Our results for grain sizes and clay abundances are comparable to those of other workers studying hemipelagic sediment. Diemer and Forsythe (1995), using both the Spectrex laser particle counter and a rapid sediment analyzer, found that silty clays from the inner trench slope of the Chile margin have an average size of ~2.0-2.4 µm. Cavin et al. (2000), using a Sedigraph, found that hemipelagic mud from the Cascadia Basin typically contains 60%-85% clay and the mean size ranges from 1 to 4 µm.
Grain-size variations in sediment from the Muroto Transect show subtle trends with depth, and these trends are at in part a function of the original depositional environment. The lower Shikoku Basin facies contains no turbidites, so grain sizes tend to be smaller and better sorted. Precipitation of authigenic clay may also contribute to the decreased particle sizes with depth because of the dissolution of disseminated volcanic ash as it alters to clay. At Site 1177, the relations of mean grain size and sorting with lithologic unit are less clear. The upper Shikoku Basin samples from this site show a higher average mean grain size than either the lower Shikoku Basin hemipelagic or turbidite facies, and the sorting in terms of standard deviation is similar in both the upper and lower units. For samples from Site 1177, the weight percentage of clay only shows a slight difference between the upper and lower Shikoku Basin units, with the upper Shikoku Basin facies samples containing only 5% less clay by weight than the lower Shikoku Basin units. It is difficult to resolve any differences between the lower Shikoku Basin hemipelagic and turbidite facies because of the small number of samples analyzed.
Changes in moisture content and bulk density also show depth-related trends. At the Muroto Transect, sediment follows a complicated compaction trend with depth. The upper Shikoku Basin facies, for example, shows nearly constant porosity with depth. This anomalous porosity-depth relation is also seen in the upper Shikoku Basin facies at Site 1177 (Shipboard Scientific Party, 2001a, 2001b, 2001c). The compaction trend with depth is also irregular at Site 1177, but porosity generally decreases with depth (Shipboard Scientific Party, 2001c). Related trends are also noted in bulk density, which generally increases with depth (Shipboard Scientific Party, 2001b, 2001c, 2001e).
Figure F7 shows the relation of weight percent clay with porosity for Sites 1173, 1174, and 1177, and the lithologic units are highlighted in each plot. It is clear that the decrease in porosity coincides with increasing depth. A similar depth dependency occurs for bulk density, void ratio, and water content. The correlations between grain size and physical properties (Fig. F6), therefore, are largely the result of the combined effects of downward-fining stratigraphy and increasing sediment compaction with depth.