Sampling was conducted in 2018 at various depths through the water column at eleven stations along the GEOTRACES Pacific Meridional Transect (GP15), carried out on R/V Roger Revelle (cruises RR1814 and RR1815). Particulate samples were collected by Phoebe Lam's group using McLane in-situ pumps (Lam et al., 2015). Two particle sizes were collected by pumping: a large particle fraction collected on a 142-millimeter (mm) 51-micrometer (μm) polyester mesh prefilters (Sefar 07-51/33) and a small particle fraction collected on paired 0.8-μm polyethersulfone Pall Supor 800 membrane filters. The filters were dried on board in a laminar flow hood, split, and placed in a polyethylene bag. Aliquots sent to Stony brook comprised 28-280 liters (L) (average ~100L) filtered for the small particle fraction and 60-330L (average ~250L) for the large particle fraction. More details on pump operation can be found in the GP15 Cruise Report (see Supplemental Files section).
The particulate filters were dried on board the ship in a laminar flow hood, split, and placed in a polyethylene bag. Aliquots sent to Stony Brook comprised 28-280 liters (L) (average ~100L) filtered for the small particle fraction and 60-330L (average ~250L) for the large particle fraction. Due to the equipment limitations and potential hazards when doing acid digestions at sea, on-board treatment was not possible for these samples. Instead, filter aliquots from Leg 1 were sent to the lab during the port stop in Hilo, Hawaii on 21 October. Leg 2 filters were sent back from Papeete, Tahiti at the conclusion of the cruise on 24 November.
In the laboratory, filters were placed in microwave digestion vessels and spiked with Polonium-209 (209Po) tracer (1.76 disintegrations per minute (dpm)) and 10 milligrams (mg) stable lead (Pb). 5 milliliters (mL) each of concentrated HCl, HNO3, and HF were then added to the vessel. The mixture was microwave-digested for 1 hour at 180° Celsius (C). After digestion, the resulting solution was decanted into a 25 mL Falcon tube, and the digestion vessels were rinsed with small aliquots of deionized (DI) water. The Falcon tubes were centrifuged for 10 minutes at 2000 rotations per minute (rpm) and the supernatant was pipetted into 50 mL Teflon beakers. The beakers were heated to almost complete dryness before 10 mL of concentrated HCl was added. This process was repeated to ensure that the HCl was the only acid present. 20 mL of 6 molar (M) HCl was added, the solution was decanted into a glass beaker and the Teflon beaker was rinsed with two aliquots of 30 mL DI water. The beakers were labeled, and ascorbic acid was added to the solution to reduce any Fe(III) to Fe(II) and prevent its plating. A silver planchet embedded in a Teflon stirring magnet was added to each sample, which was then plated at 80°C for 3 hours (Flynn, 1968; Lee et al., 2014), rinsed with DI water and acetone and counted in a Canberra/Mirion Quad Alpha spectrometer to determine initial 210Po. Residual Po was removed from solution by adding scrap silver for ~5 days and then transferring the samples to 125 ml polycarbonate bottles for storage and ingrowth of additional 210Po. The second plating was carried out after 6 to 8 months of storage. Data reduction was carried out using the methods reported in Rigaud et al. (2013).
Known Problems/Issues:
Calculation of the particulate 210Pb activity requires measurement of the concentration of stable Pb added during sample dissolution to determine the recovery of 210Pb from dissolution through plating. The sample solutions were stored for 6 to 8 months after the initial Po plating and removal of residual Po to permit ingrowth of additional 210Po from 210Pb decay. At the time of the second plating, aliquots were taken to measure the stable Pb. A discrete number of samples were completed in late 2019 to early 2020 and Pb recoveries were determined. However, Stony Brook University closed for an extended period starting in March 2020 due to the Covid-19 pandemic. Upon reopening, measurement of stable Pb in the stored aliquots showed losses in storage. As well, analytical instrumentation used to measure Pb (Element 2 ICP-MS) was no longer functioning and required extensive maintenance and repair. Because of these problems, it was decided to use average Pb recoveries for these samples, based on the Pb recoveries of samples analyzed before the shutdown.
