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doi:10.2204/iodp.proc.303306.108.2006

Physical properties

Measurements of physical properties were conducted at Site U1308 following the procedures described in “Physical properties” in the “Site U1302–U1308 methods” chapter. Two measurements of magnetic susceptibility were conducted, along with GRA density, NGR, and P-wave velocity. Moisture and density (MAD) properties were also measured on two discrete samples per core in Hole U1308A, usually at the bottom of Section 1 and the top of Section 6.

Whole-core magnetic susceptibility measurements

Site U1308 has peak magnetic susceptibility values of ~200 × 10^–5 SI and minimum values approaching instrument noise levels with most values ranging from 20 × 10^–5 to 100 × 10^–5 SI (Fig. F23). Magnetic susceptibility values from 0 to ~193 mcd show high-amplitude, high-variability changes with an average magnetic susceptibility value of 73 × 10^–5 SI, whereas magnetic susceptibility values from ~193 to 259 mcd show high-variability changes and have an average value of 54 × 10^–5 SI. In the 259–355 mcd interval, magnetic susceptibility values generally approach instrument noise levels with an average value of 6 × 10^–5 SI.

Density

Bulk density measurements taken at Site U1308 show a similar trend among holes and are variable, ranging from 1.0 to 2.0 g/cm³ (Fig. F24). Generally, the bulk density plots are highly variable and show a peak to peak match with magnetic susceptibility plots. The density values increase downhole from 0 to 193 mcd with an average value of ~1.6 g/cm³. From 193 to 259 mcd, the average density increases from 1.6 to 1.8 g/cm³ and the amplitude of the variation decreases. There is an abrupt increase in the rate of change in density in the 255 to 355 mcd interval with an average value of 1.8 g/cm³. The discrete measurements match the trends of the MST values; however, the discrete measurements are generally lower than GRA density values (Fig. F24).

Natural gamma radiation

NGR counts range from 0 to 41 cps with the majority of the values between 1 and 20 cps (Fig. F25). From 0 to 188 mcd, NGR covaries with magnetic susceptibility, showing highly variable, high amplitude values that average ~18 cps. From 193 to 355 mcd, NGR average values are lower (~14 cps) and show low variability. The 193–355 mcd interval also shows a discontinuity at ~255 mcd, which is consistent with magnetic susceptibility plots.

P-wave velocity

Both P-wave logger (PWL) and discrete P-wave sensor number 3 (PWS3) measurements were performed where possible at Site U1308. Values vary between 1500 and 1700 m/s and increase downhole (Fig. F26). We see an offset between PWL and PWS3 measurements. The exact nature of this offset is unknown and is a long-standing problem not unique to Expedition 303 (see “Physical properties” in the “Site U1302–U1308 methods” chapter).

Porosity

Porosity was calculated using GRA density measurements and spot-checked with the porosity results generated from the discrete MAD samples. Porosity values are highly variable and range from 40% to 84% (Fig. F27). As expected, porosity shows an inverse relationship to density, decreasing with depth at Site U1308 to an average minimum value of 50%.

Discussion

The magnetic susceptibility, density, and NGR records show similar patterns of high-amplitude variability that decrease downcore (Fig. F28). Two distinct changes at 193 and 255 mcd are used to define three geophysical units that are closely related to lithologic Units I and Subunits IIA, and IIB (see “Lithostratigraphy”). Geophysical Unit 1 (0 to ~193 mcd, corresponding to lithologic Unit I) is characterized by the highest values and greatest variability in magnetic susceptibility, density, and NGR records. Magnetic susceptibility and NGR generally covary within this interval, whereas density appears to be antithetic. Unit 2 (~193–255 mcd, corresponding to lithologic Subunit IIA) is marked by a decrease in amplitude of the high-frequency variability in magnetic susceptibility and NGR that is observed in Unit 1. Unit 3 sediments (255–355 mcd, corresponding to lithologic Subunit IIB) have low NGR and magnetic susceptibility values (approaching instrument noise levels) and have high densities.

High magnetic susceptibility and NGR amplitudes suggest that the delivery of terrigenous material to Site U1308 is highly variable in Unit 1. Decreased magnetic susceptibility and NGR amplitudes in Unit 2 reflect a reduction in terrigenous materials or an increase in the biogenic component of the sediments. In Unit 3, magnetic susceptibility and NGR values approach instrument noise levels, indicating the presence of little to no terrigenous material. This trend of increasing uphole values in magnetic susceptibility and NGR in Unit 2 may be related to the onset of NHG (see “Lithostratigraphy”).

Density values gradually increase downcore through geophysical Unit 1 (0 to ~193 mcd) and the upper part of Unit 2 (193–245 mcd). There is an abrupt increase in density from the base of Unit 2 to Unit 3 (245–355 mcd). We interpret the gradual increase in density in Units 1 and the upper part of Unit 2 to reflect sediment compaction. The sharper increase from the base of Unit 2 through Unit 3 reflects the early stages of the transition of nannofossil ooze to nannofossil chalk (see “Lithostratigraphy”).

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