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doi:10.2204/iodp.proc.329.201.2014

Methods

The particle size data presented in this report were measured on a Micromeritics Sedigraph 5100 at the Paleoceanography laboratory of the School of Earth and Ocean Sciences at Cardiff University (United Kingdom). The Sedigraph technique is described in detail in Coakley and Syvitski (1991); further discussions of the operations of the instrument are provided by Stein (1985), Jones et al. (1988), and Bianchi et al. (1999). The Sedigraph method assumes that particles are dispersed in a fluid and settle according to Stoke’s law. The Sedigraph determines the particle size distribution by the measurement of settling velocity and the mass fraction by relative absorption of low-energy X-ray. No specific sediment concentration is required for the analysis, providing the dispersed sample reduces the radiation beam by 40%–60%. This technique provides a rapid and accurate method (instrumental error <1%) for particle size analyses (e.g., Stein, 1985; Bianchi et al., 1999). The data are presented as a cumulative mass percent distribution in terms of equivalent spherical diameter. We used the Sedigraph because it is capable of sensing the total amount of material present and gives the whole size spectrum with satisfactory resolution >1.5 µm.

Samples were collected and freeze-dried on board the JOIDES Resolution. Further sample preparation occurred at Cardiff University. Prior to particle size analysis, samples were disaggregated in 0.2% Calgon (sodium hexametaphosphate) dispersant on a rotating carousel for 24 h. Coarse material (>63 µm) was then removed by wet sieving over a 63 µm mesh. The fine fraction was collected in a large 500 mL glass jar and left to settle before siphoning off the excess water. Both the fine and coarse fractions were dried in an oven (40°C) and weighed, allowing the weight percent in fine and coarse fractions to be calculated. Between 2 and 4 g of the fine fraction were weighed out for particle size analysis. The biogenic carbonate component was removed by slow digestion in 2 M acetic acid solution for 24 h (twice for samples with >1% CaCO₃). Biogenic silica was removed from Site U1371 samples by digestion using 6% (w/w) sodium hydroxide at 80°C (water bath) for 5 h. Samples were rinsed with deionized water, washed into a 60 mL Nalgene bottle with 0.2% Calgon solution, and placed on a rotating carousel overnight. All samples were sonicated for 1 min immediately before analysis on the Sedigraph.

Samples were run on the Sedigraph at a constant temperature of 35°C with an analysis range of ~1.5–63 µm, thus covering both the silt and clay range. Sedigraph samples were analyzed at the highest resolution possible, which includes an obligatory 11-point smoothing specified by the manufacturer, providing a total of 269 data points (cumulative mass percentage finer). Output data files were analyzed using a custom-designed program (Sedilyze) that allows the full data to be both graphically and numerically displayed.

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