Sample storage bottle lids and threads were soaked overnight in 2N reagent grade HCl, then filled with 1N reagent grade HCl to be heated in an oven at 60˚C overnight, inverted, heated for a second day, and rinsed 5X with pure distilled water. The bottles were then filled with trace metal clean dilute HCl (~0.01N HCl) and again heated in the oven for one day on either end. Clean sample bottles were emptied, rinsed with distilled water, and double-bagged prior to shipboard rinsing and filling with sample.
Trace metal-clean seawater samples were collected using the U.S. GEOTRACES sampling system consisting of 24 Teflon-coated GO-FLO bottles that had been pre-rinsed with a 24+ hour treatment of filtered surface seawater at the beginning of the cruise (see Cutter and Bruland, 2012 for more information on the sampling system). At each station, the bottles were deployed open and tripped on ascent at 3 m/min. On deck, the bottles were kept in a trace metal clean sampling van over-pressurized with HEPA-filtered air, except immediately prior to and following deployments, in which cases they were covered on both ends with shower caps to avoid deck contamination.
Samples were analyzed at least 1 month after acidification over 11 mass spectrometry sessions by a modified method based on that described by Reuer et al. (2003) as modified by Boyle et al. (2012) and further slightly modified as noted in the following.
The shallowest samples were collected using a pumping system on board a small boat deployed from the main ship. At each station a homogenized 25L carboy sample was 0.2 um supor capsule filtered on board the small boat, and subsampled into community provided bottles inside the main lab bubble. At station 10, due to issues with the pump, an unfiltered carboy sample was collected by submerging the carboy into the water from the small boat. On-board analysis determined this sample was grossly contaminated for Zn. At station 51, to reduce the time spent on the small boat during rough weather conditions, an unfiltered carboy sample was collected with the pump and filtered on board. MIT Pb samples were transferred into warm-acid leached Nalgene 2-liter polyethylene bottles with polypropylene caps, rinsed 3 times with the sample. Bottles were then capped and not further handled until returned to MIT a few months later, where they were acidified to pH 2.0 with high-purity HCl and allowed to sit for a few months before analysis.
Nobias Chelate PA1 preconcentration followed by anion exchange purification:
This method starts with a new double batch ion-exchange chelation preconcentration followed by the Reuer et al. (2003) anion exchange purification followed by isotope ratio analysis on a GV/Micromass IsoProbe multicollector ICPMS using a 50 uL/min nebulizer aspirated into an APEX/SPIRO desolvator, using post-desolvator trace N₂ addition to boost sensitivity.
Nalgene polypropylene separatory funnels (1000mL) and Corning 50 mL conical centrifuge vials were cleaned by heated submersion for 2 days at 60˚C in 1M reagent grade HCl, followed by a bulk rinse and 4X individual rinse of each vial with pure distilled water. Each funnel and vial was then filled with trace metal clean dilute HCl (~0.01M HCl) and heated in the oven at 60˚C for one day on either end. Separatory funnels and centrifuge vials were kept filled until just before usage.
The separatory funnels were rinsed with distilled water after each use and then filled with high-purity distilled water spiked with high-purity HCl (final concentration ~0.01M) between uses.
Nobias Chelate PA1 resin was cleaned with 2 methanol rinses, distilled water rinse followed by leaching with ultrapure 6M HCl for 12-24 hours. This procedure was repeated twice, followed by two one-day leaches with ultrapure 3M HNO₃ on a shaker table. The resin was then rinsed six times with distilled water to removed the nitric acid. It was then leached twice with ultrapure 0.1N HNO₃ for one day each. The final 0.1N rinse was checked for Pb blank by ICPMS and the resin only used if the blank was acceptably low. Because of the large amounts of resin used, the used resin from each sample was saved and re-washed using the following protocol: leach with 3M HNO₃ for 1 day, rinse 4x with dH₂O, rinse with 0.1M HNO₃ for 2 days, rinse with fresh 0.1 M HNO₃ for 3 days, and then then rinse with dH₂O until the pH is ~5.
1000mL polypropylene separatory funnels (Nalgene) were weighed and rinsed one time with seawater sample, then filled with ~1000-1700 ml of sample. The pH of the solution was adjusted by addition of purified ammonium hydroxide/acetic acid pH=7.98 buffer (to a final pH>4; preferably pH~4.1 to keep the buffer blank low). A pre-cleaned aliquot of Nobias Chelate PA1 resin was added, and agitated on a shaker table for one day. Then the resin was allowed to settle to the bottom of the separatory funnel and drawn off into the 50 mL centrifuge tube. A second batch of Nobias Chelate PA1 resin was then added and agitated for one day on a shaker table. Then the second batch of resin was allowed to settle to the bottom of the separatory funnel and drawn off into the same 50 mL centrifuge tube. The solution/precipitate mix was centrifuged and the supernatant solution siphoned off. Pb was released from the resin by addition of trace metal clean 0.1M HNO3 for 1-2 days, then the supernatant was transferred into a clean fluorocarbon vial and taken to dryness on a hotplate in a clean flow fume hood in a positive pressure clean lab. A PA1 resin blank was taken from a batch of resin directly placed into the eluting 0.1M HNO3 and henceforward treated as a sample.
Eichrom AG-1x8 resin was 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.
The residue from the samples and blanks was dissolved in 8 drops of high purity 1.1M HBr. The resin in the column was first cleaned with 6M HCl, equilibrated with 1.1M HBr, and then sample was loaded onto the column. The column was then washed with 1.1M HBr followed by 2M HCl and then eluted with 6M HCl. The samples in a 5 ml Savillex PTFE vial were then taken to dryness on a hotplate in a recirculating filtered air fume hood, and stored sealed until analysis.
Just before analysis, samples were dissolved for several minutes in 10ul concentrated ultrapure HNO₃. Then, an appropriate volume of ultrapure water was added (typically ~400ul) and spiked with an appropriate amount of Tl for mass fractionation correction. IsoProbe multicollector ICPMS Faraday cups were used to collect on ²⁰²Hg, ²⁰³Tl, ²⁰⁵Tl, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. An Isotopx Daly detector with a WARP filter was used to collect on ²⁰⁴Pb+²⁰⁴Hg. This Daly detector is a revised version that eliminates a reflection problem with the electronic circuitry of the previous version. Because the deadtime of the Daly detector varied from day to day, we calibrated deadtime on each day by running a standard with known ²⁰⁶Pb/²⁰⁴Pb at a high 204 count rate. The counter efficiency drifts during the course of a day, so we established that drift by running a standard with known ²⁰⁶Pb/²⁰⁴Pb (and a 204 count rate comparable to the samples) every five samples. Tailing from one Faraday cup to the next was corrected by the ²⁰⁹Bi half-mass method as described by Thirlwall (2000).
On each analytical date, we calibrated the instrument by running NBS981 and normalized measured sample isotope ratios to our measured raw NBS981 isotope ratios to those established by Baker et al. (2004). Using this method for 22 determinations of an in-house standard ("BAB") shows that for samples near the upper range of the Pb signals shown for samples (~1V), ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb can be reproduced to ~200ppm. Low-level samples will be worse than that, but generally better than 1000ppm in this data set. Because of the drift uncertainty in the Daly detector, ²⁰⁶Pb/²⁰⁴Pb for samples in the mid-to-upper range of sample concentrations will be at best reproducible to ~500ppm. Zurbrick et al. (2018) discuss the reproducibility we observed for duplicate analyses of multiple samples as a function of the Pb sample size.
We have intercalibrated Pb isotope analyses with two labs as reported in Boyle et al. (2012). Since that report, two more labs have added intercalibration data. The outcome of that intercalibration suggests that the accuracy of our measurements approaches the analytical reproducibility we note above.
