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Heat flow

Geothermal gradient

Temperature measurements were conducted using the APCT-3 during APC coring in Hole U1351A and the SET tool during XCB coring in Hole U1351B. However, it was not possible to obtain the geothermal gradient. Five temperature measurements were taken in total (Fig. F63; Table T23). The one APCT-3 measurement yielded a reliable temperature of 11.61°C at 25.1 m CSF-A (unless otherwise noted, all depths in this section are reported in CSF-A) in Hole U1351A. The four trials with the SET tool in Hole U1351B failed because the conductive cooling time after the tool penetrated the sediment was too short (<300 s) to generate reliable fitting curves (Fig. F63A). This could have resulted from tool movement caused by ship heave and/or severely disturbed sediments falling around the SET tool tip from the top of the core during measurement. Nevertheless, all five temperature measurements were used to estimate the geothermal gradient. The fitting line to temperature versus depth data is (Fig. F63B)

T(z) = 0.0141 × z + 10.209 (R2 = 0.6537),

where T(z) is in situ temperature at depth z (m CSF-A). The estimated geothermal gradient is therefore 14.1°C/km, which is much lower than the 40°–50°C/km gradient obtained from the nearby exploration well Clipper-1 (Reyes, 2007). The geothermal gradient at Site U1351 is thus likely underestimated because (1) Site U1351 is only ~15 km distant from Clipper-1, (2) sediment as thick as several kilometers at both sites prohibits local fluid circulation in the absence of conduits, and (3) the geothermal gradient at Clipper-1 was measured in a deep hole (up to 4.7 km below seafloor), so it would not have been affected by seasonal variations in bottom water temperature. 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., 2000).

Thermal conductivity

Thermal conductivity was measured in available whole-round core sections from Holes U1351A and U1351B. The TK04 system was employed using the full-space needle probe method. Measurement frequency was usually once per section with one measuring cycle at each point. This included 19 points (0.3–27.9 m) in Hole U1351A and 212 points (0.7–1004.9 m) in Hole U1351B. Ideally, measurements were to be conducted in the major lithology of each section measured. However, it was difficult to recognize the lithology of whole rounds through the core liner, so the middle of the section was chosen as a measurement point unless a void or crack was observed. Few lithologic variations occurred in each section at Site U1351, so this sampling procedure was appropriate. Probe V10701 was used, and heating power was kept to ~3 W.

Thermal conductivity data were screened when (1) measurements did not satisfy criteria suggested by the TK04 manufacturer because of poor contact between the probe and materials, (2) thermal conductivity values were close to that of water (0.6 W/[m·K]) because of sediment dilution during coring, or (3) measurements were taken in unsuitable lithologies such as shell hash. Thermal conductivity versus depth data from Holes U1351A and U1351B are consistent (Fig. F64A). The laboratory-measured thermal conductivity range is 0.962–2.233 W/(m·K) (average = 1.474 W/[m·K]) (Table T24). These values are higher than those observed at Site 1119 (Shipboard Scientific Party, 1999), although thermal conductivity data from Site 1119 were collected over a shorter depth interval (<130 meters below seafloor [mbsf]). These high conductivities may be due to high concentrations of quartz (6.5–12.5 W/[m·K]) in the fine-grained sediment, including the clay-sized fraction (see "Lithostratigraphy") and/or carbonate cementation (0.5–4.4 W/[m·K]).

Based on depth (m CSF-A), a bulk density of 2.01 g/cm3 from MAD results (see "Physical properties"), and the estimated geothermal gradient, correction for in situ conditions yielded a thermal conductivity range of 0.959–2.215 W/(m·K) (average = 1.467 W/[m·K]). This calculated in situ thermal conductivity value differs from laboratory measurements by less than ±2.2%.

Despite significant scattering, thermal conductivity generally increases linearly with depth. In particular, decreasing porosity and increasing bulk density in the uppermost 35 m may control the rapid increase in thermal conductivity. Thermal conductivity values reach a local maximum followed by a relatively sudden drop of ~15% at 35 m, where porosity and bulk density do not show such fluctuation. The cause of these variations in thermal conductivity is unclear. Significant scatter in thermal conductivity below ~90 m might be a result of locally cemented sediments with higher thermal conductivities and/or reduced thermal conductivity caused by XCB drilling disturbance.

Despite fluctuations in the topmost portion of the cored interval, data corrected to in situ conditions can be represented by the following linear fit (Fig. F64A):

λ(z) = 1.3704 + 0.0003 × z,

where z is depth (m CSF-A).

Thermal conductivity at Site U1351 varies negatively with porosity and positively with bulk density (Fig. F64B, F64C), as expected. Variation in terms of lithology could not be detected.

Bullard plot

Thermal resistance is derived based on the relationship of thermal conductivity with depth:

Ω(z) = [ln(1.3704 + 0.0003 × z) – ln(1.3704)]/0.0003.

Following the Bullard approach by assuming conductive heat flow, a linear fit of temperature versus thermal resistance is expected (Fig. F65):

T(z) = 10.176 + 0.0201 × Ω(z) (R2 = 0.6516).

This yields a heat flow of 20.1 mW/m2 at Site U1351, which is about one-third that estimated at the Clipper-1 well when adopting the same thermal conductivity as Site U1351. Care should be taken when using the above-estimated geothermal gradient and/or heat flow values because of the poor temperature data.