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Methods and materials


During Expedition 333, an ocean-floor sediment succession, mostly hemipelagic mud with intercalated ash layers, was sampled from the surface to 379.93 meters below seafloor (mbsf). A total of 277 samples (~10 mL, covering a depth interval of 2 cm) were collected from almost every 150 cm core section. Sand and volcanic ash layers were avoided during sampling; however, a few samples accidentally contain a significant amount of ash. A total of 276 samples were analyzed in this study. The average depth interval between samples is 1.38 m.

Sample preparation

About 50 mg of each sediment sample, which was not dried or crushed, was soaked with 12 mL of 5.5 g/L sodium hexametaphosphate dispersant for >24 h and then was sonicated and well dispersed by hand shaking. Stiff lumps were gently crushed with a thin wood stick within the dispersant. To check for errors originating from nonhomogeneity of the samples, three aliquots using a different part of each sample block were made for 27 samples, selected from each consecutive 10 samples (Table T1).

Grain size analysis

Grain size analysis was carried out using a Malvern Mastersizer 2000 laser diffractometer with an automated sample dispersion unit (Hydro 2000S) installed at the Kochi Core Center (Japan). Principles of the instrument are described in Sperazza et al. (2004). We adopted the procedure and analytical setting proposed by Sperazza et al. (2004) with some modification. In this study, samples dispersed in advance were poured into the dispersion unit, with pump speed and intensity of sonication set to 2000 rpm and 20%, respectively. The input amount of the samples was controlled so that obscuration fell within the 15%–20% range. The grain reflectance index and dispersant were set to 1.555 and 1.33, respectively. Particle absorption was set to 1. Each measurement run was set for 12 s, or 12,000 snaps, and repeated five times. Grain size analyses reported in this paper are the fifth result of the repetition series. Prior to accepting an analysis, we visually inspected the output from each of the five runs for consistency.

Results were compiled with Malvern’s Mastersizer 2000 software as the volume frequency of 100 size classes. The boundaries between classes are set as a geometric series from 0.02 to 2000 µm. For convenience, we used the phi (ϕ) scale in the following text, table, and figures. The grain size unit ϕ is derived as follows:

ϕ = –log2(d/d0),

where d (in millimeters) is the diameter of a particle and d0 is a reference diameter equal to 1 mm.