Sampling:

Small (1-51 µm) and large (> 51 µm) particles for particulate 210Po and 210Pb analyses were collected at 8 stations (5 super stations, 1 shelf station, and 2 full station) using McLane Research in situ pumps that had been modified to accommodate two flow paths (Ohnemus and Lam, 2015). At super stations, three casts (shallow, mid, and deep) were deployed to collect a 24-depth profile. At full stations, one shallow and one deep cast was deployed while one cast was deployed at shelf station. A Seabird 19plus CTD was deployed to verify target depths during deployment.

Filter holders (142 mm diameter) had two stages for two sized particles with multiple baffle systems for allowing even particle distribution (Bishop et al., 2012). Two-sized particles were collected on 51 µm pore size Sefar polyester mesh prefilter followed by paired 1 µm Whatman QMA quartz fiber filters. All filters and filter holders were previously acid leached as recommended in the GEOTRACES sample and sample-handing Protocols (Cutter et al., 2014).

The “dipped blank” filters were also deployed during each cast, exposed to seawater for the length of the pumping period, and processed and analyzed as regular samples. A total of 28 dipped blank filters were used for process blank subtraction.

Analytical Methodology:

At each depth, we got a ~ 16.4% subsample of the 1 µm QMA filter and a 37.5-62.5% subsample of the 53 µm polyester prefilter. Filters were digested using a mixture of strong acids (i.e. HCl, HNO3) after being spiked with a pre-calibrated 209Po tracer and stable lead carrier to monitor the losses during sample processing. The residual filter material was removed from the solution after being rinsed 3-5 times with strong acids. The solution was evaporated to near-dryness and then made in 0.5 M HCl, during which ascorbic acid was added to reduce the Fe(III). Polonium isotopes were extracted from the solution by depositing onto a silver disc (Flynn, 1968). Sliver discs (purity > 99.99%), 2 cm in diameter, were cleaned with a commercial silver polish, ethanol, and Millipore water. Only one side of the silver disc was for deposition with the other side being covered by electronic spray. After deposition, the activities of 209Po and 210Po were measured using Alpha Analyst Integrated Alpha Spectrometer (Canberra Inc.). The plating solution was then run through AG-18 anion exchange resin to remove any remaining polonium isotopes (Sarin et al., 1992), re-spiked with 209Po tracer, and stored for at least 6 months to allow for 210Po ingrowth. The samples were reanalyzed for 210Pb by re-plating of 210Po on a new sliver disc. After second plating, aliquots were taken for Pb recovery determination by measuring stable lead concentrations using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The determination of the initial activities of 210Po and 210Pb in the sample at the time of collection involved the correction for decay and ingrowth between the time of sample collection and processing, chemical recoveries of Po and Pb, and dipped blank and detector background subtraction(Rigaud et al., 2013). The uncertainty (median) of 210Pb activity at the time of collection is about 6.5% and 8.8% in small and large particles, respectively. In contrast, the uncertainty (median) of 210Po is about 5.3% and 7.1% in small and large particles, respectively.