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 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. Dissolved trace metal sample samples were filtered through 0.2 um Acropak filters into acid-cleaned 1 L LDPE bottles. Total dissolvable samples were collected unfiltered into acid-cleaned 1 L LDPE bottles. Samples were double-bagged and transported to the lab at the end of the cruise. Prior to each cruise, 1 L LDPE bottles were soaked in Citranox for one day, soaked in 10% HCl for 7 days, and rinsed multiple times with 18.2 M-Ohm Milli-Q water (Millipore).
Sample Processing and Measurement
Samples were processed via batch extraction following Conway et al. (2013) and measured by isotope dilution using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Briefly, samples were acidified to pH 1.8 using Optima grade HCl and left to equilibrate for a minimum of 2 months. Most samples were acidified for >6 months before processing.
500 uL of a stable isotope spike (containing 47Ti, 57Fe, 58Fe, 61Ni, 62Ni, 65Cu, 64Zn, 67Zn, 110Cd, 111Cd, and 207Pb) was added to the samples and allowed to equilibrate at least overnight (up to 1 week). Sample pH was adjusted to a pH of 6 using an ammonium hydroxide acetic acid buffer. Nobias PA-1 resin (stored in 1M HNO3) was rinsed in water and added to the sample, which was then shaken for at least 3 hours or overnight. Resin beads were collected on a 5 um polycarbonate filter, rinsed with water and transferred to a disposable low-pressure chromatography column (Bio-rad, stored in 1M HNO3). Extracted metals were eluted from the resin using 10 mL 1M HNO3 with a 10 ppb In internal standard added. The same process was repeated regularly with 1 L Milli-Q water as a processing blank.
Samples were measured using iCAP-TQ ICPMS (Thermo Scientific) with In used as an internal standard. For Ti, Fe, Ni, Cu, Zn, Cd, and Pb, concentrations in the resin eluent were calculated using isotope-dilution equations, then converted to sample concentrations using the mass of each sample. For other elements (Mn, Co), concentration was calculated based on comparison to a 10 ppb multi-element standard (Inorganic Ventures), assuming analogous recovery to Fe and Ni (calculated from isotope dilution equations). The samples were largely processed and measured as two sets. The medians of 72 Milli-Q blanks for set 1 (70 pM Fe, 54 pM Zn, 36 pM Ni, 13 pM Cu, 1.4 pM Pb, 1.1 pM Cd, 11 pM Mn, 2.2 pM Ti) and 44 Milli-Q blanks for set 2 (125 pM Fe, 41 pM Zn, 34 pM Ni, 26 pM Cu, 1.3 pM Pb, 1.0 pM Cd, 6 pM Mn, 14 pM Ti) were subtracted from each sample in the corresponding set.
Accuracy of dissolved metal samples was assessed by measurement of GEOTRACES community consensus standards GSC (n = 3) and GDI (n = 4), which is similar to the GD standard. For both standards, agreement with reported values was found for Fe, Cd, Ni, Cu, Zn and Pb. Concentrations for Ti were not available on the GEOTRACES website.
We observed variable recovery on Mn in both standards, likely due to the known pH sensitivity of Mn binding to the Nobias PA-1 resin. As a result, we subjected Mn measurements to additional scrutiny during quality control.
Samples were not UV-oxidized, which is essential for complete determination of dissolved Co concentration. As a result, data are reported as labile dissolved Co and total dissolvable labile Co.
