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doi:10.2204/iodp.proc.341.102.2014

Physical properties

Shipboard measurements of the physical properties of all the cores recovered were undertaken to rapidly characterize lithologic units, providing information on hole-to-hole and site-to-site stratigraphic relationships and guiding drilling efforts to recover complete stratigraphic sequences at each site. The primary objectives for physical properties measurements during Expedition 341 include the following:

1. Performing real-time assessment of sediment recovery at each site to guide drilling (see “Stratigraphic correlation”),
2. Correlating lithostratigraphy to downhole geophysical logging data,
3. Facilitating the construction of synthetic seismograms and integration with seismic data,
4. Providing porosity information to assist in understanding sediment consolidation and deformation behavior,
5. Identifying the physical parameters responsible for facies changes,
6. Developing and verifying a glacial sequence stratigraphic framework, and
7. Establishing the composite depth scale, the splice record, and initial age models at each site (see “Stratigraphic correlation”).

Physical properties were first measured on whole-round core sections. A typical core is cut into sections of no more than 1.5 m, yielding as many as six full-length core sections and a shorter seventh section in the case of maximum core recovery. Two data logging systems were run on each whole-round core section: (1) the STMSL, which measured sections immediately after they were brought onboard, and (2) the WRMSL, which was used to measure core sections that had warmed to ambient laboratory temperature (at least 4 h) for Sites U1417 and U1418. During operations at Site U1419, core expansion due to gassy sediments was determined to be a greater source of error than temperature, and thus the warming to ambient temperature procedure was abandoned. Following this, for Sites U1420 and U1421 we ran the cores at some point after warming to the point at which condensation ceased to form on the PVC liner. The linear track of the STMSL includes a GRA bulk densitometer and a magnetic susceptibility loop. The WRMSL also includes both those instruments, as well as a compressional P-wave velocity sensor.

After WRMSL scanning, the whole-round sections were measured for NGR. Core sections were then split along their length into archive and working halves. The archive half was run through a third data logging system, the SHMSL, for measurement of reflectance spectroscopy and colorimetry, magnetic susceptibility, and laser split-core surface analysis.

Discrete samples of 10 cm³ were collected from the working halves (~1/section) to measure wet bulk density, dry bulk density, water content, porosity, and grain density according to IODP moisture and density (MAD) procedures. Point measurements of P-wave velocity were collected on the working halves using the PWC (~2–3/core). Shear strength tests using the automated vane system were also conducted on the working halves (~2–3/core). A full discussion of all methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997).

Special Task Multisensor Logger

STMSL measurements were used to acquire GRA bulk density and magnetic susceptibility data at a lower spatial resolution than on the WRMSL to rapidly correlate the different holes at each site in order to guide the drilling operations and ensure complete sediment recovery. The GRA bulk densitometer and magnetic susceptibility loop on the STMSL are similar to those on the WRMSL (see “Whole-Round Multisensor Logger”), although the warming of the core over the course of measurement produces drift of up to ~5 instrument units (IU) in magnetic susceptibility measurements. The along-section spacing distance between STMSL measurements varied between 2.5 and 5.0 cm, depending on the time available to process each new core through the shipboard laboratories.

Whole-Round Multisensor Logger

After collection of STMSL data, most cores were allowed to sit in the laboratory for as long as 4 h to equilibrate with room temperature, with the exception of cores expanding because of high methane content. These cores were logged as quickly as possible to minimize vertical physical core disturbance prior to splitting. Whole-round sections were then measured at high resolution on the WRMSL. These data, including GRA bulk density, magnetic susceptibility, and P-wave velocity, advance the primary scientific objectives of the expedition by providing information on lithostratigraphic variability associated with changes in regional tectonics, glacial activity, and ocean productivity. However, this information is also critical in establishing shipboard core-to-core correlation between drill holes and for the construction of the composite depth and splice stratigraphic sequences. To optimize WRMSL performance, all sensors were set to a consistent sampling interval of 2.5 cm. Measurement times were 5 s for the GRA bulk densitometer, 3 s for the magnetic susceptibility loop (the average of 3 measurements of 1 s), and 5 s for P-wave velocity. With handling, data initializing, and track movement, a 1.5 m section took ~10 min to scan. A 9.5 m core thus took ~1 h to pass through the WRMSL. P-wave velocity measurements were not collected for XCB or RCB acquired cores because of errors generated by an incompletely filled core liner.

Gamma ray attenuation bulk density

Bulk density is a reflection of water-saturated porosity, grain size, grain density (mineralogy), grain packing, and coring disturbance. To measure bulk density, the GRA densitometer uses a 10 mCi ¹³⁷Cs capsule as a gamma ray source (with the principal energy peak at 0.662 MeV) and a scintillation detector. The narrow collimated peak is attenuated as it passes through the center of the core. Incident photons are scattered by the electrons of the sediment by Compton scattering.

The attenuation of the incident intensity (I₀) is directly related to the electron density in the sediment core of diameter (D) that can be related to bulk density given the average attenuation coefficient (in micrometers) of the sediment (Evans, 1965; Harms and Choquette, 1965). Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density is obtained through direct calibration of the densitometer using aluminum rods of different diameters mounted in a core liner filled with distilled water. Between holes, calibrations are performed from an aluminum plug with four different thickness steps of 2, 3, 4, and 5 cm mounted in a core liner filled with distilled water. In addition to this standard allowing for detection of instrument drift between holes and sites, the measurements provide a calibration curve to empirically relate the counts to density. As the instrument is volumetrically calibrated assuming the full interior diameter of the core liner is filled with sediment, any reduction in core diameter (such as can occur from XCB/RCB coring and/or clast-rich lithologies) will bias the results toward lower values (see VOLNORM in "Supplementary material"). In such cases of incomplete recovery, GRA bulk densities must be regarded as a minimum estimate.

Magnetic susceptibility

Magnetic susceptibility is a measure of the degree to which a material can be magnetized by an external magnetic field. It provides information on the magnetic composition of the sediments, which can often be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Magnetite and a few other iron oxides with ferrimagnetic characteristics have a specific magnetic susceptibility several orders of magnitude higher than clay, which has paramagnetic properties. Diamagnetic minerals (carbonate and silica) and plastics (core liner) have small negative values of magnetic susceptibility (e.g., Thompson and Oldfield, 1986). Sediments rich in biogenic carbonate and/or opal therefore have generally low to negative magnetic susceptibility values (e.g., Tauxe, 2010).

Magnetic susceptibility was measured using the noncontact pass-through “loop” sensor on the STMSL and WRMSL, with loop diameters of 90 and 80 mm, respectively. Because of its larger loop diameter, the initial data from the STMSL is of lower amplitude than the corresponding data from the WRMSL, although the half-height width of both response curves is ~4.5 cm (Fig. F19). To compensate for the reduction in response function amplitude, all raw STMSL magnetic susceptibility measurements are multiplied by an instrument unit correction factor of 1.46, derived from the measured ratio of a series of six homogenized “standards” made shipboard from varying concentrations of sepiolite drilling mud and powdered iron (Fig. F20A). These standards ranged from ~10 to ~2500 IU on the WRMSL magnetic susceptibility loop, fully encompassing the range of values observed during the expedition.

The output of the magnetic susceptibility loop sensors can be set to centimeter-gram-second (cgs) units or SI units, which are the standard units used by IODP. However, to actually obtain dimensionless SI units (i.e., volume-specific low-field magnetic susceptibility), the instrument units stored in the IODP database must be multiplied by a correction factor to compensate for instrument scaling and the geometric ratio. Identifying this correction factor requires standards of known magnetic susceptibility, which were not available on the ship. Consequently, all magnetic susceptibility values from the loops reported here are in instrument units and have not been corrected to dimensionless SI units.

On the SHMSL, magnetic susceptibility was measured via “point” sensor at 2.5 cm intervals for the majority of core sections. In some intervals where a specific lithology of interest was identified, the sampling interval was decreased to as low as 1 cm. The point sensor requires flush contact with the split core, and results are negatively biased by rough surfaces associated with clast-rich lithologies and gassy sediments. A laser surface analyzer aids in the recognition of irregularities in the split core surface (e.g., cracks and voids), and data from this tool were recorded to provide an independent check on the fidelity of SHMSL measurements. STMSL MS-POINT data are reported in SI units. When downloading the depth data from LIMS, note that for some intervals there is an 0.5 cm offset between uploaded data and measured sample point data.

We evaluated the sensitivity of the SHMSL point sensor to the STMSL and WRMSL loop magnetic susceptibility measurements via the same six homogenized standards used to evaluate the relationship between the WRMSL and STMSL loop magnetic susceptibility (Fig. F20B). The relationship between the WRMSL and the STMSL magnetic susceptibility data is potentially nonlinear and should be interpreted with caution.

P-wave logger

Compressional wave (P-wave) velocity is dependent upon the incompressibility (bulk modulus) and rigidity (shear modulus) of a material. These two moduli vary with a material’s lithology, porosity, cementation, bulk density, state of stress, temperature, and fabric or degree of fracturing. In marine sediments, velocity is additionally controlled by the occurrence and abundance of free gas and gas hydrate. Microscopic and macroscopic fracturing, as well as the presence of gas in the sediments, may attenuate the signal to the point where it is not possible to obtain data. Together with bulk density, velocity data are used to calculate acoustic impedance and reflection coefficients that can be used to construct synthetic seismograms and estimate the depths of seismic horizons.

P-wave velocities were measured on whole-round sections with the PWL on the WRMSL for cores that were acquired using the APC system. The PWL transmits a 500 kHz P-wave pulse through the core section at a specified repetition rate of 2/s. This signal is coupled to the sample by the plastic pole pieces of the transducers and by the pressure applied by the linear actuator. For the PWL, in contrast to the PWC, no water is used to improve coupling between the transducers and the liner because the pressure applied by the actuator is known to be sufficient for reliable P-wave measurements. The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the section’s long axis.

A linear voltage differential transformer is used to measure the separation of the transducer to derive a travel path length for the signal (i.e., the slightly compressed core diameter). The ultrasonic P-wave velocity is then calculated after corrections have been made for system propagation delay, liner thickness, and liner material velocity. Calibrations were performed before each hole on the PWL using an acrylic cylinder milled to four calibration sections of 69.7, 64.8, 59.75, and 54.75 mm in diameter.

Natural Gamma Radiation Logger

NGR measurements provide insights into sediment composition and thus can be used to identify lithology. The NGRL measures gamma rays emitted from whole-round core sections arising primarily from the decay of long-period isotopes uranium (²³⁸U), thorium (²³²Th), and potassium (⁴⁰K). Typically, high counts identify fine-grained deposits containing K-rich clay minerals and their adsorbed U and Th atoms. NGR data can also help to correlate the core material recovered with the downhole logs for core-log integration.

The design and basic operation of the NGR detector unit is described in Vasiliev et al. (2011). The instrument contains eight sodium iodide detectors spaced ~20 cm apart. A measurement run consists of counting two positions on each core section for a total of 16 measurements per 150 cm section, yielding a depth resolution of 10 cm. Total detector counts are routinely summed for the range of 100–3000 keV. An energy calibration for each detector was routinely performed using ¹³⁷Cs and ⁶⁰Co sources to identify peaks at 662 and 1330 keV, respectively. Background measurements were done on an empty core liner for ~20,000 s (>5 h) on approach to each site or group of sites to minimize local cosmic-ray flux contributions to the statistical error.

The quality of the energy spectrum measured in a core depends on the concentration of radionuclides in the sample but also on the counting time, with longer measurement times yielding increasingly resolved spectra. We set the instrument to count for at least 5 min in both run positions, which should equate to a measurement error of <2.5% (Vasiliev et al., 2011). Readings at the ends of core sections are automatically corrected by the NGRL software using empirically derived correction factors (Vasiliev et al., 2011).

During Expedition 341 we used two of the available edge correction files for the NGRL software, both of which consist of a lookup table at 1 cm resolution. Correction coefficients decrease along a half-Gaussian curve as the section length within the measurement window increases; in the case of less than half the detector window seeing sediment, the measurement is discarded and the edge correction is applied to the next sensor in. The appropriate correction coefficient is determined by the distance between the section end and the center of the most proximal detector; for file “ngr_edge_correction_table_0.txt,” in the case of a 0 cm offset between the outermost detectors and the edge of the core, the end detector counts are simply multiplied by 2 (see Vasiliev et al., 2011). For file “ngr_edge_correction_table_1.txt,” the lookup table of the 0 offset file is shifted 1 cm, such that the former correction for a 1 cm offset becomes the correction factor for a 0 cm offset (Fig. F21). During Expedition 341, we applied edge correction file “ngr_edge_correction_table_1.txt” through the end of Site U1418. To use this edge correction, sections were loaded ~1 cm back from the leading edge of the boat to avoid overcorrection of the uppermost measurement. However, the loading position was inconsistent between operators, so we switched to loading sections flush with the leading edge of the boat coupled with edge-correction file “ngr_edge_correction_table_0.txt” prior to occupation of Site U1419.

To determine the position of the core for the lower edge correction, the NGR detector software requires that core lengths are manually entered. The measuring tape along the sample loading boat used to determine this length was moved relative to the leading edge of the core at several times prior to occupation of Site U1419; after this was discovered, the tape was zeroed relative to the leading edge of the boat, where it remained for the rest of the expedition. Lower edge corrections with the 0 cm offset file were variably high; to minimize this effect we systematically removed 1 cm from the actual section length for the application of the edge correction, rounding to the nearest centimeter. Although this in practice reduced notable errors in the edge correction, its necessity suggests that the sodium iodide detectors are not all positioned where the software believes they should be. Preliminary evaluation with a ¹³⁷Cs point source suggests variable along-section offsets in both the positive and negative direction. Although these position errors are likely quite small, an offset of 1 cm equates to an error of >10% near the center of the detector, and this may be a contributing factor to the inconsistency of the edge corrections.

Moisture and density

After whole-round cores were split into working and archive halves, the working halves were placed on the sampling table for collection of discrete samples to determine wet and dry bulk density, grain density, water content, and porosity. In soft sediment, ~10 cm³ samples were collected with a plastic syringe. Samples were taken no more than once per core section and no less than three per core. Samples were placed in numbered, preweighed 16 mL Wheaton glass vials with a diameter matching that of the syringe for wet and dry sediment weighing, drying, and wet and dry volume measurements. The weights of wet and dry sample masses were determined to a precision of 0.005 g using two Mettler Toledo electronic balances and a computer averaging system to compensate for the ship’s motion.

Dry sample volume was determined using a hexapycnometer system of a six-celled, custom-configured Micromeritics AccuPyc 1330TC helium-displacement pycnometer. The precision of each cell is 1% of the full-scale volume. Volume measurement was preceded by three purges of the sample chamber with helium warmed to ~28°C. Three measurement cycles were run for each sample. Periodically, a solid metallic reference volume was placed sequentially in one of the chambers to check for instrument drift and systematic error. The volumes of the numbered Wheaton vials were calculated before the expedition by multiplying each vial’s weight against the average density of the vial glass. Dry mass and volume were measured after samples were heated in an oven at 105° ± 5°C for 24 h and allowed to cool in a desiccator for 15–20 min. The procedures for the determination of these physical properties comply with the American Society for Testing and Materials designation 2216 (ASTM International, 1990). The fundamental relation and assumptions for the calculations of all physical property parameters are discussed by Blum (1997) and summarized below in “Mass and volume calculation” and “Calculation of bulk properties.”

Mass and volume calculation

Wet mass (M_wet), dry mass (M_dry), and dry volume (V_dry) were measured in the laboratory. The ratio of mass (rm) is a computational constant of 0.965 (i.e., 0.965 g of freshwater per 1 g of seawater). Salt precipitated in sediment pores during the drying process is included in the M_dry and V_dry values. The mass of the evaporated water (M_water) and salt (M_salt) in the sample are given by

and

respectively, where s is the assumed saltwater salinity (0.035%) corresponding to a pore water density (ρ_pw) of 1.024 g/cm³ and a salt density (ρ_salt) of 2.22 g/cm³. The corrected mass of pore water (M_pw), volume of pore water (V_pw), mass of solids excluding salt (M_solid), volume of salt (V_salt), volume of solids excluding salt (V_solid), and wet volume (V_wet) are, respectively,

and

Calculation of bulk properties

For all sediment samples, water content (w) is expressed as the ratio of mass of pore water to wet sediment (total) mass: w = M_pw/M_wet. Wet bulk density (ρ_wet), dry bulk density (ρ_dry), sediment grain density (ρ_solid), porosity (ϕ), and void ratio (VR) are calculated as follows:

and

MAD properties reported and plotted in the “Physical properties” sections of all site chapters were calculated with shipboard-prepared Excel spreadsheets.

Section Half Measurement Gantry

Caliper P-wave velocity

For a description of the controls of P-wave velocity in marine sediments see “P-wave logger.”

We used the PWC to vertically measure P-wave velocity across the working half of split cores (x-axis), regardless of drilling method. These measurements were taken no more than once per section and no less than three times per full core. Additional PWC measurements were taken in heterogeneous sections (e.g., significant lonestone content). As with the PWL, the PWC transmits a 500 kHz P-wave pulse through the core section at a specified repetition rate of 2/s, coupled to the sample by the plastic pole pieces of the transducers and by the pressure applied by the linear actuator. Water is used to improve coupling between the transducers and the liner/sediment surface. The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the section’s long axis.

Traveltime is determined by signal processing software that automatically detects the first arrival of the P-wave signal to a precision of 50 ms. It is challenging for an automated routine to pick the first arrival of a potentially weak signal with significant background noise. The search method skips the first positive amplitude and finds the second positive amplitude using a detection threshold limit, typically set to 30% of the maximum amplitude of the signal. It then finds the preceding zero crossing and subtracts one period to determine the first arrival. To avoid extremely weak signals, minimum signal strength can be set (0.02 V) and weaker signals will be ignored. To avoid cross-talk signals from the receiver at the beginning of the record, a delay (typically 0.01 ms) can be set to force the amplitude search to begin in the quiet interval preceding the first arrival. In addition, a trigger (4 V) to initiate the arrival search process and the number of waveforms to be stacked can also be set. We used stacks of five measurements for Sites U1417–U1419. For Sites U1420 and U1421, we increased the number of stacks to 15 in order to increase the signal-to-noise ratio when measuring velocities in the mostly diamict intervals. Even with the stacking parameters, the auto-picking feature failed when the signal-to-noise ratio was high or when the picking threshold was exceeded. In some of these cases, we were able to manually pick the first arrivals.

As with the PWL, a linear voltage differential transformer is used to measure the separation of the transducer to derive a travel path length for the signal (i.e., the slightly compressed core diameter). The ultrasonic P-wave velocity is then calculated after corrections have been made for system propagation delay, liner thickness, and liner material velocity. We calibrated the system every 3–4 h, depending on when we detected significant drift. Calibrations were performed on acrylic cylinders of varying lengths: 45, 39.9, 30.03, 20.02, and 15.07 mm.

Vane shear tests

Shear strength of a material describes the point at which a significant structural failure occurs in response to an applied shear stress. During Expedition 341, sediment shear strength was measured by vane shear in the laboratory, as it is suited for measuring the shear strength of very soft to stiff marine sediments. Vane tests are useful for determining the undrained shear strength of undisturbed clay- or silt-rich samples. These shear strength tests are not suitable for coarser grained sediments or sediment containing silt or sand laminations. The typical sampling rate was one measurement per core section until the sediment became too firm for vane insertion. Samples were generally taken in undisturbed fine-grained sediment.

The automated vane system test was conducted using the Giesa automated vane shear system. This system consists of a controller and a gantry for shear vane insertion. A four-bladed miniature vane (diameter = height = 12.7 mm) was pushed carefully into the sediment of the working halves until the top of the vane was level with the sediment surface. The vane was then rotated at a constant rate of 90°/min to determine the torque required to cause a cylindrical surface to be sheared by the vane. The measurement was continued until the plot curve showed a decrease. This destructive measurement was done with the rotation axis parallel to the bedding plane. The torque required to shear the sediment along the vertical and horizontal edges of the vane is a relatively direct measurement of shear strength. Undrained shear strength, s_u, is given as a function of pressure in SI units of pascals (kPa = N/m²).

Diffuse spectral reflectance

Measurements of diffuse spectral reflectance were carried out using an Ocean Optics USB4000 spectrophotometer. This instrument measures the diffuse reflectance spectra of the split core from the ultraviolet to near-infrared range (380–900 nm) at 2 nm increments. Spectral data are also converted to the International Commission on Illumination L*a*b* color space, where L* ranges from black to white, a* from green to red, and b* from blue to yellow (e.g., St-Onge et al., 2007).

Accurate diffuse spectral reflectance measurements using the SHMSL requires flush contact between the instrument sensors and the split core. A built-in laser surface analyzer aids the recognition of irregularities in the split core surface (i.e., cracks and voids), and data from this tool were recorded in order to provide an independent check on the fidelity of SHMSL measurements and to filter the data in order to remove data irregularities caused by gaps or drilling disturbance.

Heat flow

The APCT-3 consists of electronic components, including battery packs, a data logger, and a platinum resistance-temperature device calibrated over a temperature range of 0°–30°C. Descriptions of the tool and data analysis principles can be found in Pribnow et al. (2000) and Graber et al. (2002) and references therein. The thermal time constant of the cutting shoe assembly where the APCT-3 is inserted is ~2–3 min. The only modification to normal APC procedures required to obtain temperature measurements is to hold the corer in place 5–10 min near the seafloor to record bottom water temperatures and to hold it for ~10 min in the hole after cutting the core. During this time, the APCT-3 logs temperature data on a microprocessor contained within the instrument as it approaches equilibrium with the in situ temperature of the sediments. The tool can be preprogrammed to record temperatures at a range of sampling rates. A sampling rate of 10 s was used during Expedition 341.

A typical temperature history recorded by the APCT-3 is shown in Figure F22. It consists of (a) a mudline temperature record lasting 5 min, followed by (b) a pulse of frictional heating when the piston is fired, (c) a period of thermal decay that is monitored for ~10 min, (d) a frictional pulse upon removal of the corer, and (e) a second mudline temperature measurement for 5 min. The in situ temperature is determined by extrapolating from the thermal decay that follows the frictional pulse when the piston is fired. Details of this process and the associated uncertainties are discussed in the individual site chapters.

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