Dissolved Fe and Fe-binding organic ligands in the Mediterranean Sea - GEOTRACES G04

Fe-binding dissolved organic ligands increase the solubility of Fe enabling high dissolved Fe concentrations, and hence longer residence time. These ligands had median total concentrations between [Lt] = 0.77 and [Lt] = 1.74 nEq of M Fe and conditional stability constants between logK′ = 21.57 and logK′ = 22.13 (N = 156). Median values of [Lt] were higher in the upper 100 m and its median concentration increased from west to east. The [Lt] concentrations did not relate to water mass or DFe concentration. The ligands were nearly saturated with Fe where DFe was elevated near the surface and completely saturated, ratio [Lt]/DFe ≤ 1, in patches with high DFe at depth. The high DFe concentrations in these patches are extreme, if not even maximum, concentrations as any surplus Fe with respect to the ligands will tend to precipitate. Calculated inorganic Fe concentrations in the Mediterranean had minimum concentrations of 0.23 pM and below 100 m depth median concentrations that varied between 0.68 and 1.99 pM only. This suggests that the inorganic Fe concentration is the result of a steady state between binding by organic ligands and scavenging processes. Thus scavenging will not result in lower inorganic Fe concentrations and in this way the dissolved ligand concentration determines the concentration of DFe in the Mediterranean Sea.