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doi:10.2204/iodp.proc.319.203.2015

Methods

To create a depth profile of particle sizes at Site C0009, we analyzed the particle size distribution of a total of 46 samples. The samples were primarily from 10 cm³ plug samples taken shipboard. The wet-sieve and hydrometer techniques were used, which generated particle size distributions for each sample. The procedure used at the University of Texas at Austin (USA) for this study is described below. It is slightly modified from that used in Sawyer et al. (2008). This procedure is internationally recognized as a standard in the American Standard for Testing and Materials (ASTM International, 2007) and in the British Standard Institution (British Standard Institution, 1990). These size distributions were binned into sand, silt, and clay percentages for each sample. We used a clay-silt-sand ternary diagram using the Shepard classification (Shepard, 1954) to classify the samples.

Principles of hydrometer analysis

Germaine and Germaine (2009) discuss hydrometer analysis and the physical principles of sedimentation. The terminal velocity at which spherical particles settle through a column of fluid can be described by Stokes’ law (Craig, 1992). Stoke’s law assumes that particles are (1) rigid, spherical, and smooth; (2) of similar density; (3) separated from each other; (4) do not interact during sedimentation; and (5) are large enough that Brownian motion does not control settlement. This approach is applicable for particle sizes ranging from 0.0002 to 0.1 mm (Germaine and Germaine, 2009). The general approach is to mix the sediment into a suspension and then allow sedimentation while measuring the density of the suspended sediment at a specific depth.

The diameter of the largest particle in suspension (D) is

(1)

where

• D = diameter of the particle (cm),
• µ = viscosity of water (g/[cm·s]),
• G_s = specific gravity of sediment (dimensionless),
• ρ_w = water density (g/cm³),
• g = force of gravity (cm/s²),
• L = distance the particle falls (cm), and
• t = time for fall (s).

The percent finer material (N) at reading m is

(2)

where

• N_m = percent finer material at reading m (%),
• G_s = the specific gravity of sediment (dimensionless),
• V = volume of suspension (mm³),
• M_D = dry solid mass of hydrometer specimen (g),
• R_m = hydrometer reading in suspension at time (t) and temperature, T (dimensionless),
• R_w,m = hydrometer reading in water with dispersant at the same temperature as for R_m (dimensionless), and
• m = reading number.

Samples

We analyzed 46 samples distributed across the interval between 1529 and 1591 mbsf. We analyzed samples with a wet mass between 25 and 45 g because it was determined that a mass <25 g produced inaccurate results.

Sample preparation

Samples were first manually disaggregated using a mortar and pestle. After recording the wet mass, the wet sample was mixed with 5 g of dispersing agent (sodium hexametaphosphate) and ~200 mL of deionized water and allowed to sit for 24–48 h. The mixture was further disaggregated for 1 min using a Hamilton-Beach malt mixer (ASTM International, 2007).

Once the sample was mixed, the slurry was washed through a 63 µm sieve with deionized water and a spatula. Material that was unable to pass through the sieve was dried at 110°C. The sample was then cooled and weighed to determine the percentage of sand for each sample.

The material that passed through the sieve was placed in a 1000 mL plastic cylinder and deionized water was added to create a total volume of 1000 mL. Five to six cylinders were usually tested at one time.

Hydrometer analysis

The prepared suspension was mixed thoroughly with a plunging rod for 1 min. The removal of the plunging rod marked the beginning of the sedimentation process. Two sets of hydrometer readings were obtained for the first 2 min (each at 15, 30, 60, 90, and 120 s) of sedimentation with the hydrometer remaining in the suspension. The hydrometer was then removed, rinsed and wiped dry. Readings were then taken at larger increments of time (4, 8, 16, 32, 64, etc., minutes), with the hydrometer being inserted and removed right before and after the time mark, until the largest particle in solution (Equation 1) was >0.002 m (the clay/silt boundary assumed). The temperature in the laboratory was monitored with a thermometer in a cylinder filled with deionized water and salt. At the end of the experiment, the slurry was poured into an evaporating dish and dried in an oven at 110°C to obtain the final dry mass of sediment and dispersing agent.

The hydrometer has to be calibrated prior to testing to obtain information for three factors: the meniscus rise, the effective reading depth for any particular reading, and the changes in fluid density with temperature and dispersing agent (Germaine and Germaine, 2009). For the effective reading depth (L), two relationships are required: one for situations when the hydrometer remains in the suspension continuously and one for situations when the hydrometer is inserted for the reading (Germaine and Germaine, 2009). For times ≤2 min, the effective reading depth (L) is described by

(3)

where

• L = effective reading depth for situations when the hydrometer remains in the suspension continuously (cm),
• H_r,1 and H_r,2 = dimension between the center of buoyancy and readings R_h,1 and R_h,2 on the hydrometer (cm),
• R_h = hydrometer reading in suspension (g/L) at time (t) and temperature (T), and
• c_mr = meniscus correction in units of specific gravity (dimensionless).

For times <2 min, an immersion correction (V_h/2A) was applied to the readings to account for the fact that the insertion of the hydrometer into the suspension stretches the column of fluid:

(4)

where

• L = effective reading depth for situations when the hydrometer is inserted before individual readings (cm)
• V_h = volume of hydrometer bulb up to the base of the stem (cm³), and
• A = cross-section area of cylinder (cm²).

Grain densities were not measured on the samples that we performed grain size analysis on. Instead, density was estimated from shipboard moisture and density (MAD) measurements. These measurements ranged between 2.42 and 3.01 g/cm³ (Table T2). We used the average of two or three MAD grain density measurements take near the depths of the sample that we performed grain size analysis on.

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