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doi:10.2204/iodp.proc.303306.106.2006

Physical properties

Measurements of physical properties were conducted at Site U1306 following the procedures described in “Physical properties” in the “Site U1302–U1308 methods” chapter. Two measurements of MS were conducted, along with gamma ray attenuation (GRA) density, natural gamma radiation (NGR), and P-wave velocity. Moisture and density (MAD) properties were also measured on two discrete samples per core, usually at the bottom of Section 1 and top of Section 6.

Whole-core magnetic susceptibility measurements

MS records produced during coring operations at Site U1306 are highly variable because of lithologic and/or mineralogic changes. MS measurements obtained from the “Fast Track” multisensor core logger (MSCL) and the MST present the same trends and display multiple excursions toward higher values (Fig. F28). Site U1306 has peak MS values of ~1200 × 10^–5 SI and minimum values of 200 × 10^–5, with most values ranging from 300 × 10^–5 to 900 × 10^–5 SI. The MS record is highly variable and contains a change in slope at ~100 mcd (Fig. F28). MS values increase from 300 × 10^–5 to 900 × 10^–5 from 0 to 100 mcd with an average value in this interval of ~600 × 10^–5 SI. From 100 to 340 mcd, however, most MS values vary between 500 × 10^–5 and 900 × 10^–5 with an average value of 650 × 10^–5 SI.

Density

Bulk density measurements taken at Site U1306 show a similar trend among holes and are variable, ranging from 1.4 to 2.0 g/cm³ (Fig. F29). The bulk density plots show a slope change at ~100 mcd that coincides with MS plots. Bulk density in the 0–100 mcd interval averages ~1.6 g/cm³, whereas the lower interval (100–340 mcd) shows a higher average of ~1.8 g/cm³. Discrete density measurements were also determined from Hole U1306A physical property MAD samples. The discrete measurements match the values produced by the MST (Fig. F29).

Natural gamma radiation

NGR counts range from 8 to 35 cps with the majority of the values between 10 and 20 cps (Fig. F30). NGR values show a highly variable record that loosely correlates to the density record. NGR plots also show a change in slope at ~100 mcd, which coincides with a change in slope in the MS and GRA records. NGR counts average ~15 cps from 0 to 120 mcd, whereas the average in the 100 to 340 mcd interval is ~20 cps.

P-wave velocity

Both P-wave logger (PWL) and discrete P-wave sensor number 3 (PWS3) measurements were performed at Site U1306 when possible. Because of the presence of methane gas, which disrupted velocity measurements, the velocity record is discontinuous and of poor quality. Measurements obtained at Site U1306 are fairly consistent, varying between 1500 and 1625 m/s (Fig. F31). We also see an offset between PWL and PWS3 measurements, which 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 MST GRA density measurements and spot checked with the porosity results generated from MAD samples. Porosity values are highly variable and range from 40% to 80% (Fig. F32). As expected, porosity shows an inverse relationship to density. Porosity decreases with depth at Site U1306 to an average minimum value of 50%, which may be attributed to the compaction of sediments with depth.

Discussion

MS values at Site U1306 are higher than those at Site U1305, averaging 626 × 10^–5 and 436 × 10^–5 SI, respectively (Fig. F33). Site U1305 has an average carbonate content of 12.3 wt%, whereas Site U1306 has an average carbonate content of ~3.2 wt% (see “Geochemistry”). Hence, we suggest that the carbonate content of the sediments is an important control on MS values at Sites U1305 and U1306.

The MS records three intervals of broad change at Site U1306 that may be linked to the carbonate content of the sediments (Fig. F33). The first interval from 0 to 100 mcd (Fig. F33) contains MS values that average 603 × 10^–5 SI in an interval with relatively high carbonate content (see “Geochemistry”). This upper interval is also characterized by high-frequency, high-amplitude variability of MS in the interval from ~50 to 100 mcd, whereas the interval from 0 to 50 mcd is characterized by high-frequency, low-amplitude variability. This pattern of variability is also apparent in the carbonate data. The second broad change is apparent from 120 to 220 mcd (Fig. F33), where MS values increase to a mean value of ~646 × 10^–5 and the carbonate content decreases. The lowermost interval between 220 and 340 mcd is characterized by lower MS values, averaging 618 × 10^–5, and higher carbonate values. Although the uppermost sections of Site U1306 show a clear relationship between changes in carbonate content and variability within the MS record, some intervals cannot be explained by this relationship. Further shore-based research must be conducted to completely understand the variability contained within the MS record.

GRA density is highly variable with an increasing trend downcore, probably because of compaction of the sediments (Fig. F33). There is a greater rate of increase for sediments between 0 and 100 mcd than for sediments between 100 and 340 mcd. We assume the greater rate of change in density is related to the higher carbonate content in this interval coupled with the normal increase in compaction with depth.

NGR generally mimics the GRA record at Site U1306 (Fig. F33). However, NGR does not show any clear relationship with the MS records. NGR also shows a discontinuity at ~100 mcd in which the sediments between 0 to 100 mcd have lower values, high variability, and lower amplitudes, whereas the sediments between 100 and 340 mcd have higher values, high variability, and higher amplitudes. We attribute this discontinuity in the record to the presence of higher carbonate content between 0 and 100 mcd.

One of the major objectives of Expedition 303 is to investigate millennial-scale variability beyond the reach of conventional piston coring. The high-resolution MS, NGR, and GRA records capture variability on this timescale. There are intervals at Site U1306 where the MS, NGR, and GRA records covary, and there are many intervals where the MS appears to lag or be antithetic to the GRA and NGR records, indicating a complex depositional history. Shore-based analyses are required to fully understand the causes of the variability within the physical property records and their relationship to climate change.

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