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Downhole measurements

Heat flow

Twelve APCT-3 downhole temperature measurements were made in Holes U1385A, U1385B, and U1385D. Sediment temperatures ranged from 4.78°C at 34.6 mbsf to 8.02°C at 115.5 mbsf (Table T24), giving a geothermal gradient of 39.2°C/km (Fig. F34). The bottom water temperature was 3.4°C based on the average of the minimum temperature in the 12 APCT-3 temperature profiles. The thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndmann et al. (1974) (see “Physical properties” in the “Methods” chapter [Expedition 339 Scientists, 2013]). The calculated in situ values average 1.1% below the measured laboratory values. Thermal resistance was calculated by integrating the inverse of in situ thermal conductivity over depth (Fig. F34). A heat flow of 47.5 mW/m2 was obtained from the linear fit between temperature and thermal resistance (Pribnow et al., 2000). This value is in the lower half of the normal range for heat flow on the Portuguese margin (Grevemeyer et al., 2009).