The Role of Humic Substances in Iron Complexation and Solubilization in the Arctic Ocean and Northwest Atlantic Ocean

Iron is a critical micronutrient in the ocean, playing a limiting role in primary productivity across vast regions of the global ocean and significantly impacting global CO₂ uptake. Despite its importance, inorganic iron is highly insoluble under the physicochemical conditions of seawater. However, complexation by natural organic ligands prevents precipitation and enables iron concentrations to remain in the nanomolar and subnanomolar range. Identifying and characterizing these ligands remains a challenge. Humic substances (HS), a complex and hydrophobic mix of organic compounds, are considered key candidates for constituting a significant portion of these iron-binding ligands. HS concentrations and Fe-HS complex concentrations can be measured directly in seawater using recent voltammetric protocols.

In this presentation, we will report on Fe, HS, and Fe-HS concentrations measured across the Atlantic sector of the Arctic Ocean and the Northwest Atlantic Ocean. Samples collected during three different cruises reveal the significant variability in the role of HS in iron solubility, ranging from negligible to critical, depending on the biogeochemical and physical characteristics of the water masses encountered. We will discuss the influence of the Transpolar Drift outflow, the East Greenland Current, fjord water outflows, and Icelandic sills on HS distribution and iron solubility in the region.