Previous   |    Next

doi:10.2204/iodp.proc.331.103.2011

Physical properties

Physical property measurements were made at Site C0013 to nondestructively characterize lithostratigraphic units and states of sediment consolidation.

Density and porosity

Bulk density 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 64 discrete samples were analyzed for MAD (1, 15, 20, 15, 10, 2, and 1 samples from Holes C0013B, C0013C, C0013D, C0013E, C0013F, and C0013G, respectively). Wet bulk density is roughly constant with depth (Fig. F34), as determined by both MAD and GRA, although the latter values are generally lower than the former and exhibit more scatter. This is expected of GRA-derived density, as GRA is very sensitive to incompletely filled core liners and the presence of voids and cracks.

The average bulk density in Holes C0013C, C0013D, and C0013F is ~1.9 g/cm³; a narrow higher density zone (2.5 g/cm³) exists between 8 and 9 mbsf in Holes C0013C and C0013D. In the uppermost 10 m of Hole C0013E, density is generally higher (~2.4 g/cm³), but at greater depth it is similar to that in Holes C0013C, C0013D, and C0013F. Bulk density measured in Holes C0013G and C0013H is slightly higher, ~2–2.2 g/cm³, and broadly consistent with other densities observed in the 8–12 mbsf range. Bulk density increases to values >2.5 g/cm³ below 22 mbsf in Holes C0013D and C0013E.

Porosity was calculated from MAD measurements. It is generally quite high (40%–60%) and decreases slightly with depth (Fig. F35), except within two intervals: between 8 and 9 mbsf in a high-density anhydrite-rich layer and at >22 mbsf where volcaniclastic rock dominates. Porosity drops to ~20% within both intervals.

Grain density was calculated from discrete MAD measurements and is also approximately constant with depth (~2.7–3.1 g/cm³) (open symbols, Fig. F36), except at the top of Hole C0013E, where grain densities are higher (~3.5 g/cm³) in an interval that appears to have a higher fraction of dense sulfide minerals.

Density and porosity results from the seven holes at Site C0013 reveal a coherent pattern that describes four clear lithological boundaries or units based on density. Bulk density in the uppermost Unit I (0 to ~5 mbsf; see “Lithostratigraphy”) is relatively constant at 1.7–1.9 g/cm³ for most holes (Fig. F36), with the exception of Hole C0013E (and possibly Hole C0013B), where bulk densities are relatively high in surface sediments. Lithostratigraphic Unit II is a narrower zone of higher bulk density (~2.5 g/cm³) between ~5 and 10 mbsf. Bulk densities found in Unit III (12 to ~26 mbsf) are similar to those found in Unit I. Within Unit IV (depths >25 mbsf), bulk density is also relatively high (~2.5–2.9 g/cm³) and porosity decreases sharply from ~60% to <20% (Fig. F36). These density variations appear to reflect lithological variations, which range from predominantly hydrothermally altered mud with varying amounts of sulfidic material to more cemented anhydrite breccias (~8–10 mbsf) and then to volcaniclastic breccia at depths >22 mbsf.

Density and porosity changes across lithological unit boundaries

At the Unit I/II boundary, bulk density increases and porosity decreases, consistent with a transition from a zone of water-rich hydrothermally altered mud to a more cemented anhydrite-rich layer. At the Unit II/III boundary, bulk density decreases and porosity increases; this boundary is coincident with the return to a less cemented mud-rich zone in Holes C0013C–C0013E. At the Unit III/IV boundary (~22 mbsf), bulk density increases sharply and is similar to grain density. Porosity decreases sharply as well. These changes are consistent with the transition from a water-rich anhydrite mud to a more compacted volcaniclastic breccia.

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 38 depths (1, 2, 14, 9, 10, 1, and 1 measurements in Holes C0013B, C0013C, C0013D, C0013E, C0013F, C0013G, and C0013H, respectively). Formation factors calculated for Site C0013 generally range from ~4.9 to 10 and are roughly constant with depth (Fig. F37). There is some indication that formation factor increases downcore in Holes C0013D and C0013E, but there is no clear pattern among the seven holes.

Discrete P-wave velocity and anisotropy measurements

P-wave velocity and relative anisotropy were measured only on samples indurated enough to cut sample polyhedrons, which was rarely the case. P-wave velocity determined on discrete samples from between 20 and 25 mbsf in Holes C0013D and C0013E range from 3000 to 4000 m/s (data not shown).

Thermal conductivity

Thermal conductivity was measured on whole-round cores. A total of 58 measurements were made (2, 19, 5, 19, 10, 2, and 1 measurement from Holes C0013B, C0013C, C0013D, C0013E, C0013F, C0013G, and C0013H, respectively). Values range from a low of ~0.5 W/(m·K) in Hole C0013C to high values of 3–6 W/(m·K) in Holes C0013D and C0013E (Fig. F38). The average thermal conductivity for all seven holes is 2 ± 1 W/(m·K). Thermal conductivity is relatively constant with depth. It exhibits a loose and inverse correlation with porosity (Fig. F36). At the same time that porosity decreases, thermal conductivity increases as water is forced from void spaces because the thermal conductivity of grains is greater than that of the water. Narrow zones of higher thermal conductivity are also roughly coincident with peaks in MSCL-W P-wave velocity, where higher velocities should also correspond to regions comprising less porous and denser material (Fig. F36).

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 for Site C0013 are generally constant and low (~1–2 Ωm), but there are regions of higher resistivity (5–30 and >100 Ωm) between ~7.5 and 12 mbsf in Holes C0013C and C0013D. At >27 mbsf in Hole C0013E, resistivity is quite high and ranges from 100 to 300 Ωm. There is no obvious relationship with the discrete measurements of formation factor (Fig. F39 versus Fig. F37).

Top of page   |    Previous   |    Next