Methodology:
Sampling and analytical procedures:
Study area and sample collection
Sample collection occurred during the CICLOPS (Cobalamin and Iron Co-Limitation of Phytoplankton Species) expedition (expedition code NBP18-01; chief scientist G.R. DiTullio) aboard the RVIB Nathaniel B. Palmer, December 11, 2017 – March 3, 2018 in the Amundsen Sea and Ross Sea of the Southern Ocean. Water samples were collected using trace metal sampling protocols described previously (Noble et al. 2012). A trace metal clean rosette suspended on a Kevlar line and equipped with twelve 8L X-Niskin bottles (Ocean Test Equipment) was used to collect seawater at depths ranging from 10 – 600 m.
Preparation of plasticware
Polyethylene and polycarbonate sampling and incubation bottles were rigorously cleaned to remove trace metal contaminants before use. Bottles with rinsed with Milli-Q water (Millipore), soaked for 72h in <1% Citranox detergent, rotated, soaked for an additional 72h, and then rinsed five times with Milli-Q water. Bottles were then filled with 10% HCl (Baker instra-analyzed) by volume and soaked for a minimum of one week, rotated, and soaked for another week. Bottles were then rinsed five times with dilute acid (HCl, pH 2) and stored double-bagged in plastic zip bags. All cleaning work was conducted in a Class 100 clean room.
Analyses of total dissolved Cd and Zn using isotope dilution
Samples for the analysis of total dissolved Zn, Cd, Fe, Mn, Cu and Ni concentrations were collected shipboard by pressure-filtering X-Niskin bottles through an acid-washed 142mm, 0.2µM Supor membrane filter (Pall) within 3 hours of rosette recovery using high purity (99.999%) N2 gas. Total dissolved water samples were collected into 250mL trace metal clean polyethylene bottles and were stored double-bagged in plastic zip bags. Seawater samples for 110Cd and 67Zn stable isotope uptake experiments were collected in the same way but without filtering. All sample collection occurred shipboard within a trace metal clean van containing laminar flow hoods and plastic sheeting. Samples for total dissolved metal analysis were acidified to pH 1.7 with high purity HCl (Optima, Fisher Scientific) within 7 months of sampling and were stored acidified at room temperature for over 1 year prior to analysis.
Quantification of dissolved metals in samples and reference seawater was performed for total dissolved Fe, Ni, Cu, Zn, and Cd using isotope dilution. 15mL of acidified seawater sample was spiked with 50μL of a stable isotope spike solution artificially enriched in 57Fe, 61Ni, 65Cu, 67Zn, and 110Cd. All stable isotopes were received in solid form (Oak Ridge National Laboratory). Initial dissolution and all subsequent dilutions were made using concentrated nitric acid (Optima, Fisher Scientific). Concentrations and spike ratios were verified by ICP-MS using a multi-element standard curve (SPEX CertiPrep). The composition of the isotope spike was made such that the 15mL spiked sample contained the following ratios: 57Fe/56Fe = 0.7, 61Ni/60Ni = 0.5, 65Cu/63Cu = 1, 67Zn/66Zn = 0.7, and 110Cd/114Cd = 1 and were verified with ICP-MS. These ratios were chosen to minimize the uncertainty introduced by error propagation through the isotope dilution equation (Wu and Boyle 1998; Rudge et al. 2009; Tan et al. 2020). Because it is monoisotopic, total dissolved Mn was calculated using a modified isotope dilution equation. This equation and its details can be found under the Supplemental Files section of this metadata landing page within the file titled, Modified Isotope Dilution Equation.
In this equation, 55Mnspl and 57Fespl refer to the blank corrected counts per second (cps) of 55Mn and 57Fe in the spiked sample, 57Fe spike is the concentration of 57Fe spike, 57Feslope is the slope of the external standard calibration curve (SPEX curve) relating 57Fe cps to ppb, and 55Mnslope is the slope of the external calibration curve (SPEX curve) relating 55Mn cps to ppb. Due to the acidification of seawater prior to ICP-MS analysis, Mn ICP-MS measurements do not include contributions from humic-type Mn(III)-ligand complexes (Oldham et al. 2021). Until the inclusion of Mn(III) is resolved and intercalibrated, we report these Mn values as Mn(II) and note that they are consistent with prior studies employing the same acidification technique (Sedwick et al. 2000; Noble et al. 2013; Gerringa et al. 2020).
Preconcentration of spiked seawater samples for total dissolved metal analysis was performed using the automated solid phase extraction system seaFAST-pico (Elemental Scientific) in offline concentration mode with an initial volume of 15mL and elution volume of 500µL (Bown et al. 2017; Rapp et al. 2017; Jackson et al. 2018; Wuttig et al. 2019). The seaFAST contains a Nobias-chelate PA1 resin column (ethylenediaminetriacete and iminodiacetate) suitable for the simultaneous preconcentration of several trace metals (Fe, Mn, Zn, Cu, Co, Cd, Ni) with high sensitivity and quantitative recovery (Sohrin et al. 2008; Biller and Bruland 2012). Adjusted seaFAST software settings were a 17 second load loop time and a single 10mL load cycle. Process blanks consisted of pH 2 HCl (Optima, Fisher Scientific) and were processed as samples were to account for any contamination introduced by instrument processing.
Reagents consisted of a 1.5M ammonium acetate pH 6.0 buffer made using glacial acetic acid and ammonium hydroxide (20-22%) of the highest purity (Optima, Fisher Chemical), a 1% nitric acid rinse solution (Optima grade, Fisher Chemical), and a 10% nitric acid elution buffer (Optima grade, Fisher Chemical) with 10 ppb indium (115In, SPEX CertiPrep) added as an internal standard. Solutions were prepared with 18.2 Ω Milli-Q water (Millipore). Polypropylene 15mL centrifuge tubes used in sample processing were made trace metal clean by soaking in 10% HCl for 5 days and rinsing with pH 2 HCl prior to use.
Following offline seaFAST preconcentration, multielemental quantitative analysis was performed using an iCAP-Q inductively coupled plasma-mass spectrometer (ICP-MS) (Thermo Scientific). Oxide interference on metal isotopes was minimized through the use of a cooled spray chamber and helium collision gas. Analytes were measured in single quadruple mode (kinetic energy discrimination [KED]). Concentrations of Mn, Fe, Ni, Cu, Zn and Cd were determined using a six-point external standard curve of a multi-element standard (SPEX CertiPrep), diluted to range from 1-10 ppb in 5% nitric acid. An indium standard (SPEX CertiPrep) was similarly added to these standard stocks, diluted to range 1-10 ppb. Instrument injection blanks consisted of 5% nitric acid in Milli-Q. Standard curve R2 values were ≥0.98 for all metals monitored. Method accuracy and precision were assessed using the 2009 GEOTRACES coastal surface seawater (GSC) standard which produced values consistent with consensus results.
