At Site 1231, the downhole tools employed were the Adara temperature shoe and the DVTP-P. The results of the temperature and pressure measurements at Site 1231 are described in the two sections below.
Six downhole temperature determinations were made at Site 1231 using three Adara tool deployments in Hole 1231B, two in Hole 1231D, and one in Hole 1231E. An Adara temperature shoe deployment at the seafloor yielded a value for the bottom-water temperature of 1.75°C. Table T11 summarizes all of the Site 1231 temperature measurements.
The combined downhole temperature profile exhibits a noticeable change in gradient somewhere between 41.4 and 60.4 mbsf. A relatively steep gradient in the upper part of the sediment column of 0.090°C/m changes to a more moderate gradient of 0.035°C/m in the lower part (Fig. F18A). Using the lower gradient, the extrapolated temperature at the sediment/basement interface is 8.6°C. Figure F18B shows the temperature data with two theoretical steady-state conductive temperature profiles calculated using a seafloor temperature of 1.75°C and the measured thermal conductivities from the Hole 1231B cores. Like the temperature data, the simulated profiles show a marked gradient change at 55 mbsf because of a marked change in sediment composition and corresponding large increase in thermal conductivity from ~0.7 W/(m·K) above this depth to ~1.2 W/(m·K) below (Fig. F18). The two simulated profiles bracket the data, but it is not possible to fit both the upper and lower gradients with a steady-state conductive model using the measured conductivities. Other hypotheses to explain the temperature data such as fluid flow and basement temperature variations should be explored. The two conductive profiles provide a range for the heat flow of 51-64 mW/m2.
The DVTP-P was deployed once at Site 1231 at a depth of 107.9 mbsf (after Core 201-1231A-12H). The results of this deployment may indicate minor overpressure in the formation relative to the base of the hole (Fig. F19). The expected 0.7-MPa spike was present at the start of the deployment, but the pressure dropped rapidly within a minute and then stabilized at a flat value for the rest of the 30-min run. The recorded value for the formation is 0.02 MPa higher than that recorded in the base of the hole at the end of the run. Assuming that the tool was raised ~1 m for the stop at the base of the hole, the formation may be slightly overpressured. The data quality suffers from heave during the deployment (average > 2 m). For a reasonable formation permeability of ~10-16 m2, the measured overpressure would be adequate to produce flow at a rate of ~0.4 mm/yr. On the basis of chloride data, upward flow at this site is estimated to be ~2 mm/yr (see "Biogeochemistry")