IODP Proceedings    Volume contents     Search


Physical properties

At Site U1370, 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 Imaging Logger was used to collect images of the split surfaces of the archive-half cores, and a color spectrophotometer was used to measure color reflectance on the Section Half Multisensor Logger (SHMSL). Three holes targeted the sedimentary cover, Holes U1370D–U1370F. The most complete hole for logging physical properties was Hole U1370D. The sediment recovered from different holes has not been correlated and offsets exist.

Density and porosity

Bulk density values at Site U1370 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from Hole U1370D from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). A total of 44 discrete samples were analyzed for MAD.

Bulk density values in Hole U1370D increase slightly with depth throughout lithologic Unit I (Fig. F14A). In lithologic Unit II (nannofossil ooze), density is greater than in Units I and III (metalliferous clay). The average density of Units I and III is 1.32 g/cm3, and the average density of Unit II is 1.49 g/cm3. Bulk density discrete values generally agree with GRA values of bulk density above ~30 mbsf, but beneath this depth the discrete values are significantly lower than GRA values. GRA density calibrations were checked, but the source of this offset is not known. Grain densities have a mean value of 2.38 g/cm3, with some values approaching 3 g/cm3 (Fig. F14B). Porosity generally decreases with depth and varies between ~87% and 71% (Fig. F14C).

The mean value of bulk density in Hole U1370B is 1.3 g/cm3 and is slightly higher in lithologic Unit II than in Unit I. The scattered values in Hole U1370B deeper than 15.5 mbsf likely reflect core disturbance. Gaps and scatter in bulk density values for Holes U1370C and U1370E are due to whole rounds being taken for geochemical and microbiological sampling prior to WRMSL measurements.

Bulk densities for Holes U1370D, U1370E, and U1370F are shown in Figure F15. Bulk density values in Holes U1370E and U1370F are in good agreement with those in Hole U1370D, suggesting that the offset between GRA and MAD values shown in Figure F14 are more likely due to the MAD values than the GRA values.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL in all recovered cores from Site U1370. Uncorrected values of magnetic susceptibility are presented for Holes U1370D–U1370F (Fig. F16). 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 U1370C and U1370E is due to edge effects.

Magnetic susceptibility generally increases with depth through lithologic Unit I with a peak ~5 m above the Unit I/II contact. Magnetic susceptibility values within Unit II (Holes U1370D and U1370F) are low, although about half of the core material in this unit is disturbed (i.e., Section 329-U1370D-8H-1; see “Lithostratigraphy”). Magnetic susceptibility in Unit III reaches a maximum just below the Unit II/III contact and then decreases with depth. The high magnetic susceptibility in Units I and III is likely due to the RSO content of the clay (see “Lithostratigraphy”).

Natural gamma radiation

NGR results are reported in counts per second (Fig. F17). NGR counting intervals were ~60 min per whole-core interval for Hole U1370D and decreased to 30 min per whole-core interval for Holes U1370E and U1370F. NGR counts are considered reliable. NGR at the tops of Holes U1370E and U1370F are high, indicating that the sediment/water interface was sampled.

In general, NGR counts increase with depth through lithologic Unit I, are low in Unit II, and decrease with depth through Unit III (Fig. F17). These patterns are similar to those observed in the magnetic susceptibility data (Fig. F16). Ringing is more prevalent in cores from Holes U1370E and U1370F because only short core pieces remained after whole-round sampling prior to NGR measurements.

P-wave velocity

P-wave velocity at Site U1370 was determined from measurements on whole-round sediment cores (Fig. F18) and on discrete samples from the working halves of sediment split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Only six discrete measurements of P-wave velocity were measured. These discrete values are somewhat higher than measurements made on whole rounds, which may be due to drying of core material. The mean value of all whole-core measurements is ~1510 m/s (Fig. F18B). In Hole U1370B, compressional wave velocities are relatively uniform except in the upper portion of lithologic Unit III, where it is <1500 m/s.

Formation factor

Electrical conductivity was measured on working halves of the split sediment cores from Hole U1370B. Measurements in Hole U1370B were made at a nominal interval of 10 cm. For each measurement, the temperature of the section was also noted. Surface seawater was used as a standard and measured at least twice per section (Table T4), normally prior to making measurements for that section and again around the 75 cm offset of each section. These measurements were used to compute the drift for this set of measurements (Fig. F19). Near measurement 400, the drift changed substantially, likely caused by wearing away of platinum on the electrodes. For this set of measurements, we used two linear trends to describe the drift. 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 measurements for the drift (Table T5).

In general, the formation factor in lithologic Unit I increases with depth and varies between ~1.4 and ~2.5 (Fig. F20). The formation factor in Unit II is scattered between 2 and 2.5. Within Unit III values decrease from about 2.2 to 1.6. This pattern correlates with porosity (Fig. F14C).

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method and on basement split cores using the half-space 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 either 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 Unit I and appear to decrease with depth through Unit III. The mean thermal conductivity is 0.7 W/(m·K) (Fig. F21A). In the uppermost 3 m or so, thermal conductivities collected during the KNOX-02RR site survey cruise (R. Harris, unpubl. data) are somewhat higher than the values reported but also have a mean of 0.7 W/(m·K).

Downhole temperature

Downhole temperature was measured using the APCT-3. In total, four measurements were attempted between 25.2 and 53.7 mbsf in Holes U1370D and U1370E (Table T6). 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 a period of 5 min thereafter. The average bottom water temperature is 1.24°C (Table T6). All measurements were made in a relatively extreme sea state (~5 m swell), and each temperature-time series suffers to some extent from the ship’s heave (Fig. F22). 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 large and fits to the equilibrium curve are short (Table T4). Nevertheless, all 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 93.0°C/km (Fig. F21B).

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 and thermal gradient. Using the average thermal conductivity of 0.7 W/(m·K) and thermal gradient of 3.0°C/km yields a heat flow value of 65 mW/m2. This value is consistent with conductive cooling models for crust of this age.

Color spectrometry

Spectral reflectance was measured on split archive-half sections from Holes U1370D–U1370F. Measurements from Hole U1370D are shown in Figure F23. All parameters generally decrease with depth. Values for L* are generally <50 but are >50 in lithologic Unit II (nannofossil ooze). Values for a* and b* are between 10 and 0 and 30 and 0, respectively, with maximum values in Unit II. Values for a* and b* appear modestly correlated.