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doi:10.2204/iodp.proc.303306.103.2006

Physical properties

Physical properties were measured at Sites U1302 and U1303 following the procedures described in “Physical properties” in the “Site U1302–U1308 methods” chapter. Two measurements of magnetic susceptibility were conducted, along with GRA, natural gamma radiation (NGR), and P-wave velocity (PWS3). Thermal conductivity was measured on whole cores with a frequency of one per section for each core, and moisture and density (MAD) property samples were measured on two samples per core, usually at the bottom of Section 1 and the top of Section 6.

Whole-core magnetic susceptibility measurements

Trends in magnetic susceptibility are similar between those obtained from the “Fast Track” MSCL and the MST (Fig. F34). One exception occurs in Hole U1303A between 72.8 and 76.2 mcd, where MSCL values are similar between both holes at Site U1303 but the MST record shows a different pattern. Some data are missing because of failure of the MST magnetic susceptibility loop. Peak magnetic susceptibility values range from 280 × 10^–5 to 300 × 10^–5 SI in lithologic Subunit IA (down to ~106 mcd) (Fig. F34) and coincide with peaks in GRA bulk density (Fig. F35).

Density

Site U1302 and U1303 sediments display a relatively uniform increase in bulk density and decrease in water content downcore. Discrete bulk densities agree well with the primary trends in GRA bulk density (Fig. F36). GRA density varies between 2 and 2.5 g/cm³ and is positively correlated with susceptibility in lithologic Subunit IA. Underlying lithologic Subunit IB and coinciding with a mass flow deposit, the co-variance between magnetic susceptibility and GRA density is not as clear. However, magnetic and peak density values found below the debris flow deposits at the base of Subunit IB at Site U1302 show the same covarying behavior.

Natural gamma radiation

NGR and magnetic susceptibility measurements were used to correlate among holes from the two sites (Fig. F37). These proxies provide a means of evaluating the relative amounts of clay and carbonate in the sediments. At both sites, NGR is positively correlated with both magnetic susceptibility and GRA density measurements. NGR counts range from 20 to 49 cps, with the majority of the values between 40 and 49 cps. Low NGR counts (20–22 cps) correspond to low magnetic susceptibility and GRA values, suggesting that these intervals are associated with increased carbonate sedimentation.

P-wave velocity

P-wave velocity was not measured with the MST-mounted P-wave logger (PWL) at these sites because of an instrument malfunction. However, discrete measurements were performed with the PWS3 on every section where the sediment was not disturbed by the coring process. P-wave velocities range from 1400 to 1650 m/s, averaging 1541–1566 m/s at Site U1302 and 1549 m/s at Site U1303, slightly higher than the standard velocity of sound in seawater (1531 m/s). Prominent maxima correspond to levels where peak GRA density values were measured. In Subunit IB (100–106 mcd at Site U1302), scatter in the measurements becomes more pronounced with sound velocities up to 1800 m/s (Fig. F38), coinciding with bulk density and magnetic susceptibility peaks.

Thermal conductivity

Discrete thermal conductivity measurements were made usually on Section 4 of each core from Sites U1302 (Table T34) and U1303 (Table T35). Thermal conductivity is variable throughout the cores, showing an average value of 1.03–1.06 W/(m·K) at the top of Holes U1302D, U1302E, and U1303A, increasing to values of ~1.22–1.25 W/(m·K) downcore (Fig. F36). Thermal conductivity is strongly correlated with other physical properties of the sediments, including bulk density, P-wave velocity, and porosity.

Porosity

Porosity was calculated from the MST GRA density measurements (see “Physical properties” in the “Site U1302–U1308 methods” chapter) and compared with the porosity results generated from discrete MAD samples (Fig. F39). The porosity values are highly variable and range between 30% and 80%. Highest velocities and lowest porosities are associated with clay-rich layers in this interval. Porosity also decreases gradually downhole from 60%–70% at the top to 40%–50% at the bottom of the hole.

Discussion

Physical property records allow identification of high-resolution lithologic variability that results largely from variations in the input of terrigenous and biogenic components to the region. Generally, magnetic susceptibility and density peaks coincide with increased delivery of terrigenous sediments. The NGR variations are sufficient to resolve millennial-scale events and glacial–interglacial cycles extending back to MIS 17 (Fig. F35). In some cases, low magnetic susceptibility, density, and NGR values correspond to deposition of more carbonate rich sediments (see “Lithostratigraphy”) in Subunit IA. Physical properties were used to identify the last six Heinrich events as well as several other older ice-rafting episodes by comparison with previously studied records from nearby IMAGES Core MD95-2024 and Cores HU91-045-094 (e.g., Hillaire-Marcel et al., 1994; Stoner et al., 1995, 2000) and MD99-2237 (Turon et al., 1999). These rapidly deposited detrital layers, intercalated with background hemipelagic sediments, provide a record of the effects of Laurentide Ice Sheet instability and associated paleoceanographic changes. Sites U1302 and U1303 constitute a proximal analog to the detrital stratigraphy developed for the central North Atlantic IRD belt.

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