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doi:10.2204/iodp.proc.346.107.2015

Downhole measurements

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1426A at five depths, including the mudline. In situ temperatures range from 4.00°C at 31.0 m CSF-A to 13.88°C at 116.5 m CSF-A (Table T20), with a linear downhole increase indicating that the gradient is uniform with depth (Fig. F49). A linear fit of temperature versus depth gives a geothermal gradient of 115°C/km, slightly higher than was measured during ODP Leg 127 at Site 798 (111°C/km) (Ingle, Suyehiro, von Breymann, et al., 1990). The bottom water temperature at this site is estimated to be 0.51°C, based on the average of the mudline temperature and the four APCT-3 measurements. This is slightly higher than the mudline temperature measured at previous sites.

The thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al. (1974) (see “Physical properties” in the “Methods” chapter [Tada et al., 2015b]). Thermal resistance was then calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole (Fig. F49). A heat flow of 94 mW/m² was obtained from the slope of the linear fit between in situ temperature and in situ thermal resistance (Pribnow et al., 2000). This value is in the range of the heat flows measured at Site 798 (91 mW/m² from Ingle, Suyehiro, von Breymann, et al. [1990] and 101 mW/m² from Langseth and Tamaki [1992]) and in the region (from 68 to 137 mW/m² from Ingle, Suyehiro, von Breymann, et al. [1990]).

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