North and equatorial Pacific seawater samples were collected on the GP15 expedition as part of the U.S. GEOTRACES program (RR1804 and RR1805, September to November 2018). Dissolved water samples were collected along a transect that mostly followed the 152W meridian from 76N to 20S, traveling from the Gulf of Alaska to Hawaii (Leg 1), and on to Tahiti, French Polynesia (Leg 2). Sampling locations included 20 deep full-depth stations with a bottom depth of over 4000 m, 5 shallow/coastal full-depth stations, and 10 surface stations where only the surface 1000 m was sampled.

Dissolved seawater samples were collected using a 24-bottle trace metal clean rosette equipped with 12 L Go-Flo bottles (General Oceanics), a titanium frame, and a Kevlar cable, as described in (Cutter and Bruland 2012). Surface water samples were collected from a trace metal clean towfish and pump while arriving at each station. Seawater from the Go-Flo bottles was subsampled in a trace metal clean van under positive pressure. Dissolved Co subsamples were filtered using a 0.2 um Acropack (Pall) and stored until analysis at 4 degrees C in a 60 mL LDPE bottle 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, and rinsed with lightly acidic Milli-Q water (<0.1% HCl). All Co values were analyzed at sea, but replicates of all samples were taken for future dCo analysis in the laboratory. Preserved 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, vacuum-sealed in plastic bags, and stored at 4 degrees C.

Dissolved cobalt was determined by cathodic stripping voltammetry (CSV) as originally described by (Saito and Moffett 2001) and modified by (Saito et al. 2010; Hawco et al. 2016). Measurement occurred shipboard within a trace metal clean plastic "bubble" within one week of sample collection using the Metrohm 663 VA and uAutolabIII systems equipped with a hanging mercury drop working electrode. For total dissolved Co (dCo) analysis, 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. For labile dissolved Co analysis (L-dCo), no UV-irradiation was performed. Then, 11 mL of sample seawater was aliquoted into 15 mL acid-washed polypropylene vials, and 33 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 curve was created with 4 additions of 25 pM CoCl₂ to each sample, and a linear regression of the addition standard curve allowed for the calculation of the initial amount of Co present in the sample, assuming all available Co binds to the DMG ligand. Reagent purification protocols and standard addition calculations are outlined in more detail in (Saito and Moffett 2001). Triplicate technical replicates were run on every sample to determine the precision of the method, and duplicate depths from different rosette casts were run when available.