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doi:10.2204/iodp.proc.340.205.2016

Methods

Each of the 70 visible tephra layers identified in Cores 340-U1396A-1H through 6H (excluding Core 2H) and Core 340-U1396B-2H have been analyzed for their components and particle grain size using the methods outlined below.

Components

Samples measuring ~0.5 cm³ were taken from each visible tephra layer and dried at 60°C. The samples were then washed with deionized water over a 63 μm sieve to remove the fine-grained hemipelagic component. Following this, 2.5 mL of acetic acid (20%) was added to the sample and left for up to 4 h to dissolve the biogenic carbonate. The samples were then again washed and dried in an oven at 55°C. Component analysis was performed by placing aliquots of individual samples on a gridded microscope slide and point-counting an average of 400 grains per sample. The grains were classified according to one of six categories, following the previous studies of Cassidy et al. (2013, 2014). The six categories are as follows:

1. Nonvesicular dense juvenile lava clasts,
2. Vesicular juvenile lava clasts,
3. Altered lava clasts,
4. Crystals (including glass shards),
5. Scoria, and
6. Biogenic material.

Descriptions and examples of these categories can be found in Cassidy et al. (2013). Glass shards and crystals have been combined into one category because of the difficulty of differentiating between the two components under a normal, nonpolarized microscope (Enache and Cumming, 2006; Cassidy et al., 2013, 2014) such as the Meiji EMZ-5 binocular microscope used in this study. Some studies have previously excluded the biogenic components of samples from the point counts; they are included here for completeness, as it may be possible to infer something about transport or environment effects during deposition of the tephra layer if large proportions of biogenic material are present.

Grain size

Small subsamples of each tephra layer were first washed over a 1000 μm (1 mm) sieve and then disaggregated by placing on a shaking table overnight in 25 mL of reverse-osmosis water that contained 0.05% sodium hexametaphosphate dispersant. The dispersed sediment solutions were analyzed with a Malvern Mastersizer 2000 particle size analyzer, which is capable of measuring particles with sizes between 0.02 and 2000 μm. The grain size data collected by the Malvern Mastersizer is a volume percent of the sample. Grain size data were then analyzed with the GRADISTAT software (Blott and Pye, 2001) to calculate sorting coefficients according to the Folk and Ward (1957) method.

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