Mean concentrations of DFe (upper left), PFe (upper right), TDFe (lower left) and salinity (S, lower right) are plotted as a function of ice thickness (h) and analysed for non-parametric Spearman’s rank correlation. The statistical parameters apply to all data points in a given panel. The significant increase in [DFe] with ice thickness illustrates the continuous pumping of seawater DFe into sea ice. If DFe were a passive solute, [DFe] would decrease with thickness in the same manner as S. The correlations are weak for [TDFe] and [PFe], hence they must be driven by different processes from DFe.