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doi:10.2204/iodp.proc.329.103.2011

Physical properties

At Site U1365, physical property measurements were made to provide basic information characterizing lithostratigraphic 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. For basement cores, only GRA density and magnetic susceptibility were measured. After WRMSL scanning, the whole-round sections were logged for NGR. Thermal conductivity was measured using the full-space method on sediment cores and the half-space method on split basement cores. Images were made of the split core on the Section Half Image Logger, and color spectrometry and color reflectance data of the split surfaces of the archive-half cores were collected using the SHMSL. Discrete P-wave measurements were made on split sediment cores and on cubes subsampled from basement working-half cores on the Section Half Measurement Gantry. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split sediment cores and cubes cut from the basement working-half cores. Additional discrete measurements of electrical resistivity were made on the split sediment sections to calculate formation factor.

Density and porosity

Bulk density values at Site U1365 were determined from both GRA density measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011]). A total of 36 discrete samples were analyzed for MAD, 14 samples from Hole U1365A, 8 samples from Hole U1365B, 3 samples from Hole U1365C, and 9 samples from Hole U1365E.

In general, wet bulk density values determined from whole-round GRA density measurements and measurements from discrete samples agree well (Fig. F40A). In lithologic Unit I (see “Lithostratigraphy”), wet bulk density values average 1.26 g/cm³. Between ~10 and 20 mbsf, bulk density decreases slightly from ~1.3 to 1.2 g/cm³ (Fig. F40A). No discrete subsamples could be taken in Unit II because the entire recovered interval was chert. GRA-derived bulk densities through the chert are highly scattered because of the variable filling of the core liner. Variation in wet bulk density values of Unit III is similar to variation in Unit I. Below ~20 mbsf, bulk density values tend to increase slightly with depth. In basement, the mean and standard deviation of bulk density is 2.84 and 0.08 g/cm³, respectively.

Grain density measurements were determined from mass/volume measurements on discrete samples. In lithologic Unit I, grain densities decrease with depth from ~2.5 to 1.8 g/cm³ (Fig. F40B). In Unit III (below the chert of Unit II), grain density values vary between 2.6 and 2.0 g/cm³ but show no straightforward depth-dependent trend.

Porosity measurements (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011]) were determined from mass/volume measurements on discrete samples using Method C on sediment cores and Method D on basement cores. Within lithologic Unit I, porosity varies between 88% and 77% and does not show a straightforward depth-dependent trend (Fig. F40C). In Unit III, porosity has similar values to Unit I but appears to generally increase with depth. However, this apparent increase in value is likely an artifact because there is not a corresponding change in P-wave velocity. In basement, the average and standard deviation porosity is 4% and 2%, respectively.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL for all recovered cores from Site U1365. Uncorrected values of magnetic susceptibility and are presented in Figure F41. Magnetic susceptibility values measured with these two methods are in good agreement. In the sediment (Fig. F41A, F41B), mean magnetic susceptibility is highest in lithologic Unit I, lowest in Unit II, and generally low in Unit III. Two conspicuous magnetic susceptibility highs are present, one in Unit I (between 3 and 10 mbsf) and the other in Unit III (between 71 and 72 mbsf).

Within the basement, magnetic susceptibility values measured on whole core and point measurements on working-half cores are in general agreement (Fig. F41C, F41D). The apparent variability is an artifact of discontinuous core. Between ~100 and 110 mbsf, magnetic susceptibility values appear relatively low, and between 110 and 115 mbsf values are relatively high.

Natural gamma radiation

NGR results are reported in counts per second (cps) (Fig. F42A). These values have been used to compute percent potassium through a Monte Carlo inversion (Fig. F42B). NGR counting intervals were ~1 h per whole-core interval, and NGR counts are considered reliable. In general, the potassium content tracks with NGR counts; however, notable exceptions indicate enrichments of uranium or thorium or both. For example, a very prominent NGR peak at the seafloor is not mimicked by potassium concentration because it results from high elemental concentrations from the ²³⁸U decay series (probably ²³⁰Th). At ~8.5 mbsf, a second peak is present in both NGR and potassium. Between ~20 and 40 mbsf, potassium is high, whereas NGR counts remain low, indicating a depletion in uranium and/or thorium. Lithologic Units II and III generally have low NGR counts and low potassium.

Basement NGR values and potassium content show high wave-number variability. This variability is interpreted in terms of basalt alteration; high values correlate with greater alteration (Fig. F42). NGR in the basement is typically highest in the oxidized zones at the tops and bottoms of the massive basalt flows. Background NGR and potassium increases steadily in the lowermost few meters of the cored basement, where the massive basalt is pervasively weathered.

P-wave velocity

P-wave velocity at Site U1365 was determined from measurements on sediment whole cores and mass/volume measurements on discrete samples from the working halves of sediment and basement samples taken from split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011]). In general, whole-core and discrete measurements on sediments are in good agreement for lithologic Units I and III, and no depth dependence is observed. (Fig. F43A). The mean P-wave velocity value is 1517 m/s, close to the compressional velocity of water (Fig. F43B).

P-wave velocity in the basement varies between ~4700 and 6600 m/s (Fig. F43C, F43D). Although the data appear noisy, velocity generally increases with depth.

Formation factor

Electrical conductivity was measured on working halves of the split sediment cores from Hole U1365A. Measurements were made at nominal intervals of 10 cm. For each measurement, the temperature of the section was also noted. A surface-seawater standard was measured at least twice per section, normally prior to making measurements for that section and then around 75 cm depth (Table T6). An International Association for the Physical Sciences of the Oceans standard (Table T7) was also measured, but less frequently. A comparison of measurements from these standards is displayed in Figure F44. Both sets of measurements show similar, but not identical, trends. The difference in trends is attributed to the different salinity contents of the standards. The best-fit trend to the measurements made on surface seawater is used to compute the drift in electrical conductivity measurements made in the sediment. The temperature dependence of electrical conductivity was corrected; 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 T8). In lithologic Unit I, the formation factor displays a low wave-number trend that increases with depth (Fig. F45). Variations from this trend include a conspicuous high between 3 and 7 mbsf and a trend to lower values just above Unit II. No values were measured in Unit II (i.e., chert). Measurements in Unit III decrease with depth but show higher wave-number scatter than in Unit I.

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, 2011]). Many of the Site U1365 needle-probe measurements on sediment are considered unreliable because the temperature-time series of these measurements indicate that the measurements caused fluid to convect within the samples. Convection leads to unreasonably low estimates of thermal conductivity by causing the thermal response to heating to depart from the theoretical prediction. The tendency for fluid to convect is more prevalent in lithologic Unit I than in Unit III. However, a subset of values clustered around 0.8 W/(m·K) (Fig. F46A); this value is used as a shipboard estimate of the thermal conductivity. Sediment samples were collected to measure thermal conductivity postcruise using a divided bar apparatus.

Half-space measurements were made on working-half basement cores. In general, the values are uniform with a mean and standard deviation of 1.6 and 0.2 W/(m·K), respectively.

Downhole temperature

Downhole temperature was measured using the advanced piston coring temperature tool (APCT-3). Six measurements were attempted between 24.6 and 42.0 mbsf in Holes U1365A–U1365C (Table T9; Fig. F46B). All measurements were made in lithologic Unit I.

All measurements were made in a moderate sea state (<2 m swell), and 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 each penetration. The average bottom water temperature is 1.22°C (Table T9). Temperature-time series for each temperature measurement are shown in Figure F47. 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 useable section of the temperature-time series. Tool movement is attributed to the high porosity of the sediments. 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 T9). 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 76.4°C/km (Fig. F46B).

Heat flow

Because thermal conductivity appears relatively constant and the thermal gradient is linear, we compute heat flow as the product of the thermal conductivity and thermal gradient. Using a thermal conductivity of 0.78 W/(m·K) and a thermal gradient of 76.4°C/km yields a heat flow of 58 mW/m². This value is consistent with conductive cooling models for crust of this age (Stein and Stein, 1994).

Color spectrometry

Color reflectance measurement results are presented in Figure F48. L* values are ~50 with some clusters of higher values of ~200. The majority of a* values range from approximately –2 to 10, with some higher values of ~20 between 11 and 16 mbsf. The majority of b* values ranged from –50 to 30, with both the minimum and maximum values within lithologic Unit I. Maximum values of b* decrease with depth.

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