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doi:10.2204/iodp.proc.340.205.2016

Results

Components

Results of the component analysis for the 70 visible tephra layers identified in Cores 1H through 6H in Holes U1396A and U1396B are presented in Table T1. A summary of the component analysis is presented in Table T2, which clearly shows that of the six component categories used to classify grains, samples are dominated by crystals and glass shards, with vesicular lava clasts and nonvesicular dense clasts in order of decreasing abundance. Glass shards and crystals are the most abundant component and are present in all 70 tephra layers, varying between 18% and 96% of grains per sample. Within this wide range, 58 of the 70 tephra layers contain >40% glass shards and crystals. There appears to be no pattern to the depth of tephra layers containing the highest abundances of glass and crystals within Holes U1396A and U1396B. The second most abundant component is juvenile vesicular clasts, which varies between 1.3% and 76.6%. Of the 67 tephra layers that contain juvenile vesicular clasts, 19 contain >30% juvenile vesicular clasts. As with the glass shards and crystals component, there appears to be no pattern to the depth of tephra layers containing the highest abundances of juvenile vesicular clasts within Holes U1396A and U1396B. Juvenile dense clasts are the third most abundant component, varying from 0.2 to 47.9%, with 10 samples containing >20% juvenile dense clasts. In terms of the vertical distribution through the cores, 8 of the 10 tephra layers with >20% juvenile dense clasts were found in Core 340-U1396A-6H. Of the remaining components, scoria clasts (0.5%–45.1%) were found in only 12 tephra layers. All scoria-bearing tephra layers were found within Core 6H. Several examples of samples containing high abundances of two or more components were found. For example, Sample 340-U1396A-6H-5, 144–145 cm, contains >30% vesicular juvenile components and >56% glass shards and crystals, whereas Sample 6H-7, 5–6 cm, contains 50% glass shards and crystals and 45% scoria.

Grain size

The results of the grain size analysis are presented in Figure F2, whereas summary statistics derived using the GRADISTAT software are provided in Table T3. Grain size distributions presented graphically in Figure F2 show wide variation in the nature of the distributions, from predominantly fine grained to strongly coarse grained tephras, as well as unimodal, biomodal, and distributions with multiple populations. The summary statistics presented in Table T3, derived from GRADISTAT software (Blott and Pye, 2001), show that the majority of tephra layers (59 out of 69; 86%) display either unimodal or bimodal distributions, with the rest being either trimodal (eight tephra layers) or polymodal (two tephra layers). Core 340-U1396A-6H contains most (14 out of 30; 47%) of the tephra layers with a unimodal grain size distribution.

The sorting and mean grain size of the tephra layers varies considerably, ranging from very well sorted very fine sand to poorly sorted coarse sand. However, the majority of tephra layers contain fine or medium sand (48 out of 69; 70%) and are poorly to moderately sorted (49 out of 69; 71%). The shape of the grain size distributions, as shown graphically in Figure F2, also varies considerably, with distributions covering the entire range of values skewed strongly to the fine grain sizes through those skewed strongly to coarse grain sizes. However, approximately one-third (26 out of 69) of the tephra layers have a symmetrical distribution around the mean grain size. Values for kurtosis, or degree of peakedness, of the grain size distributions show that most tephra layers (53 out of 69; 78%) display a platykurtic- or mesokurtic-shaped curve, indicating that the tails of the curves are better sorted than the center region. Platykurtic- or mesokurtic-shaped curves have a flat shape and would be associated with either symmetrical or slightly skewed grain size distributions.

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