Sample Collection
Samples were collected on 21 Hawaii Ocean Time-series (HOT) cruises, onboard the R/V Kilo Moana throughout 2020-2023. All samples were collected at Station ALOHA (22.45°N, 158.0°W) following trace metal clean procedures.
For all cruises except for October and December 2021, trace metal clean particle profile samples were collected using a powder-coated aluminum 12-place 'trace metal' rosette (Seabird) with 8 L external spring sampling bottles (Ocean Test Equipment), deployed using metal-free line (Amsteel). For the October and December 2021 cruises, samples were collected with C-Free bottles (Ocean Test Equipment) that were attached directly to the metal-free line and triggered with Teflon-coated messengers. Prior to sampling, sampling bottles were subjected to a ca. 18 hr soak in seawater collected at a test station near Oahu.
Upon recovery, sampling bottles were taken to a positive-pressure clean van onboard. Particle samples were collected onto acid-cleaned 25 mm diameter 0.2 mm polyethersulfone (Supor) filters using acid-cleaned filter holders and polyethylene tubing. Bottles were pressurized for sampling at 10-15 psi using compressed air and filtrate (~4 L) was collected for gravimetric measurement on land. Filters were stored in acid-cleaned centrifuge tubes and transported back to the lab.
Sample Processing and Measurement
In the lab, filters underwent a two-stage digestion to isolate 'labile' and 'recalcitrant' metal fractions. The 'labile' digestion followed the procedure outlined by Berger et al. (2008). Filters were submerged in 2 mL of a 25% acetic acid, 0.02 M hydroxylamine hydrochloride solution, with 10 ppb Indium added as an internal standard and recovery tracer. The filter and solution were heated in a water bath at 95C for 10 min, and allowed to return to room temperature. After a total contact time of 2 hrs, the filters were removed and the leachate was transferred to acid-cleaned PFA digestion vials (Savillex). To improve recovery, the filter was then rinsed with a few mL of high purity (18.2 M-ohm cm-1) water, which was then added back to the leachate. 100 uL of concentrated nitric acid was added to the leachate, which was then dried overnight at 110C. The sample was redissolved in 0.1 M HNO3 for ICP-MS analysis.
Following the 'labile' digestion, the filter was then subject to a total extraction following the procedures used by Planquette & Sherrell (2012). 2 mL of a digest solution containing 8 M HNO3, 2.9 M HF, and 10 ppb Y (also applied as an internal standard and recovery tracer) was added to the filters in a 20 mL PFA vial (Savillex), which was heated for 4 hours at 110°C. The filters were then removed and the sample was evaporated to dryness. 100 ul of concentrated nitric acid was added and the sample was dried again before being redissolved in 0.1 M HNO3.
Several blank filter and reagent blank analyses (containing no filter) were analyzed to estimate the metal contributions of the filters and reagents.
Digested samples were analyzed using a Thermo Scientific iCAP TQ ICP-MS at the University of Hawaiʻi at Mānoa. All elements were analyzed in triple-quadrupole mode with oxygen as the reactive gas (TQ-O2). A multi-element standard (Inorganic Ventures) was run multiple times throughout a run to monitor changes in instrument sensitivity over the course of a run.
Accuracy of particulate metals from the digestion protocol was assessed by digestion and measurement of certified reference materials BCR-414, PACS-3. Average recoveries for these standards ranged from 85-92% for Al, 80% for Ti, 79-93% for Mn, 83-93% for Fe, 70-80% for Zn, 122-139% for Cd, 97-113% for Ni, 79-84% for Cu, and 89-98% for Co.
