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.
Before each cruise, particle interceptor traps (PITs) were acid-cleaned by soaking in Citranox overnight followed by a 10% HCl soak for seven days, followed by several rinses with high purity (18.2 mega-ohm-cm) water. A brine was made by partially freezing trace-metal-clean seawater over multiple cycles until a 100 ppt salinity was achieved. Prior to deployment at sea, the traps were filled with approx. 1.8 L of trace metal clean brine and topped with approx. 0.8 L of trace-metal-clean seawater.
During deployment, PITs were attached to a surface-tethered array at a depth of 150 m and deployed over approximately 2.5 days. Three replicate traps and one capped control trap were deployed. The trace-metal PITs were deployed 1-2 m above the standard 'cross' used for collection of sinking particles by the HOT program.
Upon recovery, traps were processed in a trace-metal-clean 'van' laboratory onboard. The samples were vacuum-filtered through 1 mm x 1mm screening mesh onto acid-cleaned 47 mm 0.2 mm Supor filters. The filters were folded and stored in acid-cleaned centrifuge tubes prior to analysis. 1 L of filtrate from each sediment trap was also collected to assess potential solubilization of sinking particulate metals.
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). 5 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 5 mL 0.1 M HNO3. 'Labile' metal samples were diluted 100-fold and 'recalcitrant' metals were diluted 10-fold in 0.1 M HNO3 prior to analysis.
Particulate trace metals collected on the filter corresponding to the 'control' trap were used to determine process blanks. Several blank filter and reagent blank analyses (containing no filter) were also 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.
