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doi:10.2204/iodp.proc.337.203.2017

Results

All samples are primarily sand sized with the primary peak of the distribution in the fine-to-medium sand range (Figures F3, F4; see SAMPLES in Supplementary material). Each sample has a low and variable amount of fine-grained material represented by a long tail on the left of the distribution. Each sample from Units II and III falls within the range of 49%–97% sand, 3%–42% silt, and 0%–13% clay (Table T1). Based on the classification system of Shepard (1954), all samples fall within the range of sand or silty sand, with silty sand samples more common in Unit III (Figure F5). Median grain diameter d(0.5) ranges from 55 to 405 µm. Surface area–weighted mean diameter D[3,2] ranges from 9 to 178 µm, and volume-weighted mean diameter D[4,3] ranges from 77 to 400 µm.

The overall coarsest unconsolidated material recovered from Hole C0020A is found in the middle of Unit II between 1377 and 1630 mbsf, indicated by the highest percent sand, lowest percent silt and clay, and highest values of d(0.1) (Figure F6). Within Unit III, the percent sand decreases downhole, and silt and clay contents increase relative to Unit II. Median grain size decreases in Unit III corresponding to the increase in clay content. Sand samples from the upper 30 m of Unit III (Figure F7) in general have lower clay content, higher silt content, and higher d(0.1) compared to the lower intervals of Unit III.

Sample 337-C0020A-5R-3, 36–39 cm, run in duplicate, shows the sand-silt-clay percentages are repeatable to within 1% and d(0.5) matches within 5 µm. d(0.1) and d(0.9) repeat within 6 and 20 µm, respectively. D[4,3] and D[3,2] repeat within 6 and 13 µm, respectively.

Our grain size measurement from the uppermost sample in Unit III (1925.21 mbsf) is consistent with the grain size distribution of one sample from 1925.38 mbsf measured via sieve analysis by Ijiri et al. (2017). Overall, these results show systematic changes downhole that may provide insight to paleoenvironmental conditions and the role of the host material for microbial activity.

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