Data from Polarstern cruise PS94 in the Arctic in 2015 with chief scientist Ursula Schauer. In order to explain geochemical cycles of Fe in the ocean, M. Rijkenberg, L. Gerringa and H. Slagter a.o. sampled filtered seawater using an ultra clean titanium CTD. In these samples on board measurements were executed: dissolved Fe by flow injection analysis and the characteristics of the dissolved organic Fe-binding organic ligands (totalL); characteristics being the concentration of these ligands and the conditional binding constant of the Fe-complex (K' expressed as its logarithm: logK). Moreover in order to relate the dissolved organic Fe-binding organic ligands to humic substances, fluorescence of dissolved organic matter was measured as well as humic substances using voltammetry.

Sampling of sea water was performed using the ultraclean CTD (Rijkenberg et al, 2015).Samples for dissolved metals were acidified to a pH of 1.8 using a final concentration of 0.024M ultraclean Seastar Base-line HCl (Seastar Chemicals). DFe was measured on board using flow injection analysis (FIA) based on luminol chemiluminescence (described in Rijkenberg et al., 2018). The analytical blank was on average 0.007 ± 0.004 nM DFe (n = 32) and the average detection limit (defined as 3 s of the blank pre-concentrated for 5 s) was 0.012 ± 0.007 nM DFe (n = 32).SAFe and GEOTRACES reference materials were used.K and total ligand concentrations have been calculated after Gerringa et al. (2014) using lDFe, except for stations 101, 119, 153, 161 and 173, here DFe is used.Humid substances were determined according to Laglera et al. (2007).