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doi:10.2204/iodp.proc.338.201.2014

Methods

Particle size analysis was conducted at the University of Kentucky (USA) using the standard hydrometer method for mud-dominated samples (ASTM, 2007). Sawyer et al. (2008) present similar hydrometer analyzes on fine-grained sediments from within and without MTDs in the Ursa Basin.

Principles of hydrometer analysis

Hydrometer analysis is based on Stokes’ law, which defines the terminal velocity of a spherical particle settling through a column of fluid. Stokes’ law assumes all particles are spherical and settle at different rates based on the size of each particle. A hydrometer measures the density of the mixture at a known depth below the surface. The density of the suspension is based on the specific gravity and amount of sediment. The density of the mixture decreases as the particles settle out of the suspension. From the hydrometer readings, calculations provide the maximum particle diameter (D, in mm) at a specific time (Eq. 1) and the percentage of the original sample mass that is smaller than the particle diameter (mm) at the specific time, and thus, still in suspension (Eq. 2) (Germaine and Germaine, 2009). Variables are detailed in Table T2.

(1)

where

• H = distance particle falls (cm),
• ρ_w = mass density of water (g/cm³),
• G_s = specific gravity (dimensionless),
• µ = viscosity of fluid (mPa·s),
• g = acceleration due to gravity (cm/s²), and
• D = Diameter of particle (mm).

Nm = [Gs/(Gs – 1)] × (V/MD) × ρc(rm – rw,m) × 100,

(2)

where

• N_m = percent finer material at reading m (%),
• V = volume of suspension (cm³),
• M_D = dry soil mass of hydrometer specimen (g),
• ρ_c = mass density of water at the calibration temperature (g/cm³),
• 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.

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