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doi:10.2204/iodp.proc.346.102.2015

Physical properties

High-resolution physical property measurements were made during Expedition 346 to provide information on the bulk physical character of sediment. Such data enhance our understanding of the physicochemical context and history for oceanic deposits and augment lithologic unit characterization while facilitating the correlation of downhole logging data with discrete core measurements and core descriptions. As such, physical property data play a major role in the following tasks:

• Hole-to-hole and site-to-site stratigraphic correlation for construction of composite stratigraphic sections;

• Detection of discontinuities and lateral inhomogeneities, either caused naturally or by the drilling process;

• Obtaining information about differences in the composition and texture of sediment;

• Detection of orbital cycles and tuning to reference sections for stratigraphic purposes; and

• Identification of major seismic reflectors and construction of synthetic seismic profiles.

First, physical properties were measured on whole-round core sections. Core sections are generally 1.5 m in length, so a typical coring length (stroke) of 9.5 m yields six sections plus a shorter seventh section or two shorter sixth and seventh sections. Two core logging systems were used on each section: (1) the STMSL was used on sections immediately after they were sectioned on the catwalk, and (2) at Site U1422 the WRMSL was used on core sections that had warmed to ambient laboratory temperature (20°–21°C). The STMSL houses a GRA bulk densitometer and a magnetic susceptibility sensor loop. The WRMSL employs, in order, a GRA bulk densitometer, a magnetic susceptibility sensor loop, and a compressional P-wave velocity sensor.

Simultaneous use of STMSL and WRMSL

Analysis of IODP Site U1423 data demonstrated that thermally equilibrated and unequilibrated GRA and magnetic susceptibility data sets were equivalent to within ±1–2 instrument units. Accordingly, beginning with Site U1423 (and for all sites thereafter), the STMSL and WRMSL were used simultaneously in anticipation of exceptionally fast core recovery at shallow-water sites (300–700 m). We used both tracks in parallel to process each core immediately after recovery (unequilibrated), effectively doubling our speed. This allowed the stratigraphic correlators to assess the alignment of core gaps among holes in real time (before the next core was taken). In this manner, minor adjustments to bit depth prevented gap alignment and missing sections.

The top half of each core (e.g., Sections 1–3) was typically run on the WRMSL and the bottom half (e.g., Sections 4–7) on the STMSL using the exact same settings. These data were all recorded as WRMSL in the LIMS database. In this mode, P-wave data was typically collected only on Sections 1–3 because there is no P-wave logger on the STMSL.

After STMSL and WRMSL scanning, the whole-round sections were thermally equilibrated and logged through the NGRL. Thermal conductivity was then measured on certain whole-round sections (Section 3 of each core in one hole per site, usually Hole A) by inserting a needle probe into the section through a small hole drilled through the plastic core liner close to the middle of the section. The sections were then split into working and archive halves.

Discrete samples were collected from the working halves, primarily from Hole A, to measure wet bulk density, dry bulk density, water content, porosity, and grain density with MAD procedures. Holes B and C were sampled for MAD samples only to fill significant gaps in the Hole A sample series or if unusual sediment was recovered with no equivalent in Hole A. Automated vane shear device strength measurements on split cores were also taken for Hole A. Archive halves were measured with the SHMSL for obtaining color reflectance (via an Ocean Optics sensor) and magnetic susceptibility using a discrete point-source Bartington probe. A full discussion of all methodologies and calculations used aboard the JOIDES Resolution in the Physical Properties Laboratory is available in Blum (1997).

Special Task Multisensor and Whole-Round Multisensor Loggers

In addition to their intrinsic value for the scientific party, high-resolution STMSL and WRMSL GRA (porosity evaluation) bulk density and magnetic susceptibility data were collected for shipboard core-to-core correlation between drill holes and constructing composite stratigraphic sections. Both tasks had to be completed within a reasonable time to not encumber downstream core processing and sample collection. The quality of STMSL and WRMSL data is highly dependent on the structural integrity of the sediment (cracks, voids, biscuiting, etc.) and the presence of any gaps between the sediment and the core liner. GRA bulk density and magnetic susceptibility were measured nondestructively on all whole-round core sections. P-wave velocity was measured until the acoustic signal disappeared because of high attenuation, which typically occurs in sections disturbed by gas expansion voids and cracks.

Gamma ray attenuation bulk density

Bulk density is a function of water-saturated porosity and grain density (dominant mineralogy) and influenced by 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, provided the average attenuation coefficient (in micrometers) of the sediment is known (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. The spatial resolution was <1 cm.

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 sediment that commonly can be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Magnetite and a few other iron oxides and sometimes monosulfides with ferromagnetic characteristics have a specific magnetic susceptibility several orders of magnitude higher than clay, which has paramagnetic properties. Carbonate layers, opal, water, and plastic (core liner) have small negative values of magnetic susceptibility. Calcareous and biogenic deposits with low clay and iron-bearing mineral debris content thus have values approaching the detection limit of magnetic susceptibility instruments.

Magnetic susceptibility was measured on thermally unequilibrated cores (except at Site U1422) using the STMSL and WRMSL units in parallel, with the Bartington Instruments MS2C system. The frequency at which the magnetic susceptibility loop operates is 565 Hz for the WRMSL and STMSL (Blum, 1997). The two units used in parallel were not frequency adjusted but were shielded from interfering with one another using ~2 cm thick aluminum shielding around the STMSL unit. The output of the magnetic susceptibility sensors can be set to centimeter-gram-second (cgs) units or SI units, the IODP standard. However, to obtain dimensionless SI volume-specific magnetic susceptibility values, the instrument units stored in the IODP database must be multiplied by a correction factor (0.68) to compensate for instrument scaling and the geometric ratio between core and loop dimensions (Blum, 1997).

Compressional P-wave velocity

P-wave velocity varies with the material’s lithology, porosity, bulk density, state of stress, temperature, fabric, and/or degree of fracturing. In sediment and rock, velocity is controlled by the degree of consolidation and lithification, fracturing, and occurrence and abundance of free gas and gas hydrates. Together with bulk density, velocity data are used to calculate acoustic impedance and reflection coefficients to construct synthetic seismic profiles and to estimate the depth of specific seismic horizons.

The P-wave velocity sensor measures the ultrasonic P-wave velocity of the whole-round sample residing in the core liner. The P-wave logger transmits a 500 kHz P-wave pulse across the core section at a specified repetition rate.

Traveltime is determined by signal processing software that automatically detects the first arrival of the P-wave signal to a precision of 50 ns. Ultrasonic P-wave velocity is calculated after correcting for system propagation delay, liner thickness, and liner material velocity.

Natural Gamma Radiation Logger

The NGRL measures gamma rays emitted from whole-round core sections. Gamma rays detected by the logger arise primarily from the decay of U, Th, and K isotopes. In general, high counts identify fine-grained deposits containing K-rich clay minerals and their associated U and Th atoms. The NGR data also show stratigraphic details that aid in core-to-core correlations.

The main NGR detector unit consists of 8 sodium iodide (NaI) scintillator detectors set at a 20 cm interval surrounding the lower half of the section, 7 shielding plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by at least 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of common lead. In addition to this passive lead shielding, the overlying plastic scintillators detect incoming high-energy gamma and muon cosmic radiation and cancel this signal from the total counted by the NaI detectors.

A measurement run consisted of counting on each core section for 5–10 min (at Position 1), depending on the pace of core recovery. This yields a total of 8 measurements per 150 cm section. For some analyses, Position 2 was chosen to avoid analyzing the disturbed uppermost part of the first section in a core. Therefore, NGR logging required 30–60 min measurement time per core and another ~10 min for core, software, and data handling. Additional information may be found in Vasiliev et al. (2011) and Dunlea et al. (2013).

Thermal conductivity

After NGR measurements were completed, thermal conductivity was measured with the TK04 (Teka Berlin) system using the needle-probe method in full-space configuration for whole-round sediment cores (Von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. The probe was inserted into a 2 mm hole drilled through the liner along the splitting line of the core section. To avoid interference from airflow in the laboratory, an insulating jacket of foam rubber was placed over the core section during measurement of thermal conductivity. We also placed the core section inside a large plastic “coffin” to further reduce thermal disturbances.

Three measuring cycles (replicates) were automatically performed at each probe location (normally ~75 cm offset from the top of the section) to calculate average thermal conductivity in the core material. The instrument routinely performed a self-test, which included a drift measurement, at the beginning of each measurement cycle. Once the probe temperature stabilized, the heater circuit was closed and the temperature rise in the probe was recorded. Thermal conductivity was calculated from the rate of temperature rise while the heater current was flowing. Temperatures measured during the first 150 s of the heating cycle were fitted to an approximate solution of a constantly heated line source (for details, see Kristiansen, 1982; Blum, 1997). Thermal conductivity measurements were routinely taken in one hole per site and in one section per core, typically Section 3. Cores retrieved by XCB and RCB typically yield spurious results for thermal conductivity measurements because of the negative effects on the measurement from fluid convection through cracks in the sediment and thereby associated bad coupling of the probe to the sediment.

Moisture and density

After completion of thermal conductivity measurements, whole-round cores were split into working and archive halves. The working halves were placed on the sampling table for the 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 2 cm in diameter, which fits that of the glass vial. To accommodate the large volume of sediment cored and the Rhizon experiments that require MAD data (see “Geochemistry”), one or two MAD samples were usually taken and measured per core (usually at 5–7 cm offset from the top of Sections 2 and 5). These locations were selected to provide meaningful measurements for fluid flow by being close to the interstitial water samples in Sections 1 and 4 in addition to providing a judicious spread of samples at each site. This plan also provided the needed flexibility for Rhizon sampling and for choosing a third sample per core when needed for highly variable sediment such as upper organic-rich sediment. This third sample was chosen to complement the regular sample in Section 5 (i.e., the second sample was taken from a light layer if the regular sample was taken from the dark layer and vice versa) to cover the entire spectrum for these sediments’ physical properties. For Rhizon sampling (see “Geochemistry”), MAD samples were collected in consultation with the Chemistry Laboratory usually at a rate of one sample per section.

Samples were placed in numbered, preweighed 16 mL Wheaton glass vials for wet and dry sediment mass measurements, drying in a convective oven for 24 h, and subject to 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 when properly calibrated and operated. 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. A reference volume (calibration sphere) 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 cruise 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. The procedures for the determination of these physical properties comply with the American Society for Testing and Materials (ASTM) designation (D) 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.

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, respectively,

and

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:

Wet bulk density (ρ_wet), dry bulk density (ρ_dry), sediment grain density (ρ_solid), porosity (ϕ), and void ratio (VR) are calculated as

and

MAD properties reported and plotted in the “Physical properties” sections of all site chapters were calculated with the MADMax shipboard program.

Section Half Measurement Gantry

Shear strength measurement on split cores was performed using the automated vane shear device on the Section Half Measurement Gantry. We routinely measured one analysis per core in Hole A in Section 2 or 5. Measurements were usually taken at ~20 cm offset from top of section; however, if this interval did not provide good sediment (e.g., caused by high amounts of sand or cracks), different positions were chosen to generate viable data. Cores drilled with XCB or RCB generally often did not provide usable data because of bad sediment/liner contact and disturbed sediment, and thus were measured only in rare cases.

Vane shear strength (S_u[v]) can be determined by the torque (T) required to cause failure and a vane constant (K_v):

All vane shear strength measurements were obtained using a vane with a height of 12.7 mm and a blade length of 6.35 mm. Failure torque was determined by measuring the rotation of a torsional spring using a spring-specific relation between rotation angle and torque.

Section Half Multisensor Logger

Spectrophotometry (color reflectance) and magnetic susceptibility of the archive halves were measured with the SHMSL. Spurious results due to empty intervals, small cracks, and sediment disturbances caused by the drilling process and in places where the core surface is well below the level of the core liner were discarded. Additional detailed information about measurement and interpretation of spectral data can be found in Balsam et al. (1997, 1998), Balsam and Damuth (2000), and Giosan et al. (2002).

Color reflectance

Reflectance of visible light from the archive halves of sediment cores was measured using an Ocean Optics USB4000 spectrophotometer mounted on the automated SHMSL using a diffuse light source. Freshly split soft cores were covered with clear plastic wrap (Glad Wrap) and placed on the instrument track. Measurements with a circular footprint of 8 mm were taken at 1–2 cm spacing to provide a high-resolution stratigraphic record of color variations for visible wavelengths. The 1 cm resolution was employed for lithologic Unit I at sites in the marginal sea where this unit was color variegated. Each measurement recorded in 2 nm wide spectral bands from 400 to 900 nm was reported relative to the center of the measurement footprint. For core intervals that were sufficiently dry to run without Glad Wrap, it was noted that the resulting signal was very much cleaner than the same sections run with Glad Wrap covering.

Magnetic susceptibility

Magnetic susceptibility was measured with a Bartington Instruments MS2E point sensor (high-resolution surface scanning sensor) on discrete points along the SHMSL track. Because the SHMSL requires flush contact between the magnetic susceptibility point sensor and the split core, measurements were made on the archive halves of split cores covered with clear plastic wrap (Glad Wrap). Measurements were taken at 1–2 cm spacing (same as the reflectance measurements) and integrate a volume of 10.5 mm × 3.8 mm × 4 mm, where 10.5 mm is the length perpendicular to the core axis, 3.8 mm is the width in the core axis and 4 mm is the depth. For conversion of the instrument units stored in the IODP database, a correction factor (67/80) must be employed to correct for the relation of the sensor diameter and sediment thickness.

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