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doi:10.2204/iodp.proc.331.107.2011

Physical properties

Physical property measurements were made at Site C0017 to nondestructively characterize lithological units and states of sediment consolidation. In the following discussion, the four holes at Site C0017 (Holes C0017A, C0017B, C0017C, and C0017D) are treated as one continuous depth profile.

Density and porosity

Bulk density at Site C0017 was determined from both gamma ray attenuation (GRA) measurements on whole cores (with the multisensor core logger for whole-round samples [MSCL-W]) and moisture and density (MAD) measurements on discrete samples (see “Physical properties” in Expedition 331 Scientists, 2011b). A total of 70 discrete samples were analyzed for MAD (7, 9, 14, and 40 from Holes C0017A, C0017B, C0017C, and C0017D, respectively). Wet bulk density is roughly constant with depth to ~20 mbsf (Holes C0017A and C0017B) (Fig. F18; Table T14), as determined by both MAD and GRA, although the latter values are generally lower than the former and exhibit more scatter than previous sites. The average bulk density in Holes C0017A and C0017B is ~1.59 g/cm³. Between 20 and 60 mbsf (Hole C0017C), bulk density decreases somewhat from ~1.7 to 1.3 g/cm³. In Hole C0017D, bulk density is relatively constant from 60 to 80 mbsf (~1.5 g/cm³) and then increases to ~1.9 g/cm³ between 80 and 147 mbsf.

Grain density was calculated from discrete MAD measurements and is also approximately constant (2.6 ± 0.2 g/cm³) between 0 and 80 mbsf (data not shown). Grain density is slightly lower than average in the depth range from ~30 to 80 mbsf.

Porosity was calculated from MAD measurements. It is generally quite high (~60%) and mirrors patterns of bulk density. It is relatively constant in Holes C0017A and C0017B; it increases somewhat between 20 and ~80 mbsf (Hole C0017C and the upper part of Hole C0017D) and then decreases steadily toward values between 20% and 30% at the bottom of Hole C0017D (~147 mbsf) (Fig. F19; Table T14).

Density and porosity results from the four holes at Site C0017 reflect the basic lithology of the hole. In the uppermost 20 m, sediments are predominantly clays with some sand and pumice; between 20 and 80 mbsf is a higher incidence of pumice and correspondingly lower bulk density. From 95 to 145 mbsf, sediments are again dominated by clays and the density increases. At finer scales in the clay-dominated sections, GRA-derived density and MSCL-derived P-wave velocities exhibit small changes that reflect changes from finer grained clays to somewhat coarser grained silty clays (see “P-wave velocity and anisotropy measurements”).

Electrical resistivity (formation factor)

Formation factor is a measure of the connected pore space within sediment and is used to calculate the bulk sediment diffusion coefficient. Electrical impedance measurements were made at 87 depths (13, 14, 13, and 47 measurements in Holes C0017B, C0017C, C0017D, and C0017E, respectively). Formation factors calculated for Hole C0017 are near 3 in the uppermost 30 mbsf (Holes C0017A and C0017B); values increase with depth and are generally between ~5 and 8 (Fig. F20). Several peaks in formation factor generally occur where sediment water content is lower (bulk density is higher), but the peaks are not clearly related to any lithostratigraphic control and may be measurement artifacts.

P-wave velocity and anisotropy measurements

P-wave velocity and relative anisotropy were measured on discrete samples indurated enough to cut sample polyhedrons. This was rarely the case. MSCL-derived P-wave velocities generally increase with depth in the core as sediments become more compacted (Fig. F21). It is not always clear from the lithology what causes some of the larger peaks in P-wave velocity; however, small-scale variations often correlate with changes in grain size and density.

Thermal conductivity

Thermal conductivity was measured on whole-round cores. A total of 88 measurements were made (13, 15, 14, and 46 measurements from Holes C0017A, C0017B, C0017C, and C0017D, respectively). Average thermal conductivity in Holes C0017A and C0017B is ~1 W/(m·K). Between 20 and 60 mbsf (Hole C0017C), thermal conductivity decreases somewhat sharply from ~1 to 0.3 W/(m·K). In Hole C0017D it increases from ~0.5 W/(m·K) at 65 mbsf to 1.5 W/(m·K) between 120 and 140 mbsf (Fig. F22). Average thermal conductivity for all four holes is 1.09 ± 0.3 W/(m·K). Thermal conductivity is loosely and inversely correlated with porosity. As porosity decreases, thermal conductivity increases as water is forced from void spaces, because the thermal conductivity of grains is greater than that of water.

In situ temperature

In situ temperature was measured using the APCT3 (six of eight measurements were successful, at 18–112 mbsf) and thermoseal strips (one measurement at 151 mbsf) (Table T15; Fig. F23). For the APTC3, all temperature-time series were recorded at 1 s intervals. The APCT3 stopped at the mudline for as long as 10 min prior to each penetration, yielding eight measurements of bottom water temperature that averaged 4.87° ± 0.51°C (1σ). This range indicates that strong bottom water temperature variation exists, spatially, temporally, or both. Temperature-time series for each measurement are shown in Figure F24. Temperature records show significant frictional heating as the coring shoe penetrated the sediment, followed by subsequent temperature decay toward the in situ value. For most of the measurements, the probe was kept in the sediment for >5 min, allowing accurate extrapolation to equilibrium formation temperature based on a 1/time approximation (Table T15). Most measurements exhibit good penetration heating and initial decay curves, with two exceptions: Core 331-C0017D-4H at 84.7 mbsf, which did not have good penetration, and Core 12H at 150.7 mbsf, where recorded temperatures are greater than the calibrated maximum temperature for the APCT3 tool (55°C). Mathematical fits to the temperature-time series are also good and yield the equilibrium formation temperatures that are plotted versus depth in Figure F23. The deepest measurement at 150.7 mbsf was determined from three replicate thermoseal strips, with beads calibrated to blacken when exposed to temperatures of 75°, 80°, 85°, 90°, and 95°C. The first four beads were blackened on two of the strips, indicating exposure to a temperature between 90° and 95°C, and the first three beads were blackened on the third strip indicating a temperature between 85° and 90°C. We report this temperature somewhat conservatively as 90° ± 5°C.

Temperature and thermal conductivity at Site C0017 vary from one lithological unit to another (Fig. F24). Three broad units can be defined: from the top to 60 mbsf (Unit I), from 60 to 100 mbsf (Unit II), and from 100 mbsf to the bottom (Unit III). These correspond roughly with the upper clay layers, the middle pumice-rich layers, and the hard, indurated clay layers defined by the MAD data and the lithostratigraphy results.

Heat flow

If heat transfer proceeds via conduction and heat flow is constant, the thermal gradient is inversely proportional to thermal conductivity according to Fourier’s law (Expedition 331 Scientists, 2011b). Heat flow was estimated using the mean thermal conductivity derived for each unit (I, II, and III) (Table T16). Values increase by roughly a factor of 10 within each unit.

MSCL-I and MSCL-C imaging

MSCL-derived core images and color analyses are presented in the visual core descriptions (VCDs).

MSCL-W derived electrical resistivity

MSCL-W based resistivity data at this site are generally low (<4 Ωm), but there are regions of high resistivity at ~37 mbsf (5–30 Ωm) and at 95 and 140 mbsf (10–20 Ωm). There is no obvious relationship with the discrete measurements of formation factor (Fig. F25 versus Fig. F20).

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