Sampling
Size-fractionated particles were collected using dual-flow high-volume in-situ pumps (McLane Research Laboratories) fitted with 142 mm, multi-stage filter holders (126 mm active diameter). Size-fractions described further below were collected sequentially on independent stages of the filter holders. Pumps were deployed on a non-metallic hydrowire and programmed for active pumping durations of 145–180 min at an initial flow rate of 8 L min⁻¹. At-sea pump flowpaths and exhaust were independently metered to quantify the volume of seawater filtered.
Cruise and depth schemes
September 2022 & March 2023 (single-cast cruises): four depths per cast (base of mixed layer, 100 m, 200 m, 300 m), with March 2023 missing the 100 m depth due to mechanical pump failure.
May 2023 & August 2023 (two-cast cruises): each cast sampled the above four depths plus an additional four depths between 400 m and 900 m.
Size fractions
Large size-fraction ( > 51 μm): particles collected on a 51 μm polyester pre-filter.
Small size-fraction (0.2–51 μm and 0.8–51 μm): particles collected on paired 0.8 μm and/or 0.2 μm Pall Supor polyethersulfone filters.
All filters and pre-filters were acid-cleaned following GEOTRACES protocols (GEOTRACES group, 2021) and handled in HEPA-filtered clean environments. Upon pump recovery, each large size-fraction pre-filter was subsampled by excising a one-eighth wedge (averaging a ~36.3 L seawater equivalent) with a ceramic rotary blade (Cadence Inc.); particles were back-rinsed onto 25 mm 0.2 μm Pall Supor polyethersulfone filters using 0.2 μm-filtered surface seawater collected on a preceding trace-element cast. September 2022 large size-fraction samples had modified sample processing protocol (detailed below).
Small size-fraction filters were subsampled (excised one-eighth wedge with ceramic rotary blade, averaging 66.5 L for 0.8–51 μm and 21.5 L for 0.2–51 μm seawater equivalent) and processed in parallel.
All filter subsamples were dried at room temperature in a laminar-flow bench and stored in ultraclean polyethylene bags until laboratory analysis.
Sample types
Dipped blank filters, including the full filter sets (paired Supor filters), were deployed during the pump casts.
Dipped blank filters were collected in two different ways. For some casts, only one small size-fraction (0.2 or 0.8) for each depth of the cast was collected, dipped blank filters were placed on the second flow-head of the pump and the valve to that head was closed so no water was pulled through the filter. For some casts both 0.2 and 0.8-51 μm size-fraction filters were used so there was one of each size-fraction on each flow head of the 4 pumps. Dipped blank filters were sandwiched in a 1 µm polyester mesh filter bag, placed into acid-leached perforated polypropylene containers, and attached to a pump frame with plastic cable ties prior to deployment.
All dipped blank filters were exposed to seawater for the same amount of time, processed, and analyzed as regular samples. Dipped blank filters were treated as process blanks and used for blank subtraction, calculations of uncertainties, and determination of elemental detection limits. A total of 71 dipped blank filter sets were collected and used for blank subtraction and determination of uncertainty and detection limit (Table 1) with a total of 21 of them being collected at Station ALOHA and the remaining were collected along US GEOTRACES transect GP17-OCE.
In this dataset, data reported from the 51 μm pre-filter are referred to with LPT and LPL suffixes to indicate large particulate total and large particulate labile concentrations (>51 μm), respectively; data reported from the primary filters (Supor—0.2 or 0.8-51 μm) are from the top filter of the Supor pair only, and are referred to with 0.2 or 0.8 --SPT and SPL suffixes to indicate the small particulate total and small particulate labile concentrations for each small size-fraction filter pore size, respectively.
Sampling equipment
Dual-flow McLane Research in-situ pumps (WTS-LV). More details can be found in the patent description (https://patents.google.com/patent/US20130298702) and official website of the manufacturer (https://mclanelabs.com/wts-lv-large-volume-pump/ ).
Analytical Procedures
Particulate trace element total concentrations (0.8_SPT, 0.2_SPT and LPT)
For total digestions, a one-eighth wedge of each small size-fraction filter (representing 21.5 and 66.5 L of filtered seawater, depending on the pore size used) and half of each seawater-rinsed 25 mm 0.2 µm large size-fraction filter were placed into pre-cleaned 15 mL Teflon vials (Savillex). Digestions were performed using a 4-hour heating step at 110°C with a mixture of 50% nitric acid and 10% hydrofluoric acid, following Planquette and Sherrell (2012). All of the acids used were optima grade. Following initial digestion, samples were dried and treated twice with 1 mL of a 50% HNO₃ and 15% H₂O₂ mixture, then redried. The residue was reconstituted in 4 mL of 2% HNO₃ for analysis.
September 2022 LPT: At-sea, pre-filters were not excised or rinsed onto 25 mm 0.2 μm Pall Supor polyethersulfone filters using 0.2 μm-filtered surface seawater. 1/16th of dried polyester mesh prefilter was cut with ceramic rotary blade in land-based laboratory. Samples were soaked in ultra-pure water inside pre-cleaned 15 mL Teflon digest vial (Savillex) overnight. The next day, pre-filter was removed, and samples were digested following Planquette and Sherrell (2012).
Particulate trace element labile concentrations (0.8_SPL, 0.2_SPL and LPL)
The weak leach procedure targeted surface-reactive and biologically bound phases following Berger et al. (2008). A one-sixteenth wedge of each small size-fraction filter and the remaining half of each seawater-rinsed 0.2 µm filter were placed into pre-cleaned 15 mL Teflon vials (Savillex). Samples were leached with 25% acetic acid and 0.02 M hydroxylamine hydrochloride at room temperature for 2 hours, including a 10-minute 95°C heating step. Filters and leachate were then placed into acid-cleaned 2 mL microcentrifuge tubes to be centrifuged (14,000 rcf, 10 minutes). After centrifugation, leachates were transferred to 7 mL Teflon vials, acidified, evaporated to near-dryness, and reconstituted in 4 mL of 2% HNO₃.
September 2022 LPL: At-sea, pre-filters were not excised or rinsed onto 25 mm 0.2 μm Pall Supor polyethersulfone filters using 0.2 μm-filtered surface seawater. 1/8th of dried polyester mesh prefilter was cut with a ceramic rotary blade in land-based laboratory and placed into pre-cleaned 15 mL falcon tube and submerged in ultra-pure water. Samples were sonicated in a water bath for 1 hour. Filter was removed and samples were leached following Berger et al. (2008) protocol. Blanks for these samples are reported in Table 2.
Analysis by ICP-MS
Sample solutions were analyzed at the Rutgers University Stable Isotope Laboratory via magnetic sector field ICP-MS (Thermo Element XR). Elements were quantified using external solutions prepared from NIST atomic absorption standards in similar sample matrices. Volume-normalized concentrations were calculated using the seawater volume associated with each filter subsample. Quality assessed parameters measured on the ICP-MS, the isotope mass and resolution (LR=Low Resolution; MR=Medium Resolution) used for the concentration measurement are indicated in Table 4.
September 2022 LPL concentrations ONLY: “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” (https://www.bco-dmo.org/dataset/918811).
Assessed parameters measured on the ICP-MS, the isotope mass and resolution (LR=Low Resolution; MR=Medium Resolution) used for the concentration measurement are indicated in Table 5.
