 204-1244E-11P (Fig. F36). 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 PCSs were disassembled. The lengths of the cores were measured (Table T20), and samples were taken for analysis of physical properties (see "Physical Properties").
Gas was collected in 2.5- to 580-mL increments. The measured incremental and cumulative volumes are plotted vs. time in Figure F36. The cumulative volume of released gas varies from 595 mL (Core 204-1244E-4P) to 4530 mL (Core 11P) (Table T20). The volume of the last gas splits varies from 0 mL (Core 204-1244E-15P) to 120 mL (Core 204-1244C-14P). This observation suggests that, in some cases, not all gas 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 (1.4%-33.33% 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 (see Fig. F24).
Sediments in cores recovered by the PCS have lithology similar to sediments recovered by APC and XCB at adjacent depths (see "Physical Properties"). Porosity values measured in APC and XCB cores taken near the PCS were used to estimate the methane concentration in situ (Table T20).
The concentration of methane in situ was estimated based on data from the degassing experiments (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 23.7 to 221.2 mM (Table T20). These concentrations have been compared with the theoretical methane-solubility curve extrapolated from values that were calculated for higher pressures (depths) (Handa, 1990; Duan et al., 1992) (Fig. F37). Preliminary analysis suggests that gas hydrates have been present in relatively low concentrations in Cores 204-1244E-11P (~2% pore volume) and 15P (~0.5% pore volume). Interestingly, no evidence of the presence of gas hydrate was found in the pressure record during degassing of Core 204-1244E-11P (Fig. F36E). The presence of gas hydrate in these two cores correlates well with the observed distribution of gas hydrate at Site 1244.
The concentration of methane in cores taken ~4 m above the BSR (Core 204-1244C-14P), ~7 m below the BSR (Core 16P), and ~18 m below the BSR (Core 18P) is estimated to be below saturation. The measurements suggest that there is neither gas hydrate nor free gas in the intervals sampled near the BSR.
Based on PCS measurements, gas hydrate appears to be more abundant in the middle part of the GHSZ. This observation is consistent with visual observations on the catwalk, Cl¯ anomalies (see "Interstitial Water Geochemistry"), and well-logging data (see "Downhole Logging"). However, the gas hydrate concentration in sediments appears to be lower than that estimated by other methods. 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 only HYACINTH pressure core taken at Site 1244 was Core 204-1244E-8Y (FPC 9) at 50.70 mbsf. This core was recovered under full pressure well above the BSR (located at 124 mbsf) and in the GHSZ. Hydrate samples had been collected in the previous core (Core 204-1244E-7H) at 49.94 mbsf and were collected from the core beneath Core 8Y (FPC 9; Core 9H) at 58.08 mbsf. It was therefore anticipated that this short pressure core might contain some hydrate.
No HRC cores were taken at Site 1244.
A single FPC deployment was made at Site 1244 (Core 204-1244D-2Y; FPC 9) (see Table T21). After a number of adjustments based on experience from previous deployments, the outstanding problem was effective and reliable sealing of the lower autoclave valve. For this deployment further minor adjustments were made to the valve and plans were made to adjust the speed profile of the drill string when withdrawing the tool from the bottom.
After assembly and making up in the drill string, the tool was lowered while rotating at 20 revolutions per minute (rpm) and pumping at 160 gallons per minute (gpm), with the APC on (maximum heave = ~1 m). As the tool approached the BHA, the bit was picked up 2 m above TD and rotating was stopped. The tool was landed slowly while some flow was taking place. The pump pressure was increased to 200 psi, and 5 m slack was given on the sandline. Following this procedure, the bit was lowered to the bottom and the weight was set at ~10 kips. The pump pressure was increased to ~700 psi while the operator held his fingertips to the drill string. This sophisticated sensor system detected the pins shearing and a small amount of hammering. After hammering, the pump pressure was increased to 800 psi to ensure that the end of stroke had been reached. To withdraw the core from the formation, the drill string was lifted ~1 m at a moderate speed (290 m/hr) and then lifted for 3 m at a slower rate (100 m/hr). The tool was then lifted through the drill string on the sand line (slowly at first at a rate of 6 m/min).
Once the tool was laid on the piperacker, a visual observation indicated full closure of the lower valve. The autoclave was removed and placed in the ice trough while the data logger was analyzed. This showed that the valve had closed at the seabed. A maximum pressure of 92 kbar was recorded just prior to the autoclave being immersed in the ice bath. The core was transferred from the autoclave to the shear transfer chamber, sheared, and then transferred into the logging chamber. This proved to be a difficult operation due to the tight tolerances of the ball valves. To help the core from increasing in temperature, ice bags were laid over the transfer chambers during this operation. The logging chamber was placed in the ice bath overnight (which reduced the pressure to ~60 kbar) before being transferred to the Geotek V-MSCL for analysis.
After having been stored in an ice bath overnight, the HYACINTH logging chamber containing Core 204-1244E-8Y (FPC 9) was loaded into the Geotek Vertical Multi-Sensor Core Logger (V-MSCL). Initially, the pressure at this stage was 60 bar. Over the next 12 hr, 17 high-resolution GRA logs were recorded. Between the logging runs, the pressure was slowly and incrementally released, and 3.8 L of gas was collected and analyzed in a process similar to that performed on Core 204-1249F-2E (HRC 4) (see "Downhole Tools and Pressure Coring" in the "Site 1249" chapter). The initial gamma density profile indicated a single distinct zone of low density (at the logging interval of 31-37 cm), provisionally interpreted as a hydrate layer, which when depressurized formed a thin gas layer. Figure F38 shows the following three density profiles:
The density anomaly interpreted to be a hydrate in Run 1 can be accounted for by 0.5 cm of hydrate (crystal density = 0.92 g/cm3) in the 5-cm-diameter core. The lower gas layer, which formed at the base of the core, may indicate the possible presence of another discrete hydrate zone in the core catcher (below the level of logging). After all the gas had been removed, the core was X-rayed, run through the MST, split, and digitally imaged. Three IW samples were taken from Core 204-1244E-8Y (FPC 9), two near the suspected hydrate zones and one in the center of the core. These data fit well with the IW samples collected elsewhere in this hole (see "Interstitial Water Geochemistry"). It is also interesting to note that the sulfate data indicated no contamination from surface seawater, despite using surface seawater as the pressurizing fluid. It is anticipated that a detailed analysis of these data sets will provide pertinent information on the nature and structure of this region where gas hydrate forms at relatively low concentrations.
It is interesting to note that in the APC cores immediately above and below Core 204-1244E-8Y (FPC 9; Cores 7H and 9H), the thermal images from the IR camera show approximately six to eight low-temperature anomalies per core (see "Physical Properties"). This frequency of one to two anomalies per meter is commensurate with the two anomalies in density provisionally interpreted from Core 204-1244E-8Y (FPC 9). It is possible that all the hydrate in these regions of low concentration exists in the hydrate veins, and none is disseminated in the pore space.