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Samples and methods

Twenty-three samples were taken from sediment cores at Site U1333 (Table T1). After removing the surface part, which might be contaminated, these samples were freeze-dried for >24 h. The coarse fraction of sediment was separated from ~10 g of sediment with a 75 μm sieve and subsequently dried in an oven overnight. Microscopic observation revealed that the coarse fraction of sediment samples is dominated by microfossil fragments such as radiolarians and foraminifers with trace fish teeth. We found five samples that contain volcanic glass shards (i.e., Samples 320-U1333A-10H-4W, 22–24 cm, 11X-2W, 22–24 cm, and 12X-4W, 22–24 cm; 320-U1333B-11H-5W, 81–83 cm; and 320-U1333C-14H-6W, 92–94 cm). Volcanic glass shards were further separated from the coarse fraction of samples by handpicking under a stereoscopic microscope. These glass shards were then washed with deionized water and dried again.

In preparation for electron microprobe analysis, glass shards were mounted in epoxy resin. The surface of samples in the resin was polished as flat and smooth as possible (using 1 μm diamond paste) to avoid any influence from surface relief. The polished thin sections were cleaned in an ultrasonic cleaner and then coated with a ~20 nm carbon layer to counteract sample charging. Major element compositions were measured with a JEOL JXA-8900RL EPMA at the Japan Agency for Marine Science and Technology (JAMSTEC). Concentrations of elements such as Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe were measured by wavelength-dispersive spectrometry (WDS). Measurements were performed under conditions of 15 kV accelerating voltage, 20 nA specimen current, and 10 μm beam diameter. Synthetic and mineral materials supplied by JEOL were used as analytical standards. Matrix effects such as absorption or fluorescence influences by surrounding elements were corrected using the conventional ZAF method (e.g., Philibert, 1963; Dumcumb and Reed, 1968). We confirmed that the decrease in the Na-Kα line due to electron scattering was insignificant. Analytical precisions (2σ) based on the triplicate analyses of an in-house glass standard were <2% for Al, Si, Mg, and Ca; <4% for Na, Ti, and Fe; and ~25% for K and Mn. Differences between our results of the in-house glass standard and those of reference values measured with the FE-EPMA JXA-8800F at JAMSTEC (H. Shukuno, pers. comm., 2011) were <1% for Mg, Si, and Fe; <3% for Al, Ca, and Ti; ~6% for Na and K; and ~27% for Mn. The large errors for Mn and K are attributed to very low concentrations for these elements (<0.2%).