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doi:10.2204/iodp.proc.344.103.2013

Physical properties

At Site U1381, physical properties measurements were made to help characterize 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) and thermal conductivity was measured using the full-space method on sediment cores. After splitting the cores, 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. Electrical conductivity, P-wave velocity, and shear strength were measured on the working halves of split cores, generally once per section. Discrete physical properties measurements were taken near the base of the section to be adjacent to whole-round cores taken for interstitial water or postcruise geotechnical studies. Measurements on Unit V (basement) do not include magnetic susceptibility, NGR, MAD, electrical conductivity, P-wave velocity, and shear strength because of core disturbance.

Density and porosity

Bulk density values in Hole U1381C were determined from both GRA measurements on whole 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., 2013]). In general, wet bulk density values determined from whole-round GRA measurements and measurements from discrete samples agree well, with whole-round GRA measurements showing greater scatter (Fig. F27A). Grain density measurements were determined from mass/volume measurements on dry discrete samples within the sedimentary sequence. Grain density values average 2.7 and 2.5 g/cm³ within lithostratigraphic Units I and II, respectively (Fig. F27B). Porosity was determined from mass/volume measurements on discrete samples using MAD Method C on sediment cores (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). Porosity values slightly decrease with depth within Unit I, exhibiting very little compaction, and then increase at the Unit I/II boundary (Fig. F27C). In general, porosity values are high, averaging 75% and 78% in Units I and II, respectively. Because porosity is determined based on water mass removed during oven drying, reported porosity values are not true interstitial porosity but include water held in minerals that was removed at 105°C. MAD results from Hole U1381C show similar trends to those from Holes U1381A and U1381B but show less scatter and are more complete because of the better core recovery and reduced core disturbance of APC coring.

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 (Units I–IV) from Hole U1381C. No magnetic susceptibility track data were acquired in the basement (Unit V). Similar trends were observed in magnetic susceptibility values measured with these two instruments (Fig. F28). Hole U1381C magnetic susceptibility data are continuous from the seafloor to the basement contact, allowing greater interpretation of sediments than Expedition 334 data (Holes U1381A and U1381B; Expedition 334 Scientists, 2012). In the uppermost 10 m, magnetic susceptibility values rapidly increase and then decrease from >170 instrument units (IU) to near 25 IU. Below 10 mbsf, baseline values range between 15 and 25 IU with intermittent peaks of >100 IU. These peaks are interpreted as ash layers and become more frequent with depth down to the Unit I/II boundary.

Unit II is characterized by magnetic susceptibility values near zero with a few spikes. These spikes, which are interpreted as ash layers, are thinner than those observed in Unit I. Also in Unit II, point measurements are generally higher than volumetric measurements but record fewer of the spikes. A spike to >1000 IU in the WRMSL data at the top of Core 344-U1381C-11H (93.6 mbsf) was caused by a sheared metal pin that was later discovered in the core, and this data point is not shown on Figure F28. SHMSL point measurements do not record this spike because this section was rerun after the removal of the pin. Lithostratigraphic Units III and IV have low magnetic susceptibility (averaging 9 IU), with no spikes.

Natural gamma radiation

NGR counting periods were 10 min, and measurement spacing was fixed at 20 cm in all core sections. NGR results are reported in counts per second (cps) (Fig. F29). NGR measurements were not made in the basal sediment and basement (Cores 334-U1381C-12H and 13H) because of core disturbance. NGR counts gradually increase with depth from 10 to 40 mbsf and are highly variable between 40 and 60 mbsf (near the Unit I/II boundary). Below ~60 mbsf (most of Unit II and all of Units III and IV), NGR counts are lower than those within Unit I. The lowest counts, around 10 cps, were observed near the Unit II/III boundary and increase within Unit III.

P-wave velocity

P-wave velocity in Hole U1381C was measured on sediment whole rounds using the WRMSL and on the working halves of sediment split cores using the P-wave velocity gantry (Fig. F30). Discrete measurements of P-wave velocity average 1520 m/s in Unit I (silty clay). In Unit II, P-wave velocities are slightly higher, averaging 1540 m/s, despite the higher measured porosity (Fig. F27C). P-wave velocity increases to 1620 m/s in Unit III (calcareous clay) and decreases to 1560 m/s in Unit IV (claystone). P-wave velocity measured by the WRMSL shows a similar trend to that measured on working halves, although WRMSL P-wave velocity is slightly slower (Fig. F30B). P-wave velocity values on the y- and z-axes were measured only in the shallower interval because of poor waveforms due to brittle cracking in the deeper interval. No significant anisotropy was observed in the measured interval (Fig. F30A).

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). In general, thermal conductivity values in Hole U1381C agree with values in Holes U1381A and U1381B using the same needle-probe method (Fig. F31A), and thermal conductivity results are inversely correlated with porosity (Fig. F27C). Thermal conductivity increases with depth to 50 mbsf, decreases slightly from 50 to 60 mbsf, and then gradually increases below 60 mbsf (Fig. F31A). The overall variation in thermal conductivity with depth is small, and the data yield a mean and standard deviation of 0.83 and 0.05 W/(m·K), respectively.

Downhole temperature and heat flow

Four downhole temperature measurements were attempted using the APCT-3 between 20 and 60 mbsf in Hole U1381C (Fig. F31B). All measurements were made in a calm sea state. The APCT-3 was stopped at the mudline for as long as 10 min prior to each penetration. Equilibrium temperature values plotted as a function of depth are relatively linear and show a similar trend with depth as Expedition 334 data. Coupled with the average bottom water temperature (2.4°C; Expedition 334 Scientists, 2012), they give a least-squares gradient of 231°C/km.

We calculate heat flow of 185 mW/m² as a product of the average thermal conductivity (0.83 W/[m·K]) and the thermal gradient. This value is significantly larger than the half-space prediction of 130 mW/m² for 15 Ma crust and much larger than the observed global average heat flow of 77 mW/m² for this age crust (Stein and Stein, 1994). This high heat flow suggests significant fluid flow in the underlying crust.

Sediment strength

Sediment strength was measured both by the automated vane shear (AVS) and the Geotester STCL-5 pocket penetrometer. To compare between the two measurements, unconfined shear strength is one half of unconfined compressive strength (Blum, 1997). Strength generally increases with depth. Expedition 334 data show much lower values that those from Expedition 344, probably because of disturbance from RCB coring during Expedition 334. The AVS and penetrometer measurements both show approximately linear trends within Unit I (Fig. F32). Values become scattered below 50 mbsf, and strengths measured by pocket penetrometer become more variable than those measured by vane shear. Strength values increase with depth to 85 mbsf, ranging between ~40 and ~110 kPa (vane shear) and between ~10 and ~340 kPa (compressive strength from penetrometer). Within the interval from 85 to 98 mbsf, shear strength decreases with depth, with a minimum value of ~30 kPa (vane shear) and ~120 kPa (compressive strength from penetrometer). Below ~98 mbsf, values increase sharply with depth where calcareous clay (Unit III) and claystone (Unit IV) appear. Within Units III and IV, values increase as high as 145 kPa (vane shear) and 440 kPa (compressive strength from penetrometer).

Electrical conductivity and formation factor

Formation factor values were obtained from sediment conductivity measurements conducted in the y- and z-axes of the split core (Fig. F33). The y and z measurements are similar throughout the cored interval, indicating little anisotropy in electrical conductivity within the sediments. Within Unit I, electrical conductivity tends to decrease gradually with depth and ranges between 18 and 25 mS/cm. Within Unit II, values increase slightly, ranging between 19 and 26 mS/cm. Near 95 mbsf, conductivity reaches its highest value of 28 mS/cm. Formation factor ranges between 1.8 and 2.7 within Unit I and decreases slightly to 1.8–2.3 within Unit II. The transition seen at 58 mbsf may correspond with the boundary between silty clay (Unit I) and calcareous ooze (Unit II). The increasing conductivity trend at 95–100 mbsf corresponds with the start of calcareous clay (Unit III) and claystone (Unit IV).

Color spectrophotometry

Results from color reflectance measurements are presented in Figure F34. Reflectance L* values vary between 30 and 50 throughout Hole U1381C. Reflectance a* and b* values show clear contrast between Units I and II. Reflectance a* values in Unit I vary between ~2 and ~5, whereas in Units II–IV, values vary between 4 and 6. Reflectance b* values vary between –10 and –5 in Unit I, whereas values in the units below Unit I scatter between –10 and 0. Trends in color reflectance between 95 and 100 mbsf correspond with the start of calcareous clay (Unit III) and claystone (Unit IV). Within the basement (Unit V; basalt breccia), a* values range between 6 and 9 and b* values vary from –20 to –15.

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