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doi:10.2204/iodp.proc.307.102.2006

Physical properties

Physical properties were measured on whole-round cores and on undisturbed portions of split cores. The MST was used on whole cores for nondestructive measurements of wet bulk density, compressional wave velocity, magnetic susceptibility, and natural gamma radiation. Magnetic susceptibility was also measured on the Fast Track MSCL before the cores were equilibrated to room temperature to allow fast stratigraphic correlation and composite depth construction (see “Composite depths”) Thermal conductivity measurements were also made on whole-round cores after the cores were thermally equilibrated. Undrained shear strength, moisture and density (MAD), and compressional wave velocity (V_P) were measured at discrete intervals on split cores, usually at a frequency of two or three per section. Figure F7 shows the sequence of physical property measurements made during Expedition 307, and Table T5 lists the average sampling intervals for each of the physical property data sets collected.

Physical property measurements gathered during Expedition 307 were used to obtain (1) high-resolution records for hole to hole correlation, construction of complete stratigraphic sequences, and downhole log calibration; (2) information related to sediment composition, diagenesis, and consolidation history to help constrain the location of unconformities; and (3) data for the calculation of synthetic seismograms (i.e., compressional wave velocity and bulk density) and for the calculation of local heat flow (i.e., thermal conductivity).

Shipboard measurements

The Fast Track MSCL measurements were run shortly after cutting the whole-round cores on the catwalk to start the stratigraphic correlation of the different holes. To ensure thermal homogeneity for all other physical property measurements, data were collected after equilibrating the cores to ambient room temperature (17°–25°C). Detailed information on the physical principles underlying the sampling methods discussed here can be found in Blum (1997).

Fast Track MSCL

The Oregon State University Fast Track MSCL for measuring magnetic susceptibility on cores as soon as possible following recovery was first introduced during ODP Leg 202. During Expedition 307, we used the IODP Fast Track MSCL system that contains two magnetic susceptibility loops on a single track to speed up analysis time. This helped us make drilling adjustments aimed at ensuring the recovery of a complete stratigraphic section while allowing us to run the MST to optimize data quality. For a number of critical sections designated for microbiological sampling, sections were first run through the Fast Track MSCL (immediately after cutting on the catwalk) and then sent to the cool room for microbiological subsampling. This procedure ensured that at least some stratigraphic data was obtained form these cores.

Multisensor track

The MST consists of an automated track that moves whole-core sections through sensors measuring magnetic susceptibility, GRA bulk density, P-wave velocity (P-wave logger [PWL]), and natural gamma radiation (NGR). Approximately one whole-round per core section from Holes U1316B, U1316C, U1317A, U1317D, and U1318B was dedicated to microbiological analysis and was not run through the MST.

Magnetic susceptibility, GRA density, and NGR were measured on all cores regardless of collection method (i.e., APC, XCB, or RCB). P-wave velocities were measured only on the upper APC intervals because loss of coupling between the liner and core with XCB and RCB drilling resulted in inaccurate values.

P-wave velocity was measured at 5 cm intervals (5 times at 1 s period) using a 500 kHz compressional wave pulse at a repetition rate of 1 kHz. The transmitting and receiving transducers were aligned horizontally, perpendicular to the core axis. A pair of displacement transducers monitored the separation between the compressional wave transducers. Sediments must completely fill the liner for the PWL to provide accurate results. PWL measurements were inaccurate at Site U1317 due to high coral content in a muddy matrix and drilling with XCB and RCB of the lower consolidated sediments, which created insufficient contact between the sediments, the core liner, and the transducers.

Magnetic susceptibility was measured using a Bartington Model MS-2 meter with an 80 mm internal diameter sensor loop (88 mm coil diameter) operating at a frequency of 565 Hz and an alternating field of 80 A/m (0.1 mT) with the sensitivity range set to 1.0 Hz. The sampling interval was 5 cm, with a period of 5 times at 1 s. The long sampling period ensured acceptable readings for the usually low magnetic susceptibility of carbonate sediments. The MS-2 meter measures relative susceptibilities, which need to be corrected for volume variations. For core (d) and coil (D) diameters of 66 and 88 mm, respectively, the corresponding correction factor for d/D is 1.48 (Blum, 1997, p. 38). During data reduction, the relative susceptibility is converted to the volume-normalized magnetic susceptibility by multiplying by 1/(1.48 × 10⁵), or by 0.68 × 10^–5 SI units.

Natural gamma radiation is a product of the decay of radioactive isotopes, predominantly U, Th, and K. NGR was measured using four scintillation detectors arranged at 90° to each other and perpendicular to the core (as outlined by Hoppie et al., 1994). During Expedition 307, NGR was measured every 5 cm for a period of 5 times at 1 s. NGR calibration was performed at the beginning of the expedition. For the interval at the top of the hole in which pipe remained during downhole logging, the data can be used to complete and correct for the attenuation of the gamma ray wireline log collected through pipe. In open-hole logging sections, the wireline logging data could be used to calibrate the core data.

GRA was used to estimate sediment bulk density. This measurement is based on the principle that the ¹³⁷Cs attenuation, mainly by Compton scattering, of a collimated beam of gamma rays (produced by a ¹³⁷Cs source) passing through a known volume of sediment is related to material density (Boyce, 1976). During Expedition 307, the measurement interval was set at 5 cm (5 times at 1 s period). For each site, GRA and discrete sample bulk densities were compared for consistency.

Thermal conductivity

Thermal conductivity during Expedition 307 was measured using the needle probe technique with the TK04 system as described by Blum (1997). The single-needle probe heated continuously in “full-space configuration” for soft sediments and in “half-space configuration” for hard rock. The probe was heated at 3 W/m and the temperature rise monitored. The optimal integration time for each conductivity measurement is calculated by an algorithm in the TK04 system for time units of 150 s and an evaluation time of 240 s. Thermal conductivity was reported in units of Watts per meter degrees Kelvin (W/[m·K]), with an accuracy of 5% and a precision of 5%. Data were collected once per core (usually Section 3). For whole cores, the probe was inserted through an aperture drilled in the core liner at mid-depth in the section. Half-core rock specimens were measured for thermal conductivity using the half-space configuration. The needle probe was secured onto the flat surface of the half-core. Good coupling with the needle probes was ensured by flattening and smoothing the core surface with carbide-grit sandpaper. The samples and needles were allowed to equilibrate in a cool box. We applied this configuration for six samples from Hole U1316C (Cores 307-U1316C-5R through 9R and 11R).

Moisture and density properties

MAD measurements (water content, wet and dry bulk density, and grain density) were routinely measured using ~10 cm³ samples from split cores. Porosity and void ratio were calculated from phase-relation equations. Samples for MAD measurements were collected at a frequency of two per section (usually at 25 and 100 cm from the top of the section), taking care to sample undisturbed parts of the core and avoid drilling slurry and biscuits. Sampling frequency was increased as needed to characterize all significant lithologies.

Immediately after samples were collected, wet sediment mass (M_t) was measured. Samples were then placed in a convection oven for 24 h at a temperature of 105° ± 5°C. After drying, dry sediment mass (M_d) and dry sediment volume (V_d) were measured. Sample mass was determined on board to a precision of ±0.01 g using two Scientech 202 electronic balances to compensate for the ship’s motion. Volumes were determined using a helium five-chambered pycnometer with an approximate precision of ±0.02 cm³. The determination of water content followed the methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM International, 1989). The recommended equation for the water content calculation, which is the ratio of the pore fluid mass to the dry sediment mass (weight percent), is as follows:

where

• W_c = water content reported as a decimal ratio of percent dry weight, and
• r = salinity.

Wet bulk density (ρ) is the density of the total sample, including pore fluid. In high-porosity sediment, bulk density was calculated using the following:

where V_t is the total sample volume (~10 cm³).

Porosity (ϕ) was calculated using the following equation:

where ρ_w is the density of the pore fluid (assuming a salinity of 35‰).

The grain density (ρ_grain) was calculated from dry mass and dry volume. Both values were corrected for salt using the following equation:

where

• s = salt correction and
• ρ_salt = density of salt (2.257 g/cm³, assuming a salinity of 35‰).

Dry density (ρ_d) is the ratio of M_d to V_d and is used for calculations of mass accumulation. Dry density was calculated using the following equation:

During Expedition 307, GRA densitometer measurements on unconsolidated sediments were commonly higher than discrete density measurements. In addition, in low-porosity sediments, GRA density was usually lower, by as much as 5%, than discrete density measurements. Three explanations for these differences have been proposed (Shipboard Scientific Party, 1997):

1. The MST software does not include a correction for the attenuation effect in high-porosity sediments (Boyce, 1976; Lloyd and Moran, 1992).
2. Air trapped in the sediment-filled beakers (unconsolidated sediments) reduces the relative saturated weight and increases the relative volume measured in the pycnometer, thereby decreasing the resulting bulk density.
3. Low-porosity sediments which are semilithified to lithified have a smaller core diameter and subsequently a relatively smaller attenuating volume than the calibrated volume, which results in a lower calculated density.

To solve the first problem, GRA densities were corrected using the Boyce (1976) equation:

where

• ρ = corrected density,
• ρ_bc = GRA density,
• ρ_fc = fluid density calculated from gamma counts (1.128 g/cm³),
• ρ_g = true grain density of quartz (2.65 g/cm³),
• ρ_f = true fluid density (1.024 g/cm³), and
• ρ_gc = grain density calculated from gamma counts (2.65 g/cm³).

It is unclear how to improve the accuracy of the MAD procedure. Therefore, it is assumed that discrete measurements are more accurate, whereas GRA density gives a reliable high-resolution relative density trend.

Sonic velocity

P-wave velocity in sediments was measured on the split core (P-wave sensor 3 [PWS3]; x-direction) using vertically oriented transducer pairs (500 Hz) with the upper transducer pressed against the split surface and the lower pressed against the core liner. In case of insufficient contact between the liner and the sediments, the PWS3 was applied directly on the sediment without liner. These data were recorded, digitized, and transferred to the Janus database. Core thickness was measured using a digital caliper that was directly mounted on the transducer pair. The velocity transducers were calibrated using a series of polycarbonate standards of known length. The axial pressure applied between sample and transducer was monitored by a pressure cell. To improve the coupling between transducer and sample, distilled water was applied to the transducer head. Measurements were corrected for the additional traveltime passing through the core liner. Three PWS3 measurements were usually made per section.

Undrained shear strength

The peak undrained shear strength of the unconsolidated sediment was measured at an interval of three per section using a manual Torvane shear apparatus (strength: 1 rev = 1 kg/cm²) following the procedures of Boyce (1977). In the interpretation of shear vane measurements, a cylinder of sediment is assumed to be uniformly sheared around the axis of the vane in an undrained condition with cohesion as the principal contributor to shear strength. Departures from this assumption include progressive cracking within and outside of the failing specimen, uplift of the failing core cylinder, drainage of local pore pressures, and stick-slip behavior. The shear strength measurements were taken as close as possible to the locations of the PWS3 measurements to facilitate the correlation between these parameters.

In situ temperature measurements

In situ temperature measurements were made using an APC temperature (APCT) tool. The APCT tool fits directly into the coring shoe of the APC and consists of a battery pack, data logger, and a platinum resistance-temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool was first briefly stopped at the mudline to thermally equilibrate with bottom water. After the APC penetrated the sediment, it was held in place for 10 min as the APCT instrument recorded the temperature of the cutting shoe every 5 s. Initially, there was an instantaneous temperature rise from frictional heating caused by APC penetration. This heat, gradually dissipating into the surrounding sediments, and the equilibrium temperature of the sediments was then estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). Additional information on the APCT tool can be found in previous Initial Reports volumes (Shipboard Scientific Party, 1992, 1994).

For shallow-water sites, a longer mudline stop was required to ensure that the temperature tools had sufficient time to equilibrate to bottom water temperatures. At deeper sites, this time was reduced as the tools are able to thermally equilibrate during descent through deeper waters with very low thermal gradients. The synthetic thermal decay curve for the APCT tool is a function of the geometry and thermal properties of the probe and the sediments (Bullard, 1954; Horai and von Herzen, 1985). However, it is never possible to obtain a perfect match between the synthetic curves and the data because (1) the probe never reaches thermal equilibrium during the penetration period; (2) contrary to theory, the frictional pulse upon insertion is never instantaneous; and (3) temperature data are sampled at discrete intervals, meaning that the exact time of penetration is always uncertain. Thus, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data. The data collected >20–50 s after penetration usually provide a reliable estimate of equilibrium temperature. However, where the APC has not achieved a full stroke, leakage of drilling fluid into the formation may occur and results are not considered reliable.

Composite depths

Core recovery from a single hole is generally insufficient to obtain a complete geologic section because of core-recovery gaps between successive APC, XCB, and RCB cores, despite the often apparent 100% or more nominal recovery (Ruddiman et al., 1987; Hagelberg et al., 1995). To maximize recovery of complete geologic sections during Expedition 307, multiple holes were drilled at all sites. The degree of continuity of the recovered cores at each site was assessed by development of composite depth sections using the Splicer software, following the general methodology first used during Leg 138 (Hagelberg et al., 1992). Similar methods were used during Legs 154 (Curry, Shackleton, Richter, et al., 1995), 162 (Jansen, Raymo, Blum, et al., 1996), 167 (Lyle, Koizumi, Richter, et al., 1997), 177 (Gersonde, Hodell, Blum, et al., 1999), and Expedition 303 (Shipboard Scientific Party, 2005). This section describes the methods used to produce composite and spliced sections during Expedition 307 using Splicer software. At on-mound Site U1317, real changes in the thicknesses of stratigraphic units precluded sensible construction of a composite scale over very short distances between holes.

Composite section development

The assembly and verification of a complete composite stratigraphic section is a two-step process that requires the construction of a composite depth scale, followed by splicing.

Composite depth scale

Cores from the various holes must first be stratigraphically correlated and depth-shifted relative to each other. Such correlation enables development of a mcd scale. The mcd scale differs from the traditional (hole specific) mbsf depth scale. mbsf is based on the length the drill string (advanced on a core-by-core basis) and is often inaccurate because of ship heave (which is not compensated in APC coring), tidal variations in sea level, and other sources of error. Depths of samples/​measurements taken within the cores/​sections are then constructed based on the mbsf depth of the respective core top and the cumulative length of the sections in that core. This method of depth calculation may result in apparent overlap of cores and apparent “stratigraphic reversals” because of sediment expansion in the cores (typically 5%–15%). In contrast, the mcd scale is built by assuming that the uppermost sediment (commonly referred to as the mudline) in the first core from a given hole is the sediment/​water interface. This core becomes the “anchor” in the composite depth scale and is typically the only one in which depths are the same on both the mbsf and mcd scales. From this anchor, core logging data are correlated among holes downsection. For each core, a depth offset (a constant) that best aligns the observed lithologic variations to the equivalent cores in adjacent holes is added to the mbsf depth in sequence down the holes. Depth offsets are often chosen to optimize correlation of specific features in cores from adjacent holes.

For Expedition 307, the mcd scale and the splice are based on the stratigraphic correlation of the physical properties and biostratigraphic and lithostratigraphic data as described above. Such correlation of events, involving alignment of data present in multiple holes, provides verification of the extent of recovery of the sedimentary section. The data sets were extracted from the ship’s Janus database and converted into the correct format using the Splicer Integrator (version 303.0) and then imported into the Splicer software package (version 2.1). Splicer is an interactive, UNIX-based software package designed by Peter DeMenocal, Ann Esmay, and Suzanne O’Hara at Lamont-Doherty Earth Observatory (LDEO) specifically for IODP stratigraphic correlation purposes.

The data from Expedition 307 were culled to avoid the use of anomalous values resulting from voids, edge effects at section boundaries, and disturbed intervals in the cores. Magnetic susceptibility peaks provide the best lithologic parameter for correlation of Site U1316. Magnetic susceptibility (from MSCL or MST), natural gamma ray emission, and GRA density are useful parameters for correlation.

Splicer software allows for direct graphical and statistical comparison of data from each hole. Tie lines are drawn between correlative features present in the data (data excursions, peaks, troughs, and plateaus). The software calculates the statistical correlation over an adjustable depth range (typically ±2 m), to aid the visual correlation process. No depth adjustments (stretching or squeezing) are made. Because depth intervals within cores are not squeezed or stretched by Splicer, not all correlative features can be aligned exactly. Stretching or squeezing between cores from different holes may reflect small-scale differences in sedimentation and/or distortion caused by the coring and archiving processes. The tops of APC cores are generally stretched and the bottoms compressed, although this is dependent on lithology. In addition, sediment (especially unconsolidated mud, sand, gravel, and coral pieces at Site U1317) occasionally falls from higher levels in the borehole onto the tops of cores as they are recovered, and, as a result, the uppermost 20–40 cm of many cores is not reliable. Where possible, tie points were chosen in the middle to lower portion of cores, where the record is likely to have been least disturbed by expansion or other erroneous effects. Utilization of at least two different physical properties allows hole to hole correlations to be made with greater confidence than would be possible with only a single parameter. Core photographs and VCDs were also a useful reference source for identifying potentially correlative lithologic features within cores. Where overlapping data from other holes were unavailable, causing data gaps in the total section, the depth adjustment applied was the cumulative offset from the overlying aligned cores. The resulting adjusted depth scale is called the mcd, and the section produced by the aligned cores is termed a composite depth section.

All adjustments to the data were written to a data output file. The offset column allows conversion of sample depths from mbsf to mcd, effectively creating a sampling strategy guide. The mcd for any point within a core equals the mbsf depth plus the offset. A table is presented in each site chapter summarizing core offsets for conversion from mbsf to mcd scales.

Splicing

Splicer software allows the user to merge, or splice, the best data from the composite section to produce a single spliced record representing the complete geologic section at each site. The spliced record is constructed by patching the intervals missing in a single hole with data from adjacent holes. This process provides a single representative record of the physical properties parameters (e.g., magnetic susceptibility, spectral reflectance, or GRA bulk density) for the entire section, which is ideally suited to guide core sampling for high-resolution paleoenvironmental studies.

Splice tie points were made between adjacent holes where visually obvious features are strongly correlated. The choice of tie points (and hence of a splice) is a somewhat subjective exercise. Our method in the construction of a splice followed these three rules:

1. Where possible, avoid using the uppermost and lowermost 1 m of cores, where disturbance resulting from drilling artifacts (even if not apparent in core logging data) is most likely.
2. Attempt to incorporate those portions of the recovered core that are most representative of the overall stratigraphic section of the site.
3. Minimize tie points to simplify sampling.

The splice operation is depth-constrained so that no further core offset is possible. Because of core expansion and/or compression, the total length of the spliced record depends on which intervals of core are selected to construct it. Each splice is constructed by beginning at the mudline at the top of the composite section and working downward. As in the composite section construction, no compression or expansion of the data are possible. Adjustments to the composite or spliced sections, such as a linear compression of the mcd scale within individual core intervals, are required to align all features exactly (e.g., Hagelberg et al., 1995).

Depths in splice tables versus Janus depths

The depth of a core interval recorded for a tie point in a splice table is not always the same as the depth for the same core interval returned by most database queries. This is because the tie point depth is based on the liner length, which is measured when the cores are cut into sections on the catwalk. The cores are analyzed on the MST almost immediately after this liner-length measurement. At some later time, typically 10–36 h after being analyzed by the MST, core sections are split and analyzed further (see “Core handling and analysis”). At this time, the section lengths are measured again and are archived as “curated lengths.” General database queries return depths based on the curated liner lengths. Because the sections are usually expanding during the period between the two measurements, the curated length is almost always longer than the initial liner length. Thus, the depths associated with the MST data used to construct the splice table are not identical to the final depths the database assigns to a given interval. This leads to small differences (usually between 0 and 5 cm) between the mbsf and mcd recorded in a splice table and the depths reported in other places for the same core interval. We have chosen not to change these depths to be compatible with Janus because this would not improve their accuracy. For consistency, we recommend that all postcruise depth models use or build on mcd values provided in the Janus database.

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