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doi:10.2204/iodp.proc.314315316.207.2011

Methods

Samples were analyzed at the Center for Marine Environmental Sciences (MARUM) in Bremen, Germany, using standard laser diffraction analysis. This method was chosen because we have considerable experience using this technique on fine-grained sediment. The technique is sufficiently versatile to process a large number of samples using an Auto-Prep Station with no minimum particle size restrictions.

Principles of laser diffraction analysis

Laser diffraction size analysis is based on the principle that particles of a given size diffract light through a given angle, the angle increasing with decreasing particle size (see the superb summary by Syvitski, 1991, and references therein). A narrow beam of monochromatic light (λ = 750 nm) is passed through a sample cell containing an upward moving suspension. The diffracted light is focused onto 126 detectors. To calculate the grain-size distribution from the light intensity reaching the array of detectors, two different diffraction theories can be used. We used the Fraunhofer theory because Loizeau et al. (1994) concluded that this detects a significantly larger proportion of the clay measured by the sieve-pipette method than does the Mie theory. The Fraunhofer theory starts from the principle that there is only diffraction and no refraction; this is not entirely correct for organic matter because it may absorb some light. If particles are large compared to the wavelength of the light, the interaction can be interpreted in terms of diffraction (de Boer et al., 1987). The wavelength of the laser light therefore suggests that measurement of the clay fraction may be problematic. The size distribution is measured while the suspension is subject to continuous pumping. This ensures random orientation of most particles relative to the laser beam so that the equivalent spherical cross-sectional diameter is measured.

The Coulter LS-13320 measures grain size contents in 117 classes ranging from 0.04 to 2000 µm as a volume percent (vol%) (see more detailed discussion by Syvitski et al., 1991, and Loizeau et al., 1994). Water, which was degassed before analysis, was used as suspension fluid. Each run took 60 s and was repeated 5 times. The third run is used as statistical representative run for data quality control.

We chose the laser technique over Sedigraph grain size analysis because the latter is not capable of measuring particles <10 µm (e.g., Konert and Vandenberghe, 1997), which in a setting such as the Nankai Trough accretionary prism would pose problems because of the substantial clay fraction. An comparison between the two (and other) techniques is difficult, as has been pointed out by several authors (see Syvitski et al., 1991, and references therein). Definition of accuracy has been attempted in some of these studies, but no coherent conclusion can be drawn from the wealth of different results. However, it is beyond the scope of this paper to elaborate on the various grain size standards available and how they compare when run on different machines, not to mention which of them is best. The main argument for using the Coulter counter laser apparatus was the time efficiency, large range, and very high reproducibility of the measurements (see below).

Samples

Two types of samples were used in this study: XRD and fabric (FAB). FAB samples are normal cluster samples next to the whole-round sample intervals including interstitial water. The whole-round intervals were positioned within intact hemipelagic mud (and/or mudstone) and are leftover material after processing for clay XRD.

Sample preparation and handling

The samples were removed from the plastic sample bag to weigh an aliquot. In order to get a statistically representative subsample it was necessary to remold a larger volume of unconsolidated and partly consolidated material. Approximately 0.15 g of a sample was first disaggregated by boiling with 50 mL degassed water and ~0.1 g of sodium pyrophosphate (Na₄P₂O₇·10H₂O) in a beaker. This agent dissolves at ~80°C and supports the disaggregation of agglomerated particles in suspension (Sun et al., 2002). After boiling, the samples were put in an ultrasonic bath for at least 30 min. The ultrasonic bath is capable of hosting eight beakers at a time. After 30 min, the samples were shaken by hand (using the same beaker) to test if all particles were in suspension. If there were still some aggregates on the bottom of the beaker, the samples were placed again in the ultrasonic bath for 30 min. The samples were taken out of the ultrasonic bath and put aside for at least one day to let the particles settle.

A portion of the sample (~5 mL) was then aspirated to a test tube with a suction device to use in the Auto-Prep Station. The wheel inside this station can handle 30 samples in one batch.

Grain size was measured with a Coulter LS200 counter on bulk material, resolving grain size spectra between 0.4 and 2000 µm.

In order to check for accuracy and comparability of results to data obtained with different machines in other laboratories, we regularly run grain standards by Retsch and Coulter.

Data acquisition, processing, and statistics

The Beckman Coulter LS program used for the measurements offers an interface to set up batches and automatically saves the produced data in separate .$LS files for each run. Every third run was manually copied into an Excel file calculate statistics in quasi-real time. Mean grain sizes were calculated with GRADISTAT software (Blott and Pye, 2001) after the Folk and Ward (1957) method (i.e., M being an average value taking into account the grain sizes at the 16th, 50th, and 84th percentiles). Sorting (i.e., the standard deviation) and skewness, used as measures for the width and symmetry in the 117 grain size classes, were also calculated based on the formulas given in Folk and Ward (1957).

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