Methodology:
This work analyzed V isotope composition of previously collected seawater samples. Below is a description of the methods used for the samples collected by various cruises. Six seawater samples were anlalyzed in this study, several using multiple methods and replicate analysis. The studied samples include the North Atlantic Surface Seawater (NASS-6) reference material distributed by National Research Council of Canada (NRC - CNRC), three samples from the Bermuda Atlantic Time-series Study (BATS), one from the Gulf of Mexico, and one from the Pacific Ocean. NASS-6 is a seawater reference material collected from surface water at Sandy Cove, Nova Scotia (44°03.10'N, 64°42.20'W) in March 2007. The certified concentrations of some metals, including vanadium, are available from NRC/CNRC (https://www.nrc-cnrc.gc.ca/ eng/solutions/advisory/crm/certificates/nass_6.html). Three BATS samples were collected with Go-Flo samplers mounted on a rosette equipped with a CTD instrument on R/V Atlantic Explorer from Bermuda Institute of Ocean Sciences (BIOS), Cruise AE0908 on September 2009 (32°18.50'N 64°31.10'W). These unfiltered BATS samples are sampled at depths of 150 m, 650 m, and 750 m and acidified to pH <2 with concentrated HCl and then stored in acid-washed 10 L polypropylene cubitainers for transporting to the laboratory. Surface seawater (~2–3 m) from the Gulf of Mexico (26°60'N, 85°200'W) was collected in 2015 using a trace metal-clean towed 'fish', filtered during collection at sea using an acid-washed Acropak-200 capsule filter (0.2 mm pore size), and stored unacidified in an acid-washed cubitainer until subsampled for analyses. Samples were then acidified to 0.024 M HCl (ultrahigh purity, distilled) and allowed to sit at least three days before column chemistry. Deepwater from the North Pacific Ocean was collected at 3500 m at Station ALOHA (A Long-term Oligotrophic Habitat Assessment; ~22°450'N, 15°8'W, here denoted as S9) during the 2002 Intergovernmental Oceanographic Commission (IOC) cruise using trace metal-clean 30-L Go-Flo samplers, filtered shipboard using acid-cleaned 142-mm Nuclepore PCTE filters (0.4 mm pore size), and stored acidified (0.012 M HCl, Optima) in acid-washed 500 ml HDPE bottles since 2002. All acids were trace metal grade (e.g., Aristar) or higher, and all water was purified using a Milli-Q deionization system to ≥18.2 MΩ per cm².

Sampling and analytical procedures:
Instrumental Configuration: Instrument measurements were performed at both Woods Hole Oceanographic Institution (WHOI) and the National High Magnetic Field Laboratory at Florida State University (FSU) with similar configurations at both institutions using a Thermo Scientific Neptune Multicollector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS). Measurements were performed on the flat-topped shoulder on the lower mass side of the overlapping V and molecular interference peaks in medium-resolution mode (resolution > 4000) to resolve all interfering molecular species representing combinations of C, N, O, S, Ar and Cl (such as ³⁶Ar¹⁴N+, ³⁶Ar¹⁶O+, and ³⁸Ar¹⁴N+). Jet sample and Ni X-skimmer cones were used to obtain the highest possible V transmission efficiency. In addition, the amplifiers with 10¹⁰Ω and 10¹¹Ω resistor were applied to monitor ⁵¹V and ⁵⁰V signal, respectively. We applied dry plasma inlet system with Aridus II desolvator (CETAC Technologies). The typical sensitivity under such configuration was ~150–250 volts/ppm. The configuration requires 400 ng of V for at least one V isotope measurement.

Vanadium Purification:
WHOI Method: Four columns were used to separate vanadium from matrix elements. The first two columns consist of a large quartz column, stem of 10 cm in length and 0.6 cm in diameter, with quartz wool inserted as a porous barrier to retain the resins. The first column pre-concentrates V (and other metals) from the salt matrix of seawater and used 1.5 mL of Nobias PA-1 chelating resin (Hitachi High-Technologies, 45–90 lm mesh size) at pH ~ 6. For the chemical procedures described below, prior to adding new or more solution the previous volume was completely discharged. Nobias resin was first activated and cleaned using 3 mL methanol. Then 1 ml of 3 M HNO₃ was added and completely drained prior to another addition of 4.5 mL 3 M HNO₃ (documented as 1 + 4.5 mL hereafter) in sequence and then 1 + 4.5 mL H2O was added to wash out the acid. Before load on seawater, the resin was preconditioned with 1 + 6 mL of ammonia acetate buffer solution (pH ~ 6), which was made by mixing ammonium hydroxide, acetic acid and de-ionized water. Approximately 250 mL of seawater, which was first adjusted to pH of 6 ± 0.1 using the ammonia acetate buffer solution, was loaded onto the column. Under these conditions trace metals were adsorbed onto the resin while the major seawater salt matrixes (Na, K, Mg, Ca) were eluted. Subsequently, V and other retained metals were then eluted and collected with 1 + 15 mL of 3 M HNO₃. The eluted solution was dried down, and re-dissolved into 0.01 M HCl with 1% (volume/volume) H₂O₂. Further purification of V was achieved through exchange columns with AG 1-X8 200–400 mesh anion resins (Bio-Rad Laboratories). The anion resins were repeatedly soaked and cleaned with 6 M HCl and H2O successively for several times before use. In the second column, 1 mL of AG 1-X8 200–400 mesh anion exchange resin was loaded in the quartz columns and cleaned with 5 mL 1 M HCl. Then the resin was pre-conditioned with 3 + 3 mL 0.01 M HCl + 1% (volume/volume) H₂O₂. In this solution pentavalent V forms anionic complexes with H₂O₂ that are strongly adsorbed onto the resin. Because of its rapid dissociation, hydrogen peroxide should be added immediately prior to loading the sample onto the column. After loading the samples onto the resin bed, another 2 + 10 mL 0.01 M HCl + 1% (volume/volume) H₂O₂ was loaded to elute off the residual matrix compounds including Cr. Vanadium was subsequently eluted and collected with 1 + 5 mL 1 M HCl. The AG 1-X8 anion exchange resin was used for only once and throwing away after use. The solution was dried down, and re-dissolved into 0.5 mL 2 M HF. The final two columns were employed to ensure quantitative removal of residual Ti and Cr. They were minicolumns with 0.1 mL resin volume using AG 1-X8 200–400 mesh resin. The resin was cleaned and pre-conditioned with 1 + 1 mL of 1 M HCl and 0.5 + 0.5 mL of 2 M HF, respectively. Sample solutions were loaded on to the column, and the eluent was immediately collected. Another 0.5 mL of 2 M HF and then 0.1 + 1.3 mL of 0.5 M HCl + 0.5 M HF were also added and collected together with the sample load. The last mini-column was identical to the second column except that all volumes were scaled down by a factor of 10, which is applied to further eliminate Cr. In between each column procedure, samples were refluxed in aqua regia for at least four hours to break down minor amounts of organic resin material that had eluted together with V. The final purified V sample was dissolved in 2% (volume/volume) HNO₃ before isotope analysis.

FSU Method: Four columns were applied to separate vanadium from matrix elements and interferences. The first column was identical to that used in the WHOI method. The second column used the same general procedure as the WHOI method, while relied on different column dimensions and resin and reagent volumes. In addition, we found that Ti could also be eluted with Cr if we use 0.1 M HCl + 2% (volume/volume) H₂O₂ rather than 0.01 M HCl + 1% (volume/volume) H₂O₂ for the anion column. Thus, we further modified the anion column step at FSU. The procedure was performed in a pre-cleaned BioRad Poly-Prep chromatography column (9 cm high and 2 mL bed volume) loaded with 2 mL of AG 1-X8 200–400 mesh resin. The AG 1-X8 resin was first cleaned and preconditioned using 15 mL of 6 M HCl, 10 mL of H2O, and 4 + 4 mL of 0.01 M HCl with 2% (volume/volume) H₂O₂, respectively. The sample that had been dissolved in 10 mL of 0.1 M HCl + 2% (volume/volume) H₂O₂ was then loaded on the column. Another 1 + 14 mL of 0.01 M HCl and 2% (volume/volume) H2O2 was added to remove residual matrix elements including Cr. The V portion was subsequently eluted and collected with 14 mL of 6 M HCl + 10 mL of 2 M HNO₃. The solution was then dried down, and re-dissolved into 1 mL 0.01 M HCl with 2% (volume/volume) H₂O₂. Similar mini-columns to those used in the WHOI method with 0.1 mL AG 1-X8 200–400 mesh resin were also used in the FSU method to remove minor amounts of residual Ti and Cr. However, the columns that utilize HF were omitted from the FSU method. For the third column procedure, resin loaded onto the columns were cleaned and pre-conditioned with 1 mL 6 M HCl, 1 mL H2O, and 0.75 + 0.75 ml 0.01 M HCl with 2% (volume/volume) H₂O₂, respectively. Samples in 1 mL of 0.01 M HCl with 2% (volume/volume) H₂O₂ were then loaded and matrix elements were eluted with 0.3 mL of 0.1 M HCl with 2% (volume/volume) H₂O₂ and 1.5 mL of 0.01 M HCl with 2% (volume/volume) H₂O₂ successively. The additional step using 0.1 M HCl was found to be efficient at removing Ti. The V was subsequently eluted and collected with 1.5 mL of 6 M HCl + 1.5 mL of 2 M HNO₃. The fourth column was a repeat of the third and was employed to ensure complete removal of Cr and Ti. As described for the WHOI method, samples were refluxed in aqua regia in between all columns and dissolved in 2% (volume/volume) HNO₃ for isotopic analysis.

V_isotopes_seawater.csv