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Physical properties

Physical properties measured on core samples from Site C0021 provide insights into the evolution of MTDs and associated deformation by comparing results from nearby Site C0018, where coring was conducted during NanTroSEIZE Stage 2 Expedition 333 (Expedition 333 Scientists, 2012b). Physical properties on core samples also help calibration and correlation with LWD data (see “Logging while drilling” and “Core-log-seismic integration”).

Whole-round multisensor core logger

Whole-round cores were analyzed using the whole-round multisensor core logger (MSCL-W). The results of gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation (NGR), and electrical resistivity measurements (see the “Methods” chapter [Strasser et al., 2014a]) are summarized in Figure F30. Ultrasonic P-wave velocity (VP) measurements on Core 338-C0021B-1H are summarized in Figure F31. GRA density increases with depth similarly to bulk density of moisture and density (MAD) measurements discussed in “Moisture and density measurements (discrete cores).” Magnetic susceptibility is mostly constant in lithologic Subunit IA, except for a slight increase above ~100 mbsf and higher values of magnetic susceptibility in lithologic Subunit IB. NGR generally increases with depth, and a rapid increase with depth is observed in the intervals from 80 to 100 mbsf in Subunit IA and below 176 mbsf in Subunit IB. Electrical resistivity generally increases with depth, with a slight drop at the Subunit IA/IB boundary. Only Core 338-C0021B-1H shows good quality VP measured on the MSCL-W, whereas the other cores yield poor VP data quality because of poor contact between the liner and sediment. VP increases from 1.45 to 1.50 km/s within the top 6 m of core sediment.

Moisture and density measurements (discrete cores)

A total of 122 discrete samples from Hole C0021B were measured for MAD. MAD data from Hole C0021B are summarized in Table T13 and Figure F32. Between 1.2 and 194 mbsf, bulk density ranges from 1.40 to 1.95 g/cm3, grain density ranges from 2.6 to 2.85 g/cm3, and porosity ranges from 45% to 75%. Both bulk density and porosity change generally with depth; bulk density increases and porosity decreases. The slope of increasing and decreasing trends in bulk density and porosity, respectively, is steeper between 80 and 110 mbsf. Sand samples show higher bulk density and lower porosity than mud samples, whereas ash samples show similar values to those of mud samples.

It should be noted that MAD measurements on cluster samples, which were taken during whole-round core (WRC) sampling for IW samples and community WRC samples, were conducted on the Chikyu, whereas MAD measurements on samples taken from working halves were conducted at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) KCC during a core opening and sampling party (see “Operations”). The data show no difference between the measurements on the Chikyu and those at KCC, although different pycnometers and balances were used.

Thermal conductivity (whole-round cores and working halves)

Thermal conductivity was measured on whole-round cores from 3.2 (Core 338-C0021B-1H) to 185.3 mbsf (Core 14T) using a needle probe sensor. There is a clear boundary in thermal conductivity that corresponds to the lithologic Subunit IA/IB boundary. Thermal conductivity is generally constant at ~1.0 W/(m·K) above 177 mbsf in lithologic Subunit IA and increases to ~1.2 W/(m·K) in lithologic Subunit IB (Fig. F33).

Electrical resistivity (working halves and discrete core samples)

A total of 197 electrical resistivity measurements were conducted on working-half cores between 1.2 and 194 mbsf using the Wenner four-pin array probe. Each measurement was recorded in the dominant lithology types per section. Electrical resistivity ranges from 0.41 to 5.27 Ωm with an average of 1.02 Ωm and generally increases with depth as expected from the densification and porosity loss (Fig. F34; Table T14). The higher values at ~150 mbsf and below 178 mbsf reflect sand layers. Resistivity measured with the Wenner probe is almost the same as that of MSCL-W measurements. This is different from the data from other sites (C0002 and C0022), which show lower resistivity measured by Wenner probe than those of MSCL-W measurements (see “Physical properties” in the “Site C0002” chapter and “Physical properties” in the “Site C0022” chapter [Strasser et al., 2014b, 2014d]). This is possibly because the correct values of standard seawater resistivity were not obtained because of the use of a small container when the measurements on cores from Sites C0002 and C0022 were conducted. Unexpectedly low resistivity values obtained for cores at Sites C0002 and C0022 are probably due to overestimation of seawater impedance.

Shear strength (working halves)

Undrained shear strength of sediment from Hole C0021B was determined using a vane shear device and a pocket penetrometer (see the “Methods” chapter [Strasser et al., 2014a]). Undrained shear strength based on penetrometer measurements varies from 4.9 to 246.9 kPa, with an average of 86.7 kPa, whereas undrained shear strength based on vane shear measurements ranges from 7.7 to 178.1 kPa, with an average of 58.2 kPa (Fig. F35; Table T15). Although the data are scattered and vane shear measurements result in overall smaller undrained shear strength, both penetrometer and vane shear data can be divided in distinct intervals. Undrained shear strength of the material from the uppermost 5 mbsf (Core 338-C0021B-1H) is 20–29 kPa, which is much weaker than that of the material starting at 80 mbsf. Most likely, higher porosity and less-indurated material in the uppermost interval are responsible for this observation. Between 80 and 106 mbsf, undrained shear strength increases from ~50 kPa up to 114.5 kPa. Between 106 and 116 mbsf, the material becomes weaker again and undrained shear strength drops to 18–33 kPa. Below 116 mbsf, undrained shear strength increases with depth again, with a slope change at 150 mbsf, and reaches the highest values of 246.9 kPa (penetrometer) and 178.1 kPa (vane shear) at 170 mbsf. Undrained shear strength quickly drops to ~100 kPa (penetrometer) and ~50 kPa (vane shear) at 170 mbsf, and the values remain almost constant below that depth. Undrained shear strength increases with depth within both MTDs and decreases rapidly at the base of the MTDs. This trend was also observed in MTDs at Site C0018 (Expedition 333 Scientists, 2012b).

Color spectroscopy (archive-half cores)

The results of color reflectance measurements using the color spectroscopy logger (MSCL-C) are summarized in Figure F36. General trends are a slight increase in L* and a* and a decrease in b* with depth. L* ranges from 11 to 61, a* ranges from –6.8 to 7.3, and b* ranges from –4.2 to 10.7.