Methodology:
The following methods are provided from a manuscript currently in preparation (Burns et al. in prep.).

Sampling and analytical procedures:

Sample Collection:

Depth profile samples were collected in austral spring 2016 aboard the R/V/I/B Nathaniel B. Palmer using a SeaBird GEOTRACES style SBE32 rosette system deployed on a conducting Kevlar line (Cutter and Bruland 2012) with OceanTestEquipment, Inc. X-Niskin samplers modified for trace element sampling. Samples were filtered by gravity through 0.2 µm Pall AcroPakTM membrane filter capsules and collected in 125 mL low density polyethylene (LDPE) bottles (Nalgene; Fisher Scientific) that had been cleaned first in a soap (3% Citranox, Fisher) bath, rinsed three times with ultrahigh purity water (Milli-Q, >18.2 MΩ cm), soaked in 25% nitric acid (HNO3, Fisher, Trace Metal Grade) for at least one month, rinsed three times with ultrahigh purity water and stored filled with ~0.024 M hydrochloric acid (HCl, Fisher, Trace Metal Grade). Bottles were rinsed three times with sample seawater prior to filling. Samples were acidified to pH 1.8 (0.024 M HCl, Fisher, Optima) and stored double bagged in buckets at room temperature until analyzed at the University of South Florida.

Dissolved Trace Metals:
Extraction and pre-concentration of the dissolved samples was performed using the seaFAST-pico system (Elemental Scientific) offline (Lagerström et al. 2013; Bown et al. 2017; Rapp et al. 2017). The commercially available Nobias-chelate PA1 resin (Sohrin et al. 2008; Sohrin and Bruland 2011; Biller and Bruland 2012) in the seaFAST preconcentration column concurrently extracts the trace metals of interest in this study: Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb. To sufficiently extract dissolved Co and Cu, ultraviolet (UV) oxidation of the dissolved samples was conducted prior to seaFAST extraction (Achterberg et al. 2001; Milne et al. 2010; Biller and Bruland 2012). To accomplish this, dissolved samples were poured into acid-cleaned Teflon™ 30-mL vials (Savillex) with Teflon™ caps custom-fitted with transparent quartz window, and UV oxidized for 90 minutes at ~20 mW cm-2 in a UVO-Cleaner® (Jelight Model No. 342) after a 30-minute system warm-up.

During the seaFAST extraction process, UV-oxidized samples were buffered to a target pH range of 6.0 to 6.5 (Lagerström et al. 2013). To conserve the buffer reagent, the seaFAST buffer flow rate was adjusted in the submethod from 400-650 to 400-350 sec-μL/min. To make the ammonium acetate (NH4Ac) buffer, a solution of 5.3 M glacial acetic acid (HAc, Fisher, Trace Metal Grade) and 2.6 M ammonium hydroxide (NH4OH, Fisher, Optima) in Milli-Q was adjusted to pH 7.4 ± 0.2 with small additions of either HAc or NH4OH.

For preconcentration of the extracted trace metals, the seaFAST software method was programmed to take up one 10-mL loop of sample seawater and elute the extracted trace metals with 400 μL of elution acid. The elution acid was 0.74 M triple-distilled nitric acid (HNO3) containing 10 ppb indium (In) and rhodium (Rh) internal standards. The HNO3 was triple-distilled using a Savillex DST-1000 Acid Purification System prior to use. The eluent was eluted into acid-cleaned, 2.0 mL PVDF vials (Elemental Scientific) with TeflonTM caps (Elemental Scientific). A 0.30 M HNO3 (Fisher, Trace Metal Grade) rinse for the seaFAST autosampler probe was used between each sample.

Quality control (QC) checks were included in seaFAST runs. GEOTRACES 2008 GS and SAFe 2004 D2 reference samples were measured to assess accuracy. Additionally, QC seawater samples were run approximately every 15 samples to monitor instrument precision over time. The first QC was offshore seawater from the Antarctic Circumpolar Current (ACC), acidified to pH 1.8 (0.024 M) with Optima (Fisher) HCl. The second QC was from offshore Eastern Pacific Zone seawater, acidified to pH 1.8 (0.024 M) with Optima (Fisher) HCl.

Two sets of standard curves were made for these analyses: one set in ACC QC seawater (acidified to 0.024 M with Optima HCl) and a second set in the elution acid (0.74 M triple-distilled HNO3 containing 10 ppb In and Rh). For the mixed metal standard curves, stock solutions were made in 1.49 M Optima (Fisher) HNO3 using 1,000 ppm standards (ULTRA Scientific) of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb. Each curve was a minimum of six points and made to cover the concentration ranges of the dissolved trace metals in incubation samples.

The eluents from the seaFAST were analyzed on a Thermo Scientific magnetic sector Element XR High Resolution Inductively Coupled Plasma Mass Spectrophotometer (HR-ICP-MS). In between each sample, the autosampler probe was rinsed twice in 0.74 M TraceMetalGrade (Fisher) HNO3, to avoid sample carryover. To account for any interference of MoO+ on Cd counts, a three-point Mo calibration curve was made in elution acid and the slope of the Mo counts plotted against Cd counts was used to adjust Cd counts.

Trace metals were quantified by standard addition from the seawater standard curves. The average counts for each trace metal were normalized to the In internal standard counts per sample, to account for daily drift in ICP-MS measurements. Dissolved trace metal concentrations in each seawater sample were calculated from the seawater calibration curve slope. The In-normalized average trace metal counts were divided by the seawater standard curve slope to yield the trace metal concentrations in each eluent.

Air blanks were measured with a minimum of three replicates per seaFAST and Element XR run. For the air blanks, the seaFAST method was run as usual, but taking up air instead of acidified seawater. For the dissolved trace metal concentrations presented here, the average air blank concentrations per seaFAST run were subtracted from the dissolved sample concentrations to account for the procedural blank.

Sample analyses for dissolved trace metals were performed by Shannon Burns (USF); ORCID ID: https://orcid.org/0000-0002-1569-3060.

Quality Flags: The standard Ocean Data View qualifying flags were used (reference all flags at https://www.bodc.ac.uk/data/codes_and_formats/odv_format/). Additional notes specific to the application of these flags to this project are noted in brackets […].

1: Good Value: Good quality data value that is not part of any identified malfunction and has been verified as consistent with real phenomena during the quality control process. [See Table 1 for reference sample data.]
2: Probably Good Value: Data value that is probably consistent with real phenomena but this is unconfirmed or data value forming part of a malfunction that is considered too small to affect the overall quality of the data object of which it is a part. [Not used.]
3: Probably Bad Value: Data value recognized as unusual during quality control that forms part of a feature that is probably inconsistent with real phenomena. [Used when data appeared anomalous.]
4: Bad Value: An obviously erroneous data value. [Not used.]
5: Changed Value: Data value adjusted during quality control. [Not used.]
6: Value Below Detection Limit: The level of the measured phenomenon was too small to be quantified by the technique employed to measure it. The accompanying value is the detection limit for the technique or zero if that value is unknown. [Not used. See Table 1 for detection limits.]
7: Value in Excess: The level of the measured phenomenon was too large to be quantified by the technique employed to measure it. The accompanying value is the measurement limit for the technique. [Not used.]
8: Interpolated Value: This value has been derived by interpolation from other values in the data object. [Not used.]
9: Missing Value: The data value is missing. Any accompanying value will be a magic number representing absent data. [Not used.]
A: Value Phenomenon Uncertain: There is uncertainty in the description of the measured phenomenon associated with the value such as chemical species or biological entity. [Not used.]