Collection protocols for the South Atlantic Ocean seawater samples (Owens et al., 2017): a "trace metal-clean" titanium rosette with a PTFE-coated CTD system was used and deployed using a plasma rope to collect the seawater samples. Seawater samples were filtered through 0.2-micrometer (μm) filter capsules and transferred into 1-liter (L) HDPE bottles in a trace metal clean container. Then seawater samples were acidified at WHOI (Woods Hole Oceanographic Institution) to pH ~2 using concentrated distilled HCl.

Collection protocols for the Black Sea seawater samples (Cruise Report 64PE373 on RV Pelagia): an ultra-clean titanium CTD system was deployed to collect the seawater samples. The collected seawater was filtered with 0.2 μm Sartobran 300 filter cartridges (Sartorius) and acidified on board to pH ~2 with Baseline grade Seastar HCl. (See: https://www.geotraces.org/ga04-2/)

Here we describe the general protocols for chemical procedures. The in-lab column chromatography procedures to pre-concentrate and purify seawater vanadium (V) for concentration and isotope composition measurement are described in detail in Li et al. (under review). The chemical reagents utilized were trace-metal clean.

For seawater V concentration measurement, the column chromatography utilizing Bio-rad AG 50W-X12 cation resin was used to pre-concentrate V (Li et al., under review). The acidified seawater samples (pH ~2) were doped with H2O2 solution to contain 2% H2O2 before being loaded onto the resin column. Vanadium was yielded with weakly acidic H2O2 solution (0.01 M HCl + 2% H2O2) with the matrix metal elements being retained on the resin.

For seawater V isotope composition measurement, a four-column chromatography (Wu et al., 2019) was used to pre-concentrate and purify seawater V. Details of further improvements on this column chromatography method have been described in Li et al. (under review). The pre-cleaned Hitachi Nobias Chelate PA-1 resin and Bio-rad AG1-X8 anion resin were utilized to pre-concentrate and purify seawater V. The first column utilized Nobias Chelate PA-1 resin. Seawater samples were buffered to pH at 6.0 ± 0.1 with acetic acid and ammonia solutions before being loaded onto the column. Then V was yielded with 3 M HNO3. The following three columns utilized Bio-rad AG1-X8 anion resin to further purify seawater V, and the yielded V from the previous column was redissolved in 0.01 M HCl + 2% H2O2 solution before being loaded onto the column. Then, clean the resin column after the complete dripping of the sample solution using 0.01M HCl + 2% H2O2. Finally, V was yielded with 6M HCl and 2M HNO3.

The artificial seawater standards SW Matrix-AA and SW Matrix-BDH were made as in-lab standards for isotope composition measurement. The matrix elutes of two seawater samples were yielded after the first Nobias resin column and were separately doped with pure V solution standards, ~900 nanograms (ng) AA-V (SW Matrix-AA) and ~900 ng BDH V (SW Matrix-BDH). The two artificial seawater standards were processed parallelly with seawater samples with the same chemical procedures.