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Bulk density corrections

Bulk density is defined as the mass of a sample divided by the volume of the sample:

ρB = (mass/volume) = [(msolids + mliquids + mair)/(vsolids + vliquids + vair)]. (1)

During ACEX, mass and volume were measured on subsamples from the recovered core and used for direct determinations of ρB. At sea, these MAD measurements were made on samples from core catchers (one per core) and performed on subsamples from split cores during the onshore phase of ACEX (~1 per section) (see the “Sites M0001–M0004” chapter). In the correction procedure outlined here, only shore-based measurements on split-cores are used. In addition to direct determinations of bulk density, MAD samples provide measurements of grain density (ρG), dry density (ρD), and porosity (ϕ) (see the “Sites M0001–M0004” chapter). These index properties are defined as

ρG = (msolids/vsolids), (2)
ρD = (msolids/vtotal), and (3)
ϕ = (vpore-water/vtotal). (4)

The MSCL also provides an estimate of sediment ρB and during ACEX was run at a sampling resolution of 2 cm on all visibly intact cores (see the “Sites M0001–M0004” chapter). The advantage of the MSCL is that it provides quick, nondestructive, high-resolution measurements. However, MSCL ρB is not a direct measurement and is derived from empirical relationships. The MSCL uses a 137Cs source to pass gamma rays through the core. Bulk density is estimated by measuring the attenuation of gamma rays (primarily through Compton scattering). The degree of attenuation is proportional to density and is dependent on the Compton mass absorption coefficient of the sample. Calibration of the system uses known seawater/aluminum standards and assumes that the attenuation coefficient of the sediments is proportional to that of aluminum. This assumption is largely valid for most siliclastic sediments (Blum, 1997; Boyce, 1976). Hence, the quality of MSCL data is a function of core quality, the accuracy of the calibration, and the similarity of the Compton mass absorption coefficient of the sample to that of aluminum. Variations in any of these parameters can introduce errors into MSCL-derived ρB. To account for this variability, MAD ρB is used to derive a correction factor for MSCL ρB. Correction factors are determined separately for each core, accounting for intercore variability in the quality of MSCL data.

MAD data from onshore analyses, where a Quantachrome pentapycnometer (helium-displacement pycnometer) was used to determine the volume of the subsamples, are compared to MSCL bulk density data from equivalent depths in each core.

Only samples that have a corresponding MSCL measurement from within 2 cm are used for determining the correction factors (cf) defined as:

cf = ρB-MSCLB-MAD. (5)

Where more than a single MAD measurement exists for any core, the average correction factor for that core is calculated. If only a single MAD measurement was available, then the average correction factor for the lithologic unit/subunit is applied. This approach recognizes lithology-dependent variability in both core quality and Compton mass attenuation coefficients. MAD measurements and equivalent MSCL ρB data are given in Table T1. Correction factors for each core are listed in Table T2, and the average correction factor and grain density for lithologic units/subunits are shown in Table T3.

Dry density and porosity

Similar to MAD data, where mass and volume measurements are used to calculate ϕ and ρD, basic phase relationships can be used to derive these variables from the corrected high-resolution MSCL ρB records. The phase relationships require that ρG is known. For siliclastic sediments a ρG of 2.70–2.75 g/cm3 is often assumed. Instead of assuming the ρG, the average value is taken from MAD measurements in the corresponding lithologic unit/subunit (Table T3). More variable grain densities in lithologic Units 1.6 and 3 are attributed to the widespread occurrence of authigenic minerals such as pyrite (see the “Sites M0001–M0004” chapter; Stein et al., 2006). For lithologic Unit 3, a relatively high standard deviation is observed for the average grain density (ρG = 2.54 ± 0.14 g/cm3), which can be reduced by subdividing the unit into petrophysical Subunits 3a and 3b. The subdivision is placed between Cores 302-M0004A-23X and 27X and results in a mean ρG for petrophysical Subunit 3a of 2.43 ± 0.06 g/cm3 and for Subunit 3b of 2.67 ± 0.08 g/cm3 (Table T3; Fig. F5). This break occurs during a prolonged interval of no recovery (Fig. F2), preventing an exact depth to be associated with the division or determination of whether the break is abrupt or gradual.

Porosity and dry density are determined from the corrected ρB using the phase relationships:

ϕ = (ρG – ρB)/(ρG – ρF), and (6)
ρD = [(ρB – ρF)/(ρG – 1)]ρG, (7)

where ρG is the average grain density from the corresponding lithologic unit/subunit (g/cm3), ρB is the corrected MSCL bulk density (g/cm3), ρD is the dry density (g/cm3), ϕ is the porosity (unitless), and ρF is an assumed pore fluid density of 1.024 g/cm3.