Methodology:

Sampling and analytical procedures:

During RR1804 cruise, regular Niskin bottles (12 L GO-FLO bottles) and Seabird CTD rosette were used to collect seawater samples. Each seawater sample was collected into an acid-cleaned LDPE bottle on board and then filtered in a trace metal clean (TMC) “bubble” on the boat using an AcroPakTM 0.2µm filter and a peristaltic pump and stored in 50 mL low density polyethylene (LDPE) centrifuge tubes.

During RR1805 cruise, the trace metal clean technique was used to collect seawater samples from the upper 700 m of the water column using 5 L acid-cleaned Teflon-coated external-spring “Niskin-type” bottles (Ocean Test Equipment) on a powder‐coated trace metal clean rosette (Sea‐Bird Electronics). After collection, seawater was filtered through acid-washed 47 mm diameter 0.2 μm Supor polyethersulfone filters (Pall) and collected into acid-washed 1L LDPE bottles (Nalgene).

Seawater samples were acidified to pH 2 with concentrated, distilled HCl (0.1% v/v) in the clean lab at University of Southern California (USC). After acidification, seawater samples were stored for three months before trace metal analysis.

Metal concentration analyses were identical to those used in Hawco et al. (2020). For each sample, 15 mL seawater was transferred to a 15 mL tube, then 50 uL of an isotope spike (containing 57Fe, 62Ni, 65Cu, 67Zn, 207Pb, and 110Cd) was added to the 15 mL tube and thoroughly mixed with the sample. The samples would then sit overnight before they were preconcentrated by seaFAST system. The seaFAST system helped to preconcentrate the seawater samples and remove the salt matrix. About 10 mL of seawater was injected through the Nobias PA-1 column of seaFAST and 0.5 mL eluent (1M HNO3 containing 1 ppb In) was used to elute trace metals for concentration measurement. The trace metal concentrations were then measured by an Element 2 single collector sector field ICP-MS. Concentrations of Cd and Ni were derived by using an isotope dilution method. Concentrations of Mn and LREEs (La, Ce, Pr, and Nd) were quantified relative to the 10 ppb standard and corrected for incomplete sample loading using the recovery of Ni (Hawco et al., 2020). Trace metal concentrations were converted from nmol/L (or pmol/L) to nmol/kg (or pmol/kg) using a seawater density of 1.025 kg/L.