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doi:10.2204/iodp.proc.333.103.2012

Physical properties

The goal of physical properties measurements in Hole C0018A was to provide high-resolution data on the bulk physical properties and their downhole variations. Because Site C0018 focused on sampling MTDs inferred from seismic reflection data, the physical properties data are discussed in light of the observed MTD 6 (see “Lithology”). All physical properties measurements were made after cores had been imaged by X-ray CT and had equilibrated to room temperature (~20°C). Whole-round multisensor core logger (MSCL-W) data were collected to define natural gamma radiation, gamma ray attenuation (GRA) density, noncontact resistivity, magnetic susceptibility, and P-wave velocity. MSCL P-wave velocity data were extremely noisy and are not discussed further. Thermal conductivity was measured using the full-space needle probe method on the whole core. Penetrometer, vane shear, and electrical resistivity measurements were made shortly after the core was split, and moisture and density (MAD) analyses were performed on discrete samples collected from either the working halves of split cores or cluster samples taken adjacent to whole-round samples. Acoustic velocity and electrical resistivity on discrete samples were not measured at Site C0018 because the sediments were too soft to allow cutting appropriate samples.

MSCL-W

GRA density

GRA density was measured using the MSCL-W, based on the detection of a gamma ray beam that is produced by a cesium source. GRA density values are affected by the presence of voids and gaps between the core and core liner and thus show considerable scatter. The GRA density values will underestimate true densities where the sediment does not completely fill the liner, such as below 190.5 m core depth below seafloor (CSF), where HPCS coring ended. In general, GRA density gradually increases with depth (Fig. F30). However, GRA density decreases slightly from 58 to 70 m CSF where MAD-derived porosity increases. GRA density slightly increases within MTD 6 (127.26–188.62 m CSF) and decreases below MTD 6.

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field. Therefore, magnetic susceptibility provides information on sediment composition. Magnetic susceptibility decreases from 20 m CSF and then shows little variation between 50 and 180 m CSF, regardless of the presence of MTDs (Fig. F30). Below the base of MTD 6, magnetic susceptibility abruptly increases.

Natural gamma radiation

Natural gamma radiation gradually increases with depth. However, a step decrease occurs between 68 and 80 m CSF, which coincides with the interval of decreasing GRA density, electrical resistivity, and porosity. Natural gamma radiation shows a slight increase at 125 m CSF and remains elevated to 190 m CSF, approximately coincident with MTD 6.

Electrical resistivity

Resistivity increases from the top of Hole C0018A to 20 m CSF, likely because of decreasing porosity.

Moisture and density measurements

MAD measurements on discrete samples from Site C0018 provide a detailed characterization of grain density, bulk density, and porosity. Samples consisting predominantly of sand-sized particles were visually noted and are plotted with distinct symbols. All MAD data are provided in Table T9 and are summarized below.

From the surface to ~200 m CSF, bulk density generally increases and porosity decreases downhole, as expected for progressive burial (Fig. F31). A modest reversal in this trend is noted between 60 and 70 m CSF. In this interval, bulk density decreases, in both discrete and MSCL-W data, and porosity increases. Below 200 m CSF, bulk density and porosity remain relatively constant, averaging 1.90 g/cm³ and 0.47, respectively.

Grain density averages 2.67 g/cm³ at Site C0018. Some of the scatter in grain density data likely reflects errors in the pycnometer measurements. The scatter in grain density decreases slightly downhole as efforts were made to increase the sample volume. Bulk density and porosity show the largest scatter within the MTD zone and within the turbidites beneath the MTD zone. Reduced scatter in grain density within this zone relative to shallower measurements suggests that the bulk density and porosity scatter reflects natural variations. The scatter within the turbidites beneath the MTD likely reflects grain size variations, whereas the reason for the increased scatter within the MTD is not clear.

Strength measurements

The undrained shear strength of soft sediments in the working half of the core was measured using an analog vane shear device (Wykeham Farrance, model WF23544) and a pocket penetrometer (Geotest E284B). Strength ranges from 10.7 to 294 kPa, with an anomalously high value of 447.2 kPa when measured with the penetrometer, whereas the vane shear provides shear strength that ranges from 3 to 215.5 kPa (Fig. F32). Although there is scatter, shear strength measurements show a general trend that can be divided into successive intervals. From 0 to 30 m CSF, shear strength increases rapidly from minimal values to 50–70 kPa. The high fluid content in the uppermost meters of sediment and its progressive decrease in the first cores are likely to be responsible for this evolution. From 30 to 50 m CSF, shear strength increases to 60–90 kPa and then remains stable around these values until 120 m CSF, except between 80 and 90 m CSF where shear strength peaks at 190 kPa. Between 125 and 185 m CSF, shear strength values are higher and highly scattered with the maximum values for this site reached at the bottom of this layer. Below 190 m CSF, the values decrease and remain low and scattered. The high shear strength values between 125 and 185 m CSF may be related to MTD 6.

Electrical resistivity

Resistivity measurements were made at room temperature (25°C) with a 4-pin, 2 kHz Wenner array. Resistivity values for samples from Hole C0018A are given in Table T10. The measured values agree with the baseline values measured by the MSCL-W (Fig. F33). Measurements with the Wenner array are expected to be less affected by cracks and voids than the MSCL-W, as care was taken to select undisturbed locations. Thus, the Wenner array will provide a more accurate measure of formation resistivity than the MSCL-W. Resistivity generally increases with depth as a result of loss of pore volume (Fig. F33). A slight decrease in resistivity below 68 m CSF coincides with an increase in MAD-derived porosity and with the base of MTDs 3 and 4. A slight decrease in resistivity and increase in porosity is also observed at ~95 m CSF, coincident with the base of MTD 5. In MTD 6, resistivity increases with depth to 178 m CSF and decreases below that to the base of the MTD. The elevated resistivity observed over most of the MTD interval is consistent with observations from IODP Expedition 308 that sediments in MTDs tend to be overconsolidated (Sawyer et al., 2009). Below MTD 6, within the interval dominated by turbidites (see “Lithology”), the trend of resistivity with depth is more variable than above MTD 6.

Thermal conductivity and heat flow

The thermal conductivity results obtained for Hole C0018A are shown as a function of depth (Fig. F34). The trend of thermal conductivity with depth in Hole C0018A appears very well correlated with other physical properties, including electrical resistivity and bulk density. Therefore, thermal conductivity values can be useful not only for heat flow determination of the hole but also for an additional physical parameter, which characterizes the sedimentary formations penetrated by the hole.

The thermal gradient was estimated by linear fit of temperatures obtained by the advanced piston corer temperature tool (APCT-3), and those results are summarized in Figure F35. Two temperature measurements were taken from >150 m CSF, but they are problematic in data quality, probably due to too large frictional heat or disturbance at the coring shoe. As a result, we omitted those two points when assessing the heat flow at this site; only the four measurements from above 150 m CSF (open squares) were used. After this data screening, the mean thermal gradient value is determined as 63°C/km and the mean seafloor temperature, or the intercept temperature on the plot, is given as 1.48°C from the linear regression, whereas the measured mudline temperature at the time of APCT-3 measurements was 1.60°–2.22°C. A slightly lower intercept temperature relative to the measured mudline temperatures may reflect some natural characteristics of the thermal gradient at the topmost sediments on the seafloor because of sediment/seawater interface physical processes or inaccuracy in the estimated mudline depth during the measurements.

Heat flow is defined as the product of temperature gradient (dT/dz) at a certain depth of the earth and corresponding mean thermal conductivity (k_mean) of the formation interval where temperature gradient is measured. Thus, heat flow in Hole C0018A is estimated by a fit of the Bullard equation (method of temperature versus integrated resistance plot). The mean heat flow value at Site C0018 is 62 mW/m², which is slightly higher than the previous heat flow estimates of 42–58 mW/m² determined for IODP NanTroSEIZE Holes C0001E, C0004D, C0008A, and C0008C in this area (Expedition 316 Scientists, 2009b). Based on the determined heat flow value as well as the thermal conductivity values that we have measured onboard, a temperature profile to the bottom of Hole C0018A is synthesized and shown in Figure F36. The temperature at the bottom of the hole is estimated as 19.4°C.

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