Volcanic glass (tephra) was extracted from the investigated sediments using the flotation procedure of (17) by sieving at 15 μm followed by density extraction using different densities of sodium polytungstate heavy liquid (1.95 to 2.55 g/cm3). To detect discrete horizons, we sampled and analyzed the entire length of targeted unit 4 sediments. First, we focused on contiguous 10-cm sediment slices. Then, we zoomed in on sections that contained glass using 1-cm interval samples. Last, shards were picked for analysis from the discrete tephra horizon presented here (278.5-cm core depth) with the help of a gas chromatography syringe [cf. (18)]. Individual tephra shards were then geochemically characterized using wavelength electron microprobe analysis to measure major and minor element oxide concentrations. Analyses were carried out at the Electron Probe Microanalysis Facility at the School of Geosciences of the University of Edinburgh. For this purpose, we used a Cameca SX100 instrument that was operated at an accelerating voltage of 15 kV, with variable beam currents of 0.5 nA (Na/Al), 2 nA (Mg/Si/K/Fe/Ca), and 60 nA (P/Ti/Mn), and a beam diameter of 6 μm. Secondary glass standards (LIPARI and BCR2g) were analyzed between and within runs to monitor analytical precision (table S1).

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