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doi:10.2204/iodp.proc.317.104.2011

Heat flow

Geothermal gradient

Temperature measurements were conducted using the APCT-3 during APC coring in Holes U1352A and U1352B and the Sediment Temperature tool during XCB coring in Hole U1352B. Six temperature measurements were taken in total (Fig. F64; Table T27), and the geothermal gradient was successfully obtained from four of these (Cores 317-U1352B-10H, 15H, 20H, and 38X) within the depth interval of 93.7–313.2 m CSF-A. Unless otherwise noted, all depths in this section are reported in m CSF-A. The other two measurements in Cores 317-U1352A-4H and 317-U1352B-6H were not used because the conductive cooling time after sediment penetration was too short (<300 s) to generate reliable fitting curves (Fig. F64A), which could be the result of tool movement caused by ship heave. The fitting line to temperature versus depth data was derived from the four good results (Fig. F64B):

where T(z) is in situ temperature at depth z (m CSF-A). The estimated geothermal gradient is therefore 46.0°C/km. Note that this geothermal gradient was established for the depth interval above ~310 m, which consists of soft sediments, and might significantly decrease with depth in accordance with a rapid increase in thermal conductivity, particularly for the interval below 575 m, where rock first occurs (see "Thermal conductivity," below). For reference, the geothermal gradients at nearby ODP Sites 1120, 1124, and 1125 are 57.4°, 52.1°, and 64.9°C/km, respectively (Carter, McCave, Richter, Carter, et al., 1999). These gradients were established in soft sediments above ~130 m.

Thermal conductivity

Thermal conductivity was measured preferentially in available whole-round core sections from Holes U1352A and U1352B using the full-space needle probe method and in section halves from Hole U1352C using the puck probe method. The puck probe method was employed because the degree of induration of sediments increases with depth in Hole U1352C. Three full-space needle probe measurements were conducted in indurated sediment drilled with the RCB system: two in rather soft sediments in Cores 317-U1352C-6R and 18R and one in rock to compare with puck probe measurements in Core 317-U1352C-76R. Measurement frequency was usually once per section with one measuring cycle at each point for the full-space needle probe method and once per core with five measuring cycles at each point for the puck probe method. This included 32 points in Hole U1352A (0.2–42.1 m), 443 points in Hole U1352B (0.7–821.7 m), and 155 points in Hole U1352C (575.1–1920.6 m) (Table T28). The middle of each section was chosen as the measurement point unless a void or crack was observed (see "Heat flow" in the "Methods" chapter). Few lithologic variations occur in each section at Site U1352, so this sampling procedure was appropriate. Probes V10701 and V10819 were used, and heating power was kept to ~3 and 2 W for full-space needle probe and puck probe methods, respectively.

Thermal conductivity data were screened when (1) contact between the probe and sediment was poor, (2) thermal conductivity values were close to that of water (0.6 W/[m·K]) because of sediment dilution during coring, and (3) measurements were taken in caved-in layers such as shell hash. In most cases, the first two criteria were controlling parameters for deciding the quality of measurements. Good results were obtained at 13, 214, and 149 points in Holes U1352A, U1352B, and U1352C, respectively, covering depths of 8–42, 1–793, and 575–1921 m, respectively (Table T28). The ratio of reliable measurements to total measurements is larger for Hole U1352C, where five measuring cycles were run, than for Holes U1352A and U1352B, where only one measuring cycle was run. Therefore, we recommend increasing the number of measuring cycles at each point from one to at least three, even at the expense of decreasing measuring frequency from once per section to once per core.

Thermal conductivity measurements at Site U1352 range from 0.849 to 3.440 W/(m·K): 0.849–1.696 W/(m·K) (average = 1.305 W/[m·K]) for sediments in Holes U1352A and U1352B in the depth interval of 1–793 m and 1.572–3.440 W/(m·K) (average = 2.360 W/[m·K]) for rocks in Hole U1352C in the depth interval of 575–1921 m (Table T28). For reference, the two lowest values in Hole U1352C were measured with the full-space needle probe in sediments in Cores 317-U1352C-6R and 18R. For Core 317-U1352C-76R we compared thermal conductivity measured by the puck probe with that measured by the full-space needle probe within a drilled hole filled with thermal compound. Results were similar (within 0.6%), indicating that the difference between methods is negligible. Thermal conductivity for rocks is ~1.8 times greater than that for sediments. In the uppermost 130 m, thermal conductivity values are higher at Site U1352 than in the same interval at nearby Site 1119 (Shipboard Scientific Party, 1999b). The high conductivities at Site U1352 may be due to high concentrations of quartz (6.5–12.5 W/[m·K]) in fine-grained sediment, including the clay-sized fraction (see "Lithostratigraphy") and/or carbonate cementation (0.5–4.4 W/[m·K]).

Thermal conductivity versus depth data for Holes U1352A and U1352B are consistent (Fig. F65A). In the overlapped depth interval between Holes U1352B and U1352C (575–793 m), a gap exists between trends of values that increase with depth. This gap can be explained by sampling bias with respect to the lithologies measured in each hole: soft sediment was preferentially recovered in Holes U1352A and U1352B and hard rock was preferentially recovered in Hole U1352C. Although marlstone first occurs at 575 m (Core 317-U1352C-2H), the rock is still porous and firm at ~900 m, which is manifested in the larger amount of scatter above 900 m compared to below. Three linear trends can be recognized at other depth intervals, including a downhole decrease from 0 to 90 m and increasing trends from 90 to 661 m and 826 to 1921 m. It is unclear why values in the uppermost 90 m interval decrease with depth, because bulk density and porosity are relatively constant in the same interval. However, thermal conductivity, in general, correlates negatively with the porosity profile (see "Physical properties"), particularly for hard rock (Fig. F65B). A good positive correlation with bulk density obtained from moisture and density, Method C (see "Physical properties" in the "Methods" chapter), is shown in Figure F65C. The linear fits between thermal conductivity and depth for sediments are

and

and the linear fit for hard rock is

where z is depth (m CSF-A). Thermal conductivity in the 661–826 m depth interval was calculated using the harmonic mean for cores consisting of alternating sediment–rock layers, based on two regressions, λ_90–661(z) and λ_826–1921(z), and the ratio of sediments/rocks in each core because (1) the sediment portion of each core from Hole U1352C steadily decreases below ~661 m (Core 317-U1352C-6R) and almost disappears at 826 m (Core 317-U1352C-23R) and (2) the measured thermal conductivities in Holes U1352B and U1352C represent either sediment or rock among alternating layers of sediment or rock. The resulting linear fit for sediment/rock mixture is

where z is depth (m CSF-A). In addition to this interval, Cores 317-U1352C-2R, 27R, 53R, and 77R contain alternating layers of sediment and rock, but these were not taken into account in terms of trends.

Bullard plot

Because the thermal conductivity profile in the 94–313 m depth interval, where the geothermal gradient was established, is represented as a linear fit, λ_90–661(z), thermal resistance for the interval is derived as

where z is depth (m CSF-A).

Following the Bullard approach and assuming conductive heat flow, a linear fit of temperature versus thermal resistance is expected (Fig. F66):

where z is depth (m CSF-A).

This yields a heat flow of 57.8 mW/m² for the 94–313 m depth interval, which can be applied to the entire cored depth interval if steady state heat flow is assumed. The resulting heat flow is comparable to the regional heat flow distribution, which decreases from 100–120 mW/m² in the mountainous area to the southwest to <60 mW/m² on the west coast of New Zealand (Reyes, 2007).

Temperature profile

The temperature profile at Site U1352 was predicted using the estimated heat flow of 57.8 mW/m² from the 94–313 m depth interval and estimated thermal conductivity trends under the assumption of steady state (Fig. F67). The temperature profile based on thermal conductivity shows a large inflection at ~575 m because of a rapid increase in thermal conductivity in the lithified material. At the bottom of Hole U1352C (1927 m), the predicted temperature based on this method is ~60°C, which is ~40°C lower than that obtained by assuming a constant geothermal gradient. However, SRA data suggest a higher thermal maturity at the bottom of Hole U1352C than would be consistent with 60°C (see "Geochemistry and microbiology").

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