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doi:10.2204/iodp.proc.346.103.2015

Downhole measurements

In situ temperature and heat flow

APCT-3 downhole temperature measurements were performed in Hole U1422C at five depths, including the mudline. In situ temperatures range from 4.80°C at 33.1 m CSF-A to 15.86°C at 115.8 m CSF-A (Table T19), with a linear downhole increase indicating that the gradient is uniform with depth (Fig. F38). A linear fit of temperature versus depth gives a geothermal gradient of 134°C/km, slightly higher than the one measured during Leg 127 at Site 795 (132°C/km) (Langseth and Tamaki, 1992). The bottom water temperature at this site is estimated to be 0.35°C, based on the average mudline temperature in the four APCT-3 measurements. This low value is in good agreement with the expected temperature as most of the water column in the marginal sea is filled with cold water (<1°C).

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]). The calculated in situ values are as much as 2.6% below the measured laboratory values. Thermal resistance was then calculated by cumulatively adding the inverse of the in situ thermal conductivity values over depth intervals downhole (Fig. F38). A heat flow of 120 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 6% higher than the one calculated at Site 795 (113 mW/m²) (Langseth and Tamaki, 1992).

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