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doi:10.2204/iodp.proc.307.203.2009

Heat flow data

Deployment of the APCT provided insight into the subsurface temperature distribution. Two, three, and four discrete temperature measurements in Holes U1316A, U1317A, and U1318A were taken ranging from 25.5 to 113.7 meters below seafloor (mbsf), and there are no resolvable thermal perturbations (Fig. F2). Maximum distance and elevation difference within each paired set among three holes are 14 km and 500 m, respectively. Data from all three holes and well-constrained Hole U1318A show temperature gradients of 39°C/km and 46°C/km with regression standard errors of 0.896 and 0.087, respectively. These estimated values are normal gradients for a passive continental margin and are consistent with the average present-day thermal gradient of ~34°C/km based on three deep (>4 km) wells at Porcupine Basin (Corcoran and Clayton, 2001).

Thermal conductivity was measured on one section of unsplit soft-sediment core, usually at 75 cm, using the TK04 measurement system. The depth profile of thermal conductivity for Sites U1316, U1317, and U1318 is shown in Figure F3. No systematic variation among the holes could be resolved. The average thermal conductivity within intervals that coincides with measured temperatures is 1.4 W/(m·K) (Fig. F4). Heat flow is estimated at 56 mW/m² using observed thermal gradient and thermal conductivity from three holes.

Thermal conductivity is primarily dependent on variations in sediment bulk density, which is related to other sediment physical properties such as velocity; therefore, these data sets are well correlated (Fig. F5). Average gamma ray attenuation (GRA) density at Site U1316 changes at 64 mbsf with an amount of scatter that coincides with a drop in thermal conductivity. Carbonate content increases from 17 wt% in the upper 50 mbsf to 60 wt% over a short interval between ~50 and 55 mbsf and is relatively constant at an average of 28 wt% between ~65 and 119 mbsf (see the “Site U1316” chapter). This pattern also corresponds to the depth profile of thermal conductivity. At Site U1317, GRA density increases gradually with depth in the upper ~150 mbsf and drops at ~150 mbsf. This pattern is not well correlated with thermal conductivity and carbonate content. At Site U1318, GRA density slightly increases in the upper 92 mbsf with some peaks and troughs. Two sharp reductions in density were observed at 92 and 132 mbsf, which are well correlated to variations in thermal conductivity. Carbonate concentrations are relatively low and uniform throughout the upper part from 0 to ~86 mbsf and increase at 86 mbsf. This appears to relate to changes in thermal conductivity and GRA density.

Downhole temperature and thermal conductivity measurements from Sites U1316, U1317, and U1318 of Expedition 307 and Hole 981C of Ocean Drilling Program Leg 162 (Shipboard Scientific Party, 1996) were used to derive the heat flow through the continental margin off Ireland. As shown in Figure F1, these data have been used to investigate the thermal state of the continental margin, with the global heat flow compilations of Pollack et al. (1993). The estimated heat flow value of 56 mW/m² is quite average among passive continental basins and there may be no thermal perturbations. This may support the results from initial geochemistry and microbiology (see the “Expedition 307 summary” chapter), in which a role for hydrocarbon fluid flow at Challenger Mound is not obvious.

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