Dissolved Iron Concentration and the Solubility Inferred by Humic-Like Fluorescent Dissolved Organic Matter in the Intermediate Water in the North Pacific Including the Marginal Seas

The marginal seas have been found to be important external sources of dissolved iron (Fe) in the North Pacific through circulation of intermediate water. Here, we show comprehensive spatial distributions of dissolved Fe concentrations and Fe(III) solubilities inferred from humic-like fluorescent dissolved organic matter (FDOMH) over the North Pacific, including the marginal seas, the Sea of Okhotsk, and the Bering Sea. FDOMH was used as a proxy of chemical speciation of dissolved Fe in the intermediate and deep waters where liner relationships were previously observed between FDOMH and Fe(III) solubility. When dissolved Fe concentration exceeds Fe(III) solubility, Fe(III) solubility is assumed to be equivalent to concentration of FDOMH-Fe complexes, and excess dissolved Fe concentration above the Fe(III) solubility is assumed to be colloidal Fe which is not complexed with FDOMH. In the intermediate water, the dissolved Fe concentration exceeded the Fe(III) solubility in the marginal seas, while excess Fe(III) solubility was evident downstream of the intermediate water circulation, suggesting that the major dissolved Fe chemical form derived from shelf and slope sediments in the marginal seas changed from colloidal Fe to FDOMH-Fe complexes. With the previous findings, namely the dominance of labile particulate Fe in total Fe in the intermediate water of the Sea of Okhotsk and the Bering Sea, we hypothesized that the average size of sediment-derived Fe decreases during transportation by intermediate water, most likely due to reversible scavenging with the highest removal rate for labile particulate Fe and the lowest removal rate for FDOMH-Fe complexes.