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doi:10.2204/iodp.proc.322.211.2015

Methods and materials

Sampling and sample handling

The whole-round (WR) specimens that we tested from Sites C0011 and C0012 consist of hemipelagic mud and mudstone with varying degrees of consolidation. Sample depths range from 36 to 748 meters below seafloor (mbsf), with the distribution covering lithostratigraphic Units I through IV (Table T1). The dips of bedding surfaces relative to the horizontal plane (Fig. F2) are as steep as 63°, as shown by nearby measurements on split core (see the “Site C0011” and “Site C0012” chapters [Expedition 322 Scientists, 2010b, 2010c] and the “Site C0011” and “Site C0012” chapters [Expedition 333 Scientists, 2012b, 2012c]). The WR samples were capped and taped in their plastic core liners on board the D/V Chikyu, sealed with wet sponges in aluminum vacuum bags to retard moisture loss, and stored at ~4°C until immediately prior to trimming.

To extract each specimen, the plastic core liner was cut lengthwise along two lines 180° apart using a hacksaw. Cylinders for tests in the vertical (along-core) direction were trimmed using a wire saw and soil lathe. The length after trimming was 3.15–7.11 cm and averaged 5.78 cm (Table T1), as measured by caliper to a resolution of 0.03 cm. The diameter was 3.67–6.54 cm and averaged 4.31 cm. Whenever possible, a second cylinder for tests in the horizontal (cross-core) direction was trimmed to comparable dimensions from material immediately below the cylinder for vertical flow. Unfortunately, most of the deeper WR specimens that were recovered by rotary core barrel (RCB) during Expedition 322 broke while trimming because of ubiquitous drilling-induced microcracks (Table T1). In other cases, the intact material was too sparse to trim two cylinders. One specimen (322-C0012A-17R-2) cracked during the test, which compromised results. Two samples (322-C0011B-3R-4 and 322-C0012A-8R-3) could not be trimmed for a horizontal flow test because of fracturing. During Expedition 333, the hydraulic piston coring system (HPCS) and the extended punch coring system (EPCS) were deployed at shallower depths, which resulted in much better core quality. Sample 333-C0011D-32X-6, however, was too friable to trim.

For most samples, we calculated values of initial (pretest) and post-test porosity from measurements of gravimetric water content (Table T1). This was done by oven drying the trimmings at 105°C in accordance with shipboard protocols (see the “Methods” chapter [Expedition 322 Scientists, 2010a] and the “Methods” chapter [Expedition 333 Scientists, 2012a]) and by assuming 100% pore water saturation. Values of grain density were imported from shipboard measurements of the closest adjacent specimen (see the “Site C0011” and “Site C0012” chapters [Expedition 322 Scientists, 2010b, 2010c] and the “Site C0011” and “Site C0012” chapters [Expedition 333 Scientists, 2012b, 2012c]). A correction for pore water salt content was applied using

where

• W_c = corrected dry weight,
• M_t = total mass of saturated specimen,
• M_d = mass of dried specimen, and
• r = salinity (permil).

For salt corrections on pretest trimmings, we assumed an average interstitial salinity value of 35‰; for post-test trimmings, we assumed a value of 25‰ to approximate the simulated seawater that was used to saturate specimens.

Constant-flow apparatus

Yue et al. (2012) provides a thorough description of the instrumentation and testing procedures for permeability at the University of Missouri (USA). To summarize, the system consists of an acrylic confining cell, a constant-flow syringe pump, one differential pressure transducer to measure hydraulic head difference between the specimen top cap and bottom cap, and an air/water interface panel for regulating the confining fluid pressure and backpressure (Fig. F3). Signals from the differential pressure transducer permit calculations of hydraulic head difference (Δh) at a precision of ±1 cm H₂O over a range of ±1000 cm H₂O. A digital interface also records values of effective isotropic confining stress (σ′) and elapsed time. A syringe pump (KDS Scientific, Model 260) simultaneously injects and extracts pore fluid from opposite ends of the specimen. The flow pump holds one syringe (Hamilton GasTight Series 1000) to infuse pore fluid into one end, and another syringe withdraws an equal volume of fluid from the other end at the same rate. During the tests described here, volumetric flow rate (Q) ranged from 3.0 × 10^–5 cm³/min to 1.0 × 10^–3 cm³/min.

Backpressure saturation

Prior to our tests, all permeant lines and porous stones are saturated with simulated seawater (25 g NaCl to 1 L tap water). After placing a specimen on the pedestal, the top cap is attached, and a latex membrane is added to encase the cylinder using a vacuum membrane expander. The confining chamber is then sealed, and the cell is filled with tap water. Saturation is achieved by ramping pore-fluid backpressure to 0.48 MPa (70 psi) using the panel board (air/water interface) while also ramping the confining pressure to maintain an effective isotropic confining stress of 0.034 MPa (5 psi). The elevated backpressure is maintained for at least 24 h. We confirm saturation by increasing the confining pressure to 0.55 MPa (80 psi) and measuring the corresponding pore pressure (u) response, which yields Skempton’s B-value (B = Δu/Δσ). Specimens are “saturated” if B ≥ 0.95 (Table T1) or if a B-value < 0.95 remains constant for more than 48 h. After saturation, the cell pressure is increased to consolidate the specimen at an isotropic effective stress of 0.55 MPa. Pore water drains during consolidation from both the top and bottom of the specimen by opening valves on the confining cell. The volume of expelled pore water is measured using the backpressure pipette and monitored for equilibrium to calculate the corresponding volume change of the specimen.

Constant-flow tests

We completed 10 successful constant-flow tests on cylinders trimmed parallel to the core axis and 8 tests on cylinders trimmed perpendicular to the core axis (Table T1). The ideal protocol consists of two tests from top to bottom (denoted subsequently as a negative flow value) and two tests bottom to top (denoted as a positive flow value) (Fig. F4). We monitor the transient response from the differential pressure transducer. Unfortunately, the pressure transducer behaved erratically during some of the tests, and after the necessary repairs were completed, those specimens had degraded too much to yield reliable replicate tests (Table T2). Plots of applied discharge velocity (v) and steady-state hydraulic gradient (i_s) allow for visual assessments of consistency and linearity (see the “Appendix”). For tests to be regarded as reliable, the coefficient of determination (R²), calculated by least-squares linear regression of those values, must be >0.9835.

Data analysis

We calculated the value of hydraulic conductivity (K, in units of m/s) for each specimen using Darcy’s law:

where

• Q = applied volumetric flow rate (cm³/s),
• i_s = steady-state hydraulic gradient,
• Δh_s = steady-state head difference,
• ΔL = length over which head difference occurs (initial height of the specimen), and
• A = cross-sectional flow area (initial specimen area).

The corresponding value of discharge velocity is computed using: v = Q/A. Conversion of hydraulic conductivity to values of intrinsic permeability (k, in units of m²) takes the permeant properties into account:

where

• µ = viscosity of permeant at room temperature (0.001 Pa·s),
• ρ = density of permeant (1027 kg/m³), and
• g = gravitational acceleration (9.81 m/s²).

Imaging microfabric

Yue et al. (2012) provides a thorough description of the procedures used at the University of Missouri (USA) to image and characterize microfabric. To summarize, a razor blade is used to cut oriented specimens while trimming cylinders for the vertical and horizontal flow-through tests (Fig. F5). Wet, uncoated, and unfixed surfaces are imaged using an FEI Quanta 600 FEG scanning electron microscope (SEM), which operates in environmental mode (ESEM) at 30 kV with the specimen chamber pressure set at 700 Pa. Water vapor (~98% humidity) from a built-in reservoir keeps specimens from losing moisture with the cooling stage set to 2°C. We use a gaseous backscattered electron detector, spot = 3.0, and a working distance of ~10 mm. This combination generates an imaging resolution of ~4 nm, and dimensions of the field of view are ~145 × 130 µm at 2000× magnification (Fig. F6).

The gray mode TIF images from ESEM are processed using “ImageJ” software (rsbweb.nih.gov/ij/index.html), which isolates the apparent dimensions of objects in a 2-D image. We generally count between 100 and 500 grains per image (depending on particle size) to calculate statistics for preferred grain orientation (Table T3). Each particle orientation (azimuth of the apparent long axis) is assigned to an angle between 0° and 180°. For the vertical cut surface, the core axis is oriented at 90°. Rose diagrams were constructed using “Rozeta” software (www.softpedia.com/get/Science-CAD/Rozeta.shtml), which automatically assigns azimuths to bins at 10° intervals. Cumulative frequency curves are then used to obtain graphical solutions of standard deviation (d) according to Folk and Ward (1957) statistics:

where ϕ₈₄, ϕ₁₆, ϕ₉₅, and ϕ₅ represent the azimuth (in degrees) at the eighty-fourth, sixteenth, ninety-fifth, and fifth percentiles. In this context, the largest possible value of d is 72.3° (i.e., a case in which ϕ₁₆ and ϕ₅ = 0° and ϕ₈₄ and ϕ₉₅ = 180°). To compare each standard deviation to this maximum d value we calculate the “index of microfabric orientation”:

With random arrangements of particles, the cumulative curve is relatively flat (slope < 0.75) near the median, the value of d is >35°, and i is <0.5. As particles attain better parallel alignment, azimuths cluster more tightly, the slope of the cumulative curve steepens (slope > 1.00) near the median, the value of d is <25°, and i is >0.65 (Yue et al., 2012). The ratio of i_h to i_v permits quantitative comparison of imaging surfaces that were cut parallel and perpendicular to the core axis and allows for numerical comparisons with the anisotropy of permeability.

We tried a second method to quantify microfabric more rigorously by calculating the mean angle (α) of the angular distribution with 180°-periodic circular statistics (Fisher, 1995; Berens, 2009). First, the mean resultant vector is calculated from the summation of unit vectors representing particle orientation. The distribution of particle orientation is 180°-periodic, so the angle of the unit vector is twice the angle measured on the particle and, hence, the mean angle α is half the angle of the mean resultant vector. The resultant vector length R is between 0 and 1 and constitutes a proxy for the directionality of the distribution; a value of 0 coincides with a uniform angular distribution, and a perfectly aligned distribution results in a value of 1. The probability that data reflect a uniform distribution of orientation (p-value) is then estimated from the Rayleigh test (Fisher, 1995; Berens, 2009). The mean angle α is considered significant only if the p-value is less than 0.05. Use of this statistical approach appears generally optimistic because few images display a clear fabric based upon visual assessment. We recognized weak preferred orientations on vertical sections of two samples: 333-C0012D-9H-3, 120 cm, which displays subhorizontal bedding and a mean angle of 166° (p = 0.00341), and 333-C0012C-9H-7, 51 cm, which displays steeper bedding dips and a mean angle of 53° (p = 0.0431).

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