Water samples for dissolved 210Pb and 210Po were collected at 12 stations along the 152°W meridional transect and processed on board through the initial plating of Po. For each sample, 10 liters (L) of water were collected using the Scripps Ocean Data Facility (ODF) rosette. Samples were filtered through Acropak filter cartridges (0.8/0.2 micrometers (um)) and acidified to pH ~2 with 40 milliliters (mL) of 6N HCl. They were then shaken well to homogenize. Surface samples were taken by the "fish" and were not filtered upon collection. Thus, the 210Po and 210Pb activities in samples marked "0" depth are total activities. At sea, samples were taken through the initial plating of 210Po. To each sample, iron (10 milligrams Fe, as FeCl₃ solution) was added along with 10 milligrams (mg) of Pb and 1.76 disintegrations pre minute (dpm) of 209Po tracer (NIST 4326a). The mixture was shaken and left for 24 hours for tracer/carrier equilibration. Iron hydroxide was precipitated by adding concentrated NH₄OH to raise the pH to 8-9. The Fe(OH)₃ precipitate was allowed to settle for 24 hours, after which the sample was filtered through a 1.2 um Versapor filter. The precipitate was dissolved in 20 mL of 6N HCl, and the solution was diluted with DI water to make 80 mL of 1.5 N HCl. Ascorbic acid was added to the 1.5N HCl solution and polonium was plated onto silver planchets (Flynn, 1968; Lee et al., 2014) mounted in Teflon planchet-holders fitted with magnetic stir bars. Plating proceeded on magnetic-stirrer hot plates heated to 80°C for 3 hours. Planchets were then removed from solution, rinsed, and allowed to dry. Dried Po-plated planchets were returned to the shore-based laboratories (Stony Brook University; Florida International University) for alpha counting using Canberra/Mirion passivated implanted planar silicon (PIPS) detectors.
The residual plating solutions were transferred to 125 mL polycarbonate bottles for transport back to Stony Brook University. Due to the long transit back from the Pacific (~3 months), it was decided to eliminate any residual 209Po and 210Po left in the sample after plating by suspending a piece of scrap silver in the sample for 5 days. The silver was then removed, with the time of removal noted. An additional aliquot of 209Po was added to the stored samples, and after ~6 months of storage in the shore-based laboratory (Stony Brook University), Po was plated again using the procedure described above. The 210Po activity obtained at the second plating was then used to back-calculate the activity of 210Pb in the sample at the time of sampling. The calculations outlined by Rigaud et al. (2013) were followed to calculate both the initial 210Po and 210Pb activities.
The scrap silver clean-up step was checked upon sample return to the laboratory by replating several samples without any additional 209Po added. It was found that 5.0 ± 1.6% of the initial Po remained, and correction was made to the calculation of 210Pb activities to account for residual 210Po and 209Po. Additionally, to check the clean-up procedure, two samples were purified by ion exchange after their return to the lab. Agreement was excellent (within 5%).
Recovery of the Pb carrier added to the samples before precipitation was determined after the initial 210Po plating. For this purpose, an aliquot of each stored solution was taken for total Pb yield by Atomic Absorption Spectroscopy or Inductively-Coupled-Plasma Mass Spectroscopy.
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