Seawater samples were collected using Teflon-lined GO-FLO bottles (General Oceanics) on a trace metal clean sampling rosette. The GO-FLO bottles were moved into a trace metal clean van after seawater collection and each sample was filtered through an acid cleaned 0.2 μm capsule filter (PolycapTC, Whatman) into an acid washed, sample rinsed, 1 L low density polyethylene (LDPE) bottle and acidified with 4 mL of 6 M ultrapure HCl (final concentration 0.024 M).

After collection, samples were shipped to University of California at Santa Cruz (UCSC) and stored at room temperature until they were analyzed. Typically 500 to 1000 g of each seawater sample was extracted by Nobias Chelate-PA1 resin (HITACH, Japan) for seawater matrix removal and Pb pre-concentration (Biller and Bruland, 2012; Sohrin et al., 2008). The Pb in the seawater was then recovered by eluting the column with 3 mL 1 M HNO3 and measured using a Thermo Element XR high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) at UCSC for concentration determination. To estimate recovery yield, Pb free seawater (pretreated with the Nobias Chelate-PA1 resin) was spiked with varying amounts of a Pb standard and processed with every sample batch. The Pb concentrations of these standard spiked seawater samples were compared to standards of similar concentration prepared in 2% HNO3. The Pb blank for the full procedure was 0.33±0.16 pmol kg−1. Method accuracy and precision were assessed relative to GEOTRACES SAFe, S, and D1 reference seawater samples. After Pb concentration measurements were made, samples were dried on a hot plate in preparation for Pb isotope ratio determination. All work was performed in a class 1000 clean lab inside class 100 laminar flow hoods.

Lead isotope ratios in seawater samples were measured using a Thermo Neptune Plus multi-collector ICP-MS the University of California, Davis. Detailed information about measurement conditions and instrument settings are described by (Erhardt, 2013). Samples were dried down and then brought up in 2% HNO3 to a concentration of at least 3 ppb, NBS SRM 997 Tl solution was added to obtain a Tl/Pb ratio of 0.2 to correct for the mass fractionation using an exponential law correction. The diluted samples were self-aspirated using a 50 μL min−1 PFA nebulizer. An ESI APEX-IR desolvating system was used to increase sensitivity with the Neptune Plus configured with a jet sample cone and X-style skimmer cone. Samples (in sets of 5) were bracketed with a 5 ppb solution of the NBS SRM981 Pb standard. The bracketing standard was used to correct for instrumental mass bias and the mass bias correction was applied to the measured samples. A 2% HNO3 blank was analyzed after each sample with the analyzed blank subtracted from each sample to ensure no sample crossover contamination.