1. Sampling

Size-fractionated (0.8-51um; >51um) particles were collected using dual-flow McLane Research in-situ pumps (WTS-LV) and 142mm “mini-MULVFS” filter holders (Bishop et al., 2012; Lam et al., 2018; Lam et al., 2015; Xiang and Lam, 2020). One filter holder/flowpath was loaded with a 51 um Sefar polyester prefilter and followed by paired 0.8 um Pall Supor800 polyethersulfone filters (“Supor-side”). A 150 um Sefar polyester mesh was placed underneath all 51 um prefilters to facilitate filter handling but not analyzed. All filters and filter holders were acid leached before use based on the recommended methods in the GEOTRACES sample and sample-handling protocols (Cutter et al., 2010). Samples were processed in a laminar flow hood at sea. After collection, each 51 um/150 um prefilter pair and 0.8 um Supor pair was transferred to an acid-leached 150mm PET petri dish, bagged in ziploc bags, and frozen at -20°C at sea and shipped back to UCSC frozen.

In the lab, a 1/8 slice of the frozen 51 um prefilter was rinsed with filtered TM-clean surface OSP seawater by vacuum filtration onto a 25mm 0.8 um Supor filter in a laminar flow hood. The 25mm Supor filters were dried until ready for analysis. Half of the frozen paired 142mm Supor filters were dried in a laminar flow hood until ready for analysis.

2. Analytical Procedures

The digestion method of particulate minor and trace elements is based on a refluxing method (Planquette and Sherrell, 2012) with light modifications as described in (Xiang and Lam, 2020). In brief, the Supor filter was adhered to the wall by surface tension in a 15 mL flat-bottom screw-cap Savillex vial to avoid immersion. After 4-h refluxing at 110 ˚C with an ultrapure (ARISTAR® or Optima grade) 50% HNO3/10% HF (v/v) mixture, digestion acids were transferred into secondary vials and heated to near dryness. The residue was heated in 50% HNO3/15% HF (v/v) to dryness at 110 ˚C. The final residue was re-dissolved with 2 ml 5% HNO3 spiked with 1 ppb In. Two certified reference materials (BCR-414 and PACS-2) were digested routinely alongside the samples to assure the quality of each digestion. Sample solutions were analyzed using an Element XR high-resolution ICP-MS (Thermo Scientific) at the UCSC Plasma Analytical Facility. Elemental concentrations were standardized using multi-element, external standard curves prepared from NIST atomic absorption-standards in 5% HNO3. Instrument drift and matrix effects were corrected using the internal 1ppb In standard and monitored using a mixed element run standard. Concentrations were determined using external standard curves of mixed trace elements standards.