At Site 1230, the downhole tools employed were the Adara temperature shoe, DVTP, DVTP-P, WSTP, and PCS. The results of the temperature and pressure measurements at Site 1230 are described in the two sections below. A sample of bottom water was successfully collected with the WSTP from 10 to 20 m above the seafloor at the site of Hole 1230A. Chlorinity data showed that this sample deviated <7% from IAPSO standard seawater (see "Interstitial Water" in "Biogeochemistry").
Abundant gas voids in sediment indicated that significant loss of methane occurred during core recovery at Site 685 (Shipboard Scientific Party, 1988). Because methane comprises an important product and reactant in biogeochemical reactions, the PCS was deployed 10 times at Site 1230 to retain methane otherwise expected to vent during the wireline trip. The ultimate scientific objective of this exercise is the postcruise construction of an in situ methane concentration profile (see, e.g., Dickens et al., 1997). Initial results of PCS deployments at Site 1230 are presented in Dickens et al. (this volume).
Three downhole temperature determinations were made at Site 1230 using two DVTP deployments in Hole 1230A and one in Hole 1230B. Two Adara temperature shoe deployments at the seafloor yielded values for the bottom-water temperature of 1.74°-1.75°C. The WSTP deployment successfully recorded a value of 1.9°C for water temperature 10-20 m above the seafloor. Figure F26 shows the data from the successful DVTP deployments at 33.3, 100, and 254.6 mbsf. Three other deployments at 73.5, 79.9, and 148.3 mbsf resulted in records that could not be used. Table T12 summarizes all of the Site 1230 temperature measurements.
The results of the DVTP and Adara temperature measurements are displayed in Figure F27. The combined downhole temperatures yield a linear gradient of 0.0343°C/m in the upper 255 m of the sediment column (Fig. F27). The extrapolated temperature at the bottom of Hole 1230A, at 277.3 mbsf, is 11.2°C. Multiplying the gradient by an average thermal conductivity of 0.83 W/(m·K) (Fig. F25) gives a conductive heat flow estimate of 28 mW/m2 at Site 1230. This heat flow estimate is identical to the value estimated for Site 685 by the Leg 112 Shipboard Scientific Party (1988). Figure F27B shows the temperature data with a theoretical steady-state conductive temperature profile calculated using a constant heat flow of 28 mW/m2, a seafloor temperature of 1.74°C, and the measured thermal conductivities from the Hole 1230A cores. To illustrate that the steady-state conductive profile cannot match the curvature of the temperature data, a best-fit parabola is also shown. Preliminary calculations indicate that the curvature in the downhole temperature profile can be explained by upward fluid flow of 1-10 mm/yr (Fig. F27C). This rate is comparable to model estimates for flow rates in the Peru accretionary prism (Kukowski and Pecher, 1999). On the basis of the DIC profile, an upper bound on the flow rate is ~1 mm/yr (see "Biogeochemistry").
The failure rate for DVTP measurements attributed to formation conditions at Site 1230 was 50%, compared to 80% at Site 1229, 50% at Sites 1227 and 1228, and 0% at Sites 1225 and 1226. To understand the causes of the failures, the conditions of the formation at each location were evaluated by noting the recovery, lithology, and disturbance of the subsequent core (Table T12). The differences in outcome do not appear to be related to either lithology or core recovery. Instead, the degree of disturbance in the subsequent core may be significant. Although the cause of core disturbance on recovery is not known, areas of multiple fractures, voids, and crumbly fabric were common in the intervals with high methane concentrations and gas hydrates (see Fig. F34, and "Gas Hydrate"). Moreover, the three failed deployments were located at 70-80 and 148-150 mbsf, where the interstitial water lithium concentrations indicate the highest hydrate concentrations (see "Interstitial Water" in "Biogeochemistry"). On the basis of these observations, the best strategy for obtaining high-quality temperature data in hydrate-bearing formations may be to identify depths of highest hydrate concentrations in the first hole and avoid these depths by deploying the tool in subsequent holes.
The DVTP-P was deployed twice at Site 1230 with one successful run at a depth of 102.5 mbsf (after Core 201-1230A-14H) (Fig. F28). The results of this deployment indicate overpressure in the formation relative to the base of the hole. This deployment did not detect a pressure spike at the start because the tool was pushed in extremely slowly over several minutes. The slow deployment strategy was used to increase the penetration depth of the tool without exceeding acceptable pressure on bit in the relatively stiff formation. The success of this strategy is evident from the 0.14-MPa difference between formation pressure and the hydrostatic pressure measured in the base of the hole for 5 min at the end of the deployment. For a reasonable formation permeability of ~10-16 m2, the measured overpressure is adequate to produce flow at a rate of ~5 mm/yr.
The APC-M tool was deployed in Hole 1230B continuously from Cores 201-1230B-3H through 13H. The recovered data from this run showed that the tool and data logger initially functioned correctly. Data from the APC-M tool will be analyzed postcruise.