3.5 Cr(III) samples
Dissolved chromium redox speciation (Cr(III)) samples were processed at sea using magnesium hydroxide (Mg(OH)2) co-precipitation, then analysed by isotope dilution ICP-MS on land at the University of Bern following the procedures of Janssen et al. (2020) (external [Cr(III)] 1σ = 10% RSD, Janssen et al., 2020).