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doi:10.2204/iodp.proc.333.105.2012

Physical properties

The goal of physical properties measurements in Holes C0012C, C0012D, C0012E, C0012F, and C0012G was to provide high-resolution data on bulk physical properties and their downhole variations. 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. P-wave velocity data from the MSCL-W were of poor quality and will not be discussed. Thermal conductivity was measured using the full-space needle probe method on whole-round cores or the half-space line source method on working halves of cores. The full-space method was used on samples from Holes C0012C and C0012D, and the half-space method was used on those from Holes C0012E, C0012F, and C0012G because the cores were too hard to allow insertion of the needle probe. 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. Discrete resistivity and P-wave velocity measurements in the x-, y-, and z-directions were performed on sample cubes cut from working halves of cores below 500 m core depth below seafloor (CSF) because the upper sediments were too soft to allow cutting appropriate samples. For basalt samples, modified procedures were used (for details, see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012a]).

MSCL-W

GRA density, magnetic susceptibility, electrical resistivity, and natural gamma radiation were measured using the MSCL-W. The results of MSCL-W measurements on cores from Expeditions 322 (Expedition 322 Scientists, 2010) and 333 are summarized with lithologic units (dashed lines) in Figure F33. GRA density, electrical resistivity, and natural gamma radiation all increase slightly with depth, whereas magnetic susceptibility is divided into a zone of gradual decrease with depth and a zone of high variability at the base corresponding to lithologic Unit II. The variation patterns of GRA density, electrical resistivity, magnetic susceptibility, and natural gamma radiation generally show good correlation with lithologic unit boundaries.

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 (Fig. F33). GRA density remains fairly constant from 7 to 65 m CSF, consistent with MAD-derived porosities. Subsequently, an increase occurs between 70 and 100 m CSF, consistent with a decrease in porosity. GRA density dramatically increases again from 140 to 180 m CSF, coinciding with the existence of volcaniclastic sandstones and tuffaceous sandstones (see Lithology columns in Fig. F3). In general, GRA density values are affected by the presence of voids and gaps between the core and core liner, and thus data from Expedition 333 HPCS coring show less scatter than data from Expedition 322 RCB coring.

Magnetic susceptibility

Magnetic susceptibility gradually decreases downhole within lithologic Unit I, with the exception of an increase between 46 and 76 m CSF (Fig. F33). This zone corresponds to ash layers (see Lithology columns in Fig. F3). Spikes of high magnetic susceptibility occur from 140 to 180 m CSF. These high-value magnetic susceptibility spikes correlate with volcanic material.

Natural gamma radiation

Natural gamma radiation remains relatively constant between the seafloor and 50 m CSF (Fig. F33). Then, natural gamma radiation sharply decreases and then increases between 55 and 90 m CSF. Within lithologic Unit II (150 m CSF and below), natural gamma radiation shows large variation.

Electrical resistivity

Electrical resistivity generally increases with depth (Fig. F33). From the seafloor to 50 m CSF, a low-resistivity section is observed. Then, resistivity abruptly increases between 50 and 65 m CSF with a decrease in natural gamma radiation. A small increase occurs between 150 and 178 m CSF, where volcaniclastic sandstones and tuffaceous sandstones exist.

Moisture and density measurements

MAD measurements on discrete samples from Site C0012 provide a detailed characterization of bulk density and porosity. All MAD data from Expedition 333 are provided in Table T7, and results from Expeditions 322 (Expedition 322 Scientists, 2010) and 333 are combined in Figure F34. From the surface to ~10 m CSF, bulk density generally increases and porosity decreases downhole, as expected for progressive burial (Fig. F34). Below 10 m CSF, bulk density slightly decreases and then remains relatively constant to ~70 m CSF. A sharp increase in bulk density (decrease in porosity) occurs between ~70 and 100 m CSF, after which porosity increases between 100 and 130 m CSF and decreases again between 130 and 170 m CSF. Below 170 m CSF, data show a steady compaction trend, with some scatter in sand-rich units, to the base of the borehole. The sharp porosity decrease between 70 and 100 m CSF could thus be a transition similar to that observed at Site C0011 below 240 m CSF. Comparison of Site C0011 and C0012 porosity data below the anomalous regions (e.g., below 240 m CSF) indicates that Site C0012 porosity is generally lower than porosity from similar depths at Site C0011 (Fig. F35). A possible explanation for the lower porosity at Site C0012 is removal of overlying material by erosion or slope failure. This interpretation would be consistent with the observed time gap of ~2 m.y. found at ~10 mbsf (see “Paleomagnetism”).

Within the basalt, measured porosity varies from 0.09 to 0.37 and bulk density varies from 2.15 to 2.64 g/cm³. Directly below the sediment/basalt interface, values are scattered and then generally decrease to ~590 m CSF, below which they generally increase. Measured grain density varies from 2.75 to 2.95 g/cm³.

Strength measurements

Strength in soft sediments was measured using an analog vane shear device (Wykeham Farrance, model WF23544) and a pocket penetrometer (Geotest E284B) in the working half of the core from 0 to 145 m CSF. Beneath this depth, high sediment strength did not allow measurements with these devices. Strength globally increases with depth and decreasing porosity (Fig. F36). In spite of scattering, the strength data present boundaries that are consistent with those of other physical properties. Shear strength progressively increases from 0 to 70–80 m CSF. Shear strength rapidly increases from 60 to 100 m CSF and then remains stable to 140 m CSF. At 140 m CSF, the shear strength suddenly increases again by 50 kPa, and measurement had to be stopped at ~150 m CSF because sediments were too strong.

P-wave velocity

P-wave velocity was measured on discrete cubes cut from working halves of cores below 500 m CSF. Velocities were measured in the x-, y, and z-directions, and horizontal- and vertical-plane anisotropies were calculated (see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012a]). In red clay above the sediment/basement boundary (~525 m CSF), P-wave velocity in the z-direction (Fig. F37A; Table T8) is nearly constant at ~2000 m/s. In the basement, P-wave velocity generally increases with depth from ~3000 to ~5000 m/s.

Vertical-plane anisotropy decreases from ~15% to 0% over the red clay (Fig. F37B), indicating that P-wave velocity in the z-direction is less than in either of the horizontal directions. This is consistent with a transversely isotropic medium in which bedding planes are roughly horizontal. Within the basement, anisotropy remains at or near 0%, indicating a more isotropic medium.

Electrical resistivity

Resistivity measurements were made with a four-pin, 2 kHz Wenner array. Resistivity values for samples from Site C0012 are given in Table T9. The measured values agree with the baseline values measured by the MSCL-W. Partial desiccation of the core occurred from 55 to 62 m CSF; these values are not plotted and have been removed from Table T9. Resistivity generally follows the trend of porosity values, indicating that the formation resistivity is a response to changes in pore volume (Fig. F38). Values are nearly constant from 12 to 78 m CSF, reflecting the trend of nearly constant porosity over this interval. At 78 m CSF, resistivity begins increasing with depth and follows the trend of porosity to the base of the cored interval.

The relationship between porosity and resistivity can be illustrated with a log-log plot of porosity and resistivity (Ellis and Singer, 2007) (Fig. F39); resistivity was matched with nearest neighbor porosity values (within 30 cm) from MAD measurements. The data from the zone of anomalously high porosity between 12 and 78 m CSF plot below the trend defined by the rest of the data. This could be due to the presence of a cementing phase (Ellis and Singer, 2007) or a change in clay mineralogy (Henry, 1997).

Electrical resistivity was measured on discrete cubes cut from working halves of cores below 500 m CSF. Measurements were performed in the x-, y-, and z-directions, allowing computation of horizontal and vertical anisotropies (see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012a]). Resistivity in the z-direction increases with depth from 3.3 to 27 Ωm (Fig. F40A; Table T10). There is a cluster of elevated resistivity values near the sediment/basement interface. Error may be large for high-resistivity samples, such as from the basalt, because the measured value may be affected by the amount of water applied to the sample surface. The observation that the x-direction is consistently greater than the y-direction (Table T10) would be consistent with some type of measurement error. Vertical-plane anisotropy is constant at about –50% in the red clay above the basement (Fig. F40B). Higher resistivity in the z-direction in the sediments is consistent with a transversely isotropic medium in which the bedding planes are approximately horizontal.

Thermal conductivity and heat flow

Thermal conductivity data obtained on whole-round core measurements using a needle probe sensor in Holes C0012C and C0012D are shown together as function of depth (Fig. F41). The trend of thermal conductivity with depth in Holes C0012C and C0012D is correlated very well with those of other physical properties, for example, resistivity and bulk density (MAD). This observation suggests that porosity is the major controlling factor for those different physical properties, which include thermal conductivity at this site.

Thermal gradient values are evaluated from measurements using the advanced piston corer temperature tool (APCT-3) taken at 10 depths in Holes C0012C and C0012D together (Fig. F42). The mean thermal gradient value determined is 135°C/km. The mean seafloor temperature, or the intercept temperature at the seafloor on the plot, is 2.85°C, which is slightly higher than the measured mudline temperatures observed during the 10 temperature measurements using the APCT-3.

Heat flow is defined as the product of thermal gradient (dT/dz) and corresponding mean thermal conductivity (k_mean) of the depth interval where temperature gradient is measured. Thus, heat flow in Holes C0012C and C0012D is estimated using the results from the two holes. The estimated heat flow value at this site is 141 mW/m², amounting to ~1.5 times as high as that of 89.5 mW/m² determined for Hole C0011C. Based on the determined heat flow value of 141 mW/m² as well as the thermal conductivity values that we obtained onboard by core measurements and data from Expedition 322 (Expedition 322 Scientists, 2010), a temperature profile to the bottom of this hole was synthesized and is shown in Figure F43. Temperature extrapolated to the top of basement (at 526 m CSF) is estimated to be ~64°C.

Measurements of thermal conductivity of the core samples taken for depths >500 m CSF were also made, and data are shown in Figure F44.

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