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doi:10.2204/iodp.proc.344.106.2013

Physical properties

At Site U1380, physical property measurements were made to help characterize the lithostratigraphic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured using the Whole-Round Multisensor Logger (WRMSL). After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR). Following core splitting, a digital imaging logger and a color spectrophotometer were used to collect images of the split surfaces, and magnetic susceptibility was measured on the archive-half cores. Moisture and density (MAD) were measured on discrete samples collected from the working halves of the split sediment cores, generally once per section. Half-space thermal conductivity, P-wave velocity, and strength were measured on the working halves of split cores, generally one to two per core, depending on recovered length and condition. To examine overall trends from the seafloor to total depth, we also used data from Site U1378, which was drilled during Expedition 334 and is located ~1 km from Site U1380.

Density and porosity

Bulk density values in Hole U1380C were determined from both GRA measurements on whole-round cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). In general, wet bulk density values determined from discrete samples are higher than the whole-round GRA measurements, and both sets of measurements show similar downhole trends (Figs. F24, F25A). Grain density measurements were determined from mass/volume measurements on dry discrete samples within the sedimentary sequence. Grain density values averaged 2.7 g/cm³ (Fig. F25B). Porosity was determined from mass/volume measurements on discrete sediment samples using MAD Method C (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). Bulk density and porosity data appear to follow a steady compaction trend from the surface to ~550 mbsf, with porosities yielding a best-fit exponential relationship of porosity = 0.68exp(–0.0008 × depth), where depth is in mbsf. Between 550 and 555 mbsf, bulk density increases from ~2.0 to 2.1 g/cm³ and porosity decreases from 43% to 35%, falling below the compaction trend (Fig. F25C). This decrease may reflect lithologic changes or differences in burial history at the Unit I/II boundary (see “Lithostratigraphy and petrology”). At 700 mbsf, bulk densities peak at 2.2 g/cm³ and porosities reach their lowest values of 25% to 26%.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the Section Half Multisensor Logger (SHMSL) for all recovered sediments from Hole U1380C. Magnetic susceptibility values measured with these two methods show similar patterns (Fig. F26). Magnetic susceptibility is relatively low (generally <50 IU) between 400 and 550 mbsf (Unit I), consistent with the observed abundance of noniron-bearing clays.

Between 550 and 700 mbsf (uppermost 150 m of Unit II), magnetic susceptibility values are more variable, ranging from ~40 to >500 IU. Most positive excursions seem to be associated with coarser grained material, and the highest excursions are associated with recovered tephra layers near 690 mbsf. Between 700 and 770 mbsf (the rest of Unit II), magnetic susceptibility is consistently low (40–50 IU), despite the continued occurrence of coarse-grained layers. Unit III (below 770 mbsf) is characterized by positive excursions in magnetic susceptibility, similar to the upper portion of Unit II. Section 344-U1380C-49R-2 (~780 mbsf) contained metal fragments from the core catcher, creating a spike exceeding 4000 IU in the WRMSL data. These fragments were removed before running the section through the SHMSL.

Natural gamma radiation

NGR counting periods were 10 min. Measurement spacing was fixed at 20 cm for Sections 344-U1380C-2R-2 (~440 mbsf) through 12R-9 (~545 mbsf) and at 10 cm for Sections 344-U1380C-13R-1 to the bottom of the hole. Total NGR results above 480 mbsf are consistent with NGR data from Hole U1380A obtained during Expedition 334 (Fig. F27), generally ranging between 15 and 25 cps. NGR values increase with depth between 480 and 550 mbsf (Unit I/II boundary) ranging as high as 30 cps, decrease at this lithologic boundary, and are relatively constant throughout Units II and III, generally ranging between 15 and 20 cps. The highest counts, as high as 38 cps, were observed at the lowermost part of Unit I, and the lowest counts, around 10 cps, were observed between 650 and 700 mbsf.

P-wave velocity

P-wave velocity in Hole U1380C was measured on the working halves of sediment split cores using the P-wave caliper measurement (Fig. F28). P-wave velocity averages 1700 m/s in Unit I, increasing from ~1650 m/s at ~450 mbsf to ~1900 m/s at 550 mbsf. Just below the Unit I/II boundary, P-wave velocity jumps to ~2000 m/s and increases to ~2250 m/s at the bottom of Unit II. Conspicuously higher P-wave velocities up to 2500 m/s were observed at 700 mbsf and are associated with calcareous silt (Cores 344-U1380C-30R and 31R). In Unit III, P-wave velocity is ~2350 m/s.

Thermal conductivity

Thermal conductivity measurements were conducted on split cores using the half-space method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). In general, thermal conductivity values in Hole U1380C agree with values in Hole U1378B using the same half-space method (Fig. F29A). Thermal conductivity varies inversely with porosity (Fig. F25C). Thermal conductivity increases with depth from 450 to 550 mbsf, steps to higher values at the Unit I/II boundary, and then increases slightly with depth to 720 mbsf. Thermal conductivity decreases at 720 mbsf, remains nearly constant to 771 mbsf, and increases at the Unit II/III boundary. Overall thermal conductivity ranges from 0.81 to 1.64 W/(m·K). The mean and standard deviation of thermal conductivity are 1.31 and 0.145 W/(m·K), respectively.

Downhole temperature and heat flow

The temperature-depth relation was evaluated using the Bullard (1939) method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]) and the measured temperatures and the estimated heat flow value of 0.0442 W/m² from Hole U1378B (Expedition 334 Scientists, 2012a). We compared results using two models for the variation of thermal conductivity with depth, k (z). The first model assumes a linear increase of thermal conductivity with depth given by a least-squares fit (Model 1); the second is based on the thermal resistance determined from each measured conductivity value (Model 2). Mudline temperatures measured in Hole U1378B average 12.3°C, which is higher than the shallowest in situ temperature measurement (9.94°C at 33 mbsf). It is likely that these elevated mudline temperatures reflect warming by circulation of surface seawater during drilling. As a result, bottom water temperatures were estimated by a least-squares method for each model of thermal conductivities. Models 1 and 2 yielded values of 7.64° and 7.76°C, respectively, for the sediment/water interface and 41.32° and 41.59°C, respectively, at the bottom of Hole U1380C (Fig. F29B).

Sediment strength

Compressive sediment strength was measured using a third-party needle penetrometer (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). Three measurements were taken near the P-wave velocity measurement in each section (Fig. F30). Above ~550 mbsf, compressive strength varies from ~340 to 1100 kPa. Below ~550 mbsf, results become more variable, ranging between ~220 and ~6000 kPa, but generally increase with depth. No distinct change occurs at the Unit I/II boundary. An apparently cemented sample at ~605 mbsf shows an extremely high value of 12000 kPa.

Color spectrophotometry

Results from color reflectance measurements are presented in Figure F31. Reflectance L* values generally vary between ~23 and ~45 throughout Hole U1380C, with scattered values as low as 14.5 to as high as 67. Reflectance a* and b* values show a clear shift at 496 mbsf. Reflectance a* values above 496 mbsf vary between ~2 and ~7, whereas below ~496 mbsf, values vary between –3 and 3, with several values ranging up to ~22. Reflectance b* values vary between –17 and –8 above ~496 mbsf, whereas values below ~496 mbsf generally range between –7 and 4.

Core-seismic correlation

Although it is not possible to provide an interval velocity profile and a synthetic seismogram because poor hole conditions prevented completion of logging operations (see “Operations”), three potential horizons of core-seismic correlation were identified (Fig. F32). The largest change in all physical property values except color reflectance occurs at the boundary between Units I and II, where the major seismic reflector at the top of the acoustic basement lies at 550 mbsf. The second major change in physical properties occurs at ~700 mbsf and is identified by a strong positive spike of magnetic susceptibility, a positive spike of P-wave velocity, and a slight negative excursion of NGR. This horizon could correlate to a strong landward-dipping reflector at ~700 mbsf. The other notable change in both magnetic susceptibility and P-wave velocity at the Unit II/III boundary could correlate to a moderately strong seismic reflector at 770 mbsf. These results suggest that the conspicuous seismic reflectors are due to lithologic changes.

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