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doi:10.2204/iodp.proc.329.109.2011

Physical properties

At Site U1371, physical properties measurements were made to provide basic information characterizing lithologic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured with the Whole-Round Multisensor Logger (WRMSL) on whole-round core sections. After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR). Thermal conductivity was measured using the full-space method on sediment cores. Discrete P-wave measurements were made on split sediment cores using the Section Half Measurement Gantry. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split sediment cores. Additional discrete measurements of electrical resistivity were made on the split sediment sections to calculate formation factor. The Section Half Image Logger and Section Half Multisensor Logger (SHMSL) were used to collect images and color spectrometry of the split surfaces of the archive-half cores. Three holes targeted the sedimentary cover, Holes U1371D–U1371F. The most complete hole for logging physical properties was Hole U1371D. The holes have not been correlated and offsets exist.

Density and porosity

Bulk density values at Site U1371 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from Hole U1371D from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). A total of 66 discrete samples were analyzed for MAD.

Bulk density values in Hole U1371D are relatively constant with depth through lithologic Unit I (clay-bearing diatom ooze) (Fig. F12A). Density in Unit II (clay) increases and then decreases with depth. Bulk density discrete values are consistent with GRA density values through Unit I but are significantly lower than GRA density values in Unit II.

Grain densities have a mean value of 2.64 g/cm³ in lithologic Unit I and 2.40 g/cm³ in Unit II (Fig. F12B). Porosity generally decreases with depth and varies between ~80% and 70% through Unit I (Fig. F12C). Porosity declines rapidly in Unit II, with minimum values of ~55%.

Bulk density GRA values in Holes U1371E and U1371F show similar patterns to those observed in Hole U1371D (Fig. F13). Whole-round sampling prior to WRMSL measurements caused edge effects, leading to noisier records.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL and point measurements were made on the SHMSL in all recovered cores from Site U1371. Uncorrected values of magnetic susceptibility are presented for Holes U1371D–U1371F (Fig. F14). Point measurements from archive halves are much more scattered than whole-core measurements, and peak-to-peak variability is greatly subdued in the point measurements. Notably, point measurements of magnetic susceptibility in areas of core disturbance show the largest scatter, with means that are either high or low relative to the bulk of the data. The spatial resolution of the WRMSL magnetic susceptibility loop is ~5 cm, and the observed ringing in Holes U1371C and U1371E is due to edge effects.

Magnetic susceptibility of the clay-bearing diatom ooze (lithologic Unit I) lies within a relatively narrow range. Local highs in this unit correlate to zones of hardground (see “Lithostratigraphy”). Magnetic susceptibility of clay (Unit II) generally increases with depth. The high magnetic susceptibility values in Unit II are likely due to the RSO content of the clay.

Natural gamma radiation

NGR results are reported in counts per second (Fig. F15). NGR counting intervals were ~30 min per whole-core interval for Hole U1371D and decreased to 20 min per whole-core interval for Holes U1371E and U1371F. NGR counts are considered reliable. NGR counts at the tops of Holes U1371D and U1371F are high, indicating that the sediment/water interface was sampled. The top of Hole U1371E was sampled prior to NGR measurements.

In general, NGR counts increase slightly with depth through lithologic Unit I and increase at a greater rate through Unit II. These patterns are similar to those observed in the GRA density and magnetic susceptibility data. Ringing is more prevalent in cores from Holes U1371E and U1371F because only short core pieces remained after whole-round sampling prior to NGR measurements.

P-wave velocity

P-wave velocity at Site U1371 was determined from measurements on whole-round sediment cores (Fig. F16) and on discrete samples from the working halves of sediment split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Only 11 discrete measurements of P-wave velocity were measured. These discrete values are somewhat higher than measurements made on whole-round core. This difference may be due to drying of core material. The mean P-wave velocity value of all whole-core measurements is ~1510 m/s (Fig. F16B). In Hole U1371D, compressional wave velocities are relatively uniform except in the upper portion of lithologic Unit I, where there is a conspicuous region of somewhat higher velocities.

Formation factor

Electrical conductivity was measured on working halves of the split sediment cores from Hole U1371D. Measurements in Hole U1371D were made at a nominal interval of 20 cm. For each measurement, the temperature of the section was also noted. Surface seawater was used as a standard and measured twice per section (Table T4), normally prior to making measurements for that section and then between sections. These measurements were used to compute the drift for this set of measurements (Fig. F17; Table T5). The first set of measurements through measurement 500 shows a sharp decline in values measured on the standard. This steep decline is attributed to the unusually rapid wear of the platinized electrodes caused by the abrasive lithologic character of Unit I. Near measurement number 500, the probe was replatinized and measurements of the standard briefly returned to high values. Following this return, the values measured on the standard again quickly dropped. This drift stabilized though lithologic Unit II. Because of the three distinct trends in measurements made on the standard, the drift is computed as three piecewise linear trends. The temperature dependence of electrical conductivity was corrected and all reported measurements correspond to a temperature of 20°C. Electrical conductivity measurements were transformed to a dimensionless formation factor by dividing the drift by the measurements (Table T6).

In general, formation factor in lithologic Unit I is relatively constant except for a conspicuous low between ~85 and 95 mbsf (Fig. F18). Formation factor increases with depth in Unit II although conspicuous departures from this trend are present.

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). In general, measurements appear reliable, although values <0.6 W/(m·K) were culled from the analysis. Values <0.6 W/(m·K) are attributed to poor contact between the probe and the sediment or convection that leads to unreasonably low estimates of thermal conductivity by causing the thermal response to heating to depart from the theoretical prediction. Values are relatively constant with depth through lithologic Units I and II. The mean thermal conductivity is 0.7 W/(m·K) (Fig. F19A). For the uppermost ~3 mbsf, thermal conductivities collected during the Cruise KNOX-02RR site survey (R. Harris, unpubl. data) are somewhat more scattered than the values reported here; however, they also have a mean of 0.7 W/(m·K).

Downhole temperature

Downhole temperature was measured using the APCT-3. Five measurements were attempted between 35.9 and 92.9 mbsf in Hole U1371D (Table T7). All measurements were made in lithologic Unit I.

All temperature-time series were recorded with a sample interval of 1 s. The temperature tool was stopped at the mudline for up to 10 min prior to the first measurement and then for 5 min thereafter. The average bottom water temperature is 1.11°C (Table T7). All measurements were made in a moderate sea state (<3 m swell) and each temperature-time series suffers to some extent from the ship’s heave (Fig. F20). Significant frictional heating occurred on all penetrations of the APCT-3, with the temperature-time records exhibiting characteristic probe penetration and subsequent decay. Tool movement was observed in all temperature records as sudden shifts in temperature both before and after the usable section of the temperature-time series. Tool movement is attributed to the ship’s heave. The effective origin time of the frictional heat pulse was estimated by varying the assumed origin time until the thermal decay pulse best fit a theoretical curve. As a result of tool movement, delay times are quite large and fits to the equilibrium curve are short (Table T7). Nevertheless, all of the measurements appear to be reliable. Equilibrium temperatures plotted as a function of depth are relatively linear. Coupled with the average bottom water temperature, they give a least-squares gradient of 74.0°C/km (Fig. F19B).

Heat flow

Because thermal conductivity appears relatively constant in lithologic Unit I and the thermal gradient is linear, heat flow is computed as the product of the thermal conductivity 0.7 W/(m·K) and thermal gradient (74.0°C/km) yielding value of 52 mW/m². This value is a little less than conductive cooling models for crust of this age.

Color spectrometry

Spectral reflectance was measured on split archive-half sections from Holes U1371D–U1371F. Measurements from Hole U1371D are shown in Figure F21. Values for L* are generally constant through lithologic Unit I and decline through Unit II. Values for a* and b* are generally correlated through Units I and II. These values are well correlated with the redox potential.

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