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doi:10.2204/iodp.proc.303306.104.2006

Physical properties

Measurements of physical properties were conducted at Site U1304 following the procedures described in “Physical properties” in the “Site U1302–U1308 methods” chapter. Two measurements of magnetic susceptibility were conducted, along with gamma ray attenuation (GRA) density, NGR, and P-wave velocity. Thermal conductivity was measured on whole cores in Hole U1304A with a frequency of one per section for each core. Moisture and density (MAD) properties were also measured on two discrete samples per core, usually at the bottom of Section 1 and the top of Section 6.

Whole-core magnetic susceptibility measurements

The magnetic susceptibility records derived during coring operations at Site U1304 show a highly variable record that results from lithologic and/or mineralogic changes. Magnetic susceptibility measurements obtained from the Fast Track MSCL and the MST present the same trends and display multiple excursions toward higher values (Fig. F28).

Site U1304 has peak magnetic susceptibility values of ~400 × 10–5 SI and minimum values approaching the instrument noise level, with most values ranging from 40 × 10–5 to 160 × 10–5 SI. The magnetic susceptibility record shows two main trends at Site U1304 (Fig. F28). The first trend runs approximately from 0 to 115 mcd with average values ranging from 80 × 10–5 to 120 × 10–5 SI. The second trend is seen from 115 to 275 mcd and has average values of 160 × 10–5 to 140 × 10–5 SI. Site U1304 also contains abundant diatom mats that caused the magnetic susceptibility values to approach the noise levels of the MSCL and MST (see “Lithostratigraphy”). These mats had highly variable thickness from centimeter to meter scale.

GRA density

Bulk density measurements taken at Site U1304 show a similar trend among holes and are variable, ranging from 1.2 to 1.8 g/cm3 (Fig. F29). The bulk density record shows a very similar trend to that of magnetic susceptibility. Bulk density between 0 and 120 mcd averages ~1.4 g/cm3, whereas the lower interval between 120 and 275 mcd shows a higher average of ~1.5 g/cm3. The diatom layers cause density values to drop to ~1.2 g/cm3. Discrete density was also determined from Hole U1304A physical property MAD samples. The discrete measurements match the values produced by the MST (Fig. F29).

Natural gamma radiation

NGR counts range from 0 to 35 cps with the majority of the values between 7 and 21 cps (Fig. F30). NGR values also mimic the trends seen in the magnetic susceptibility and GRA data. NGR counts average ~7 cps from 0 to 120 mcd and ~14 cps from 120 to 275 mcd.

P-wave velocity

Both P-wave logger (PWL) and discrete (P-wave sensor number 3 [PWS3]) velocity measurements were performed at Site U1304 when possible. Some core sections could not be measured because of the presence of diatom mats that disrupted the velocity measurements due to severe signal attenuation. Measurements that could be obtained at Site U1304 are fairly consistent, varying between 1500 and 1600 m/s (Fig. F31). There is a general increase in velocity from the top (~1525 m/s) to the bottom (~1580 m/s) of the core. We also see an offset in PWL and PWS3 measurements that is a long-standing problem not unique to Expedition 303.

Thermal conductivity

Discrete thermal conductivity measurements were made usually on Section 4 of each core at Site U1304 (Table T26). Thermal conductivity is highly variable ranging from 0.7 to 1.1 W/(m·K). There appears to be a slight correlation between thermal conductivity and magnetic susceptibility, GRA, and NGR data sets, but low resolution of the thermal conductivity data does not allow further conclusions.

Porosity

Porosity was calculated using MST GRA density measurements and spot-checked with porosity results generated from discrete MAD samples. Porosity values are highly variable and range between 60% and 90% (Fig. F32). As expected, porosity shows an inverse relationship to density. Porosity decreases only slightly with depth at Site U1304. This trend may be attributed to the presence of diatom layers causing the interval density values to be reduced to a value lower than that of the surrounding silty clay layers.

Discussion

The spliced record generated at Site U1304 shows a discontinuity in the magnetic susceptibility, GRA, and NGR records at ~115 mcd (Fig. F33). The discontinuity is characterized by higher values and higher-frequency variability below the discontinuity and lower values and lower-frequency variability above. An age estimate for this discontinuity is ~0.7 Ma (see “Biostratigraphy” and “Paleomagnetism”).

Diatom mats interbedded with silty clay layers at Site U1304 influence the magnetic susceptibility and NGR data (Fig. F33). The effect is more readily seen from 0 to 115 mcd where the average magnetic susceptibility and NGR values are lower and contain less variability and where carbonate content averages 43 wt% (see “Geochemistry”). Although the 115–275 mcd section contains more diatom mats, this is not particularly apparent in the physical property records (see “Lithostratigraphy”). We assume that the non-diatom-rich sediments between 125 and 275 mcd are better able to mask the presence of the diatoms because of their higher values of magnetic susceptibility and NGR and lower carbonate content (~30 wt%).