Dataset: FK160115 Dissolved Cobalt

Dissolved seawater samples were collected on the ProteOMZ FK150116 expedition (Jan-Feb 2016) using a 12-bottle trace metal clean rosette equipped with 8 L X-Niskin bottles (Ocean Test Equipment), a titanium frame, and a Kevlar cable, as described in (Cutter and Bruland 2012). Seawater from the Go-Flo bottles was subsampled in a trace metal clean plastic "bubble" equipped with HEPA filters providing positive air pressure. Dissolved Co subsamples were filtered using a 142 mm, 0.4 um polycarbonate plastic sandwich filter (Geotech Environmental Equipment) and stored until analysis in the laboratory in a 60 mL LDPE bottle (Nalgene) that had been soaked for ~1 week in Citranox, an acidic detergent, rinsed with Milli-Q water (Millipore), soaked for ~2 weeks in 10% trace metal grade HCl (Optima), and rinsed with lightly acidic Milli-Q water (<0.1% HCl). To preserve the samples for future analysis after the cruise, all samples were filled entirely with no remaining headspace, stored with one oxygen-absorbing satchel (Mitsubishi Gas Chemical, model RP-3K) per 60 mL water sample, heat-sealed in plastic bags, and stored at 4⁰C.

Dissolved cobalt (dCo) samples were analyzed in Nov-Dec 2017 by cathodic stripping voltammetry (CSV) as originally described by (Saito and Moffett 2001) and modified by (Saito et al. 2010; Hawco et al. 2016). One portion of the samples (stations 1-10) were analyzed in the Saito laboratory in Woods Hole, MA, and the remaining samples (stations 11-14) were analyzed at sea on the CICLOPS NBP-1801 cruise in the trace metal plastic "bubble". This was done so that all of the samples could be run together using the same batch of DMG and EPPS reagents.

Dissolved Co was measured using a Metrohm 663 VA and uAutolabIII system equipped with a hanging mercury drop working electrode. All reagents were run through treated Chelex-100 resin columns (BioRad) to remove trace metal contaminates, except DMG which was purified by recrystallization. The Chelex was resin prepared as described in (Price et al. 1988/89) and reagents were purified as described in Saito et al., 2002. 0.2 um filtered water samples were UV-irradiated in acid-washed quartz tubes for one hour using a water-cooled UV irradiation system (Metrohm 705 UV Digestor) to destroy natural ligand-bound Co complexes. Then, 11 mL of sample seawater was aliquoted into 15 mL acid-washed polypropylene vials, and 100 uL of 0.1 M dimethyglyoxime (DMG, Sigma Aldrich) and 130 uL of 0.5 M N-(2-hydroxyethyl)piperazine-N-(3-propanesulfonic acid) (EPPS, Sigma Aldrich) buffer was added. The samples were then processed on an autosampler (Metrohm 858 Sample Processor), which added 8.5 mL of the sample solution and 1.5 mL of a 1.5 M NaNO₂ reagent (Merck) to a Teflon cup for electrochemical analysis. The mercury electrode performed a fast linear sweep from -1.4 V to -0.6 V at a rate of 5 V s⁻¹, which reduced the Co bound in the Co(DMG)₂ complex from Co(II) to Co(0) and produced a Co reduction peak at -1.15 V (Saito and Moffett 2001) with a height linearly proportional to the amount of dCo present in the sample. A standard addition curve was generated for each sample analyzed with 4 automated additions of a 25 pM CoCl₂ (Fisher Scientific) standard made fresh in for this sample batch.