The Pb sub-samples were collected by the group of Dr. Clifton Buck using the high-volume sampling system described by Morton et al. (2013). The sampling method and filter preparation are identical to that described in the "GEOTRACES Cookbook" (Cutter et al., 2014). The samples were sealed in clean plastic and shipped to MIT for analysis.
Pb isotopic measurements by the isotope dilution method were carried out using a 205Pb tracer solution (ET535) that was prepared and calibrated as part of the EARTHTIME project (Condon et al., 2015; McLean et al., 2015). These measurements were carried out by undergraduate student Avery Wang and Research Scientist Jahander Ramezani.
Filters were leached inside a Pb clean fluorocarbon vial with 0.1 M nitric acid at 100° Celsius for 24 hours. The solutions were taken to dryness, and then dissolved in 400 microliters (µL) of 1.1M HBr. Samples were purified by anion exchange chromatography on Eichrom AG-1x8 resin (cleaned by three batch rinses with 6N trace metal clean HCl for a ~12 hours on a shaker table, followed by multiple washes with distilled water until the pH of the solution was above 4.5; resin was stored at room temperature in the dark until use) using 1.1M HBr and 2M HCl to wash out impurities prior to collecting the sample with 6M HCl. Eluted samples were analyzed on a IsotopX Thermal Ionization Mass Spectrometer using repeated runs of standard NBS981 to account for mass fractionation.
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