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doi:10.2204/iodp.proc.303306.105.2006

Physical properties

Physical property data at Site U1305 were measured on both whole-round sections and discrete samples for split core sections. Whole-core susceptibility was measured every 5 cm at Site U1305. P-wave velocity (P-wave sensor number 3 [PWS3]) was measured once per section for all cores where possible. We collected samples from each core in Hole U1305A for discrete moisture and density (MAD) measurements. Most cores from Site U1305 were unaffected by core disturbance, showing similar records across all holes.

Whole-core magnetic susceptibility measurements

The magnetic susceptibility records are highly variable because of lithologic and/or mineralogic changes and show multiple excursions toward high values (Fig. F28). Magnetic susceptibility measurements were obtained from the "fast track" magnetic susceptibility core logger (MSCL) and the multisensor track (MST). Both records present the same trends. Peak magnetic susceptibility values from the MST range from 860 × 10^–5 to 1300 × 10^–5 SI and coincide with peak gamma ray attenuation (GRA) density values (1.7–2 g/cm³), suggesting rapid and frequent deposition of different-sourced sediments and indicating important mineralogical changes at this site. The peaks in magnetic susceptibility commonly correspond to sand-silt laminae (see “Lithostratigraphy”). Lower magnetic susceptibility values (200 × 10^–5 to 300 × 10^–5 SI) commonly coincide with the presence of light-colored, silt-sized detrital carbonate layers (e.g., Core 303-U1305A-12H), which have been described as typical glacial deposits in this region (e.g., Hillaire-Marcel et al., 1994; Stoner et al., 1995b, 1996) and are partially related to ice-rafting events. Higher variability in magnetic susceptibility peaks in the lower part of the cored interval may indicate increased transport of magnetic particles to Site U1305.

Density

The GRA density values covary with the magnetic susceptibility records, with the main peaks characterized by values around 1.7–2.2 g/cm³ (Fig. F29). Low density values oscillate between 1.4 and 1.5 g/cm³, reaching 1.2 g/cm³ between 304 and 309 mcd. Discrete bulk density values fall generally within the higher range of GRA density values, in particular below ~70 mcd (Core 303-U1305A-9H). However, one exception was identified (162–169.98 mcd) where the discrete measurements are higher than the MST-derived values.

Natural gamma radiation

Natural gamma radiation (NGR) counts range from 14 to 51 cps with the majority of the values between 21 and 38 cps (Fig. F30). The peak values usually reflect zones enriched in clay minerals. Even at the bottom of the drilled interval, which is dominated by coarser sandy silt laminae within silty clay sediments (see “Lithostratigraphy”), high values of NGR suggest that these intervals contain significant amounts of clay minerals. The NGR variability does not correspond to the magnetic susceptibility and GRA density records.

P-wave velocity

P-wave velocities range from 1500 to 1600 m/s at the top of the sedimentary sequence (upper ~50 mcd) (Fig. F31). Both velocity records from the MST (P-wave logger [PWL]) and the split-core measurements (PWS3) show an increase in velocity between 16.6 and 27 mcd. Below ~50–60 mcd, the scatter in measurements increases. At Site U1305 we experienced several problems measuring acoustic velocities, both with the MST-mounted PWL and the PWS3. Signal strength deteriorated completely from ~60 to 74 mcd (around Cores 303-U1305A-8H and 9H and 303-U1305B-7H and 8H), often generating readings with values lower than that of seawater. Elevated methane concentrations may be the cause of P-wave signal attenuation, as there is a rapid increase in methane concentrations at ~50–60 mcd in all holes at Site U1305 (see “Geochemistry”). We also see an offset in PWL and PWS3 measurements that is a long-standing “calibration” problem not unique to Expedition 303 (see “Physical properties” in the “Site U1302–U1308 methods” chapter).

Porosity

Discrete MAD sample measurements from Hole U1305A show a steady increase in bulk density from 1.4 to 1.8 g/cm³. The GRA density-derived porosity (see “Physical properties” in the “Sites U1302–U1308 methods” chapter) and discrete pycnometer measurements (Fig. F32) agree well. Although highly variable, porosity decreases from values of 81%–78% to ~40%–43% at the bottom of Holes U1305A and U1305B.

Discussion

The physical property data document the rapid and frequent changes in the sediment composition in response to different paleoceanographic, biological, and ice sheet changes. Three main types of sedimentation regimes are proposed to explain the physical property relationships (Fig. F33). Interglacial periods are characterized by higher deposition rates than glacial intervals and by high magnetic susceptibility and density values. Glacial periods are characterized by low magnetic susceptibility, low GRA density, and low NGR counts, possibly due to the deposition of fine detrital carbonate layers within a silty clay-dominated sediment. NGR increases toward the transition between detrital carbonate and terrigenous-dominated layers, suggesting a relative increase in the clay component, which seems to be a likely cause for the offset between high NGR values and the magnetic susceptibility and GRA peaks. Two intervals where physical properties are relatively invariant with low magnetic susceptibility and low density values (60–100 and ~190–209 mcd, respectively) correspond to more carbonate-rich sediments (see “Lithostratigraphy”).

Although this is not a region of high productivity, the high sedimentation rates, particularly during interglacial intervals, may have contributed to rapid burial of organic matter in the sediments and the subsequent consumption during methanogenesis. This process gave rise to a high percentage of trapped gas in the pores after retrieval. With respect to physical properties, the most clearly observed effect of the elevated gas concentration was difficulty in measuring acoustic velocities from ~60 to 75 mcd (Cores 303-U1305A-7H, 8H, and 9H and 303-U1305B-8H and 9H). In addition, one would expect gas to result in anomalous porosity estimates.

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