Temperature measurements were made at three subbottom depths in Hole 1165B from Cores 188-1165B-4H, 10H, and 14H using the Adara temperature tool. A fourth temperature measurement was attempted for Core 188-1165B-7H; however, because of the failure of the Adara tool, no data could be obtained for this depth. Using the TFIT software package, measured temperature data were extrapolated to a stabilized sediment temperature (Table T16; Figs. F72, F73, F74) for each APC deployment of the Adara temperature tool. The seafloor temperature was determined as the lowest mudline temperature from each measurement before probe penetration. The measured temperatures are plotted against depth in Figure F75.
A straight line was plotted through the data to determine an average thermal gradient of 53.4°C/km for the uppermost 126 m of sedimentary section. A mean measured thermal conductivity of 0.915 W/(m·°C) (see "Physical Properties") was multiplied by the thermal gradient to provide an estimated heat flow of 51.4 mW/m2. Assuming a conductive steady-state system and constant heat flow in the sedimentary section, measured conductivities from the needle probe (see "Physical Properties" in the "Explanatory Notes" chapter) were used to provide estimated downhole temperatures (Fig. F75). The deepest temperature estimate at 970.5 mbsf was 43.6°C, which is ~10°C less than the linearly extrapolated temperature value. Using the conductivity-estimated temperature profile, a secant thermal gradient of 45.2°C/km was determined. Both the heat flow and secant thermal gradient lie within normal thermal ranges for postrift passive margin regions (Allen and Allen, 1990).
Depths to the base of the gas hydrate stability zone (GHSZ) were determined using the ODP Pollution Prevention and Safety Panel (PPSP) hydrate stability equation (PPSP, 1992), which was modified for seawater. Using the linearly extrapolated thermal gradient of 53.4°C/km, the depth to the base of the GHSZ was determined to be 440 mbsf. However, using variable interval thermal gradients from Table T16, the depth to the base of the GHSZ was estimated to be 460 mbsf.