Decoupling of dissolved iron and iron‐binding humic‐like substances along the West Antarctic Peninsula

Abstract

The chemical speciation of iron (Fe) has important ramifications for the stabilization, transport, and bioavailability of this critical nutrient. Organic ligands mediate dissolved Fe biogeochemistry such that ligand concentration and molecular composition help shape dissolved Fe distributions. Electroactive Fe‐binding humic‐like substances (“humic substances”) are one such class of ligands, a mixture thought to be predominantly comprised of degradation products of marine and terrestrial origin. Here, we examine the distributions of humic substances across large environmental gradients in dissolved Fe along the West Antarctic Peninsula in early austral spring. We found that the distributions of humic substances and dissolved Fe were decoupled. Concentrations of total Fe‐binding humic substances were tightly correlated with Fe bound to humic substances, or ambient Fe‐bound humic substances, but not with Fe, and total Fe‐binding humic substances were undersaturated in Fe across the study region. In the low Fe (< 1 nM) waters of the Drake Passage, ambient Fe‐bound humic substances accounted for 10–100% of the dissolved Fe pool. Increases in Fe near the continent, however, were not associated with increases in humic substances. Rather, humic substances remained unsaturated even in elevated Fe (2.3–9.5 nM) waters inshore and ambient Fe‐bound humic substances only accounted for 2–49% of the dissolved Fe pool. Our results suggest distinct sources of Fe and humic substances near the West Antarctic Peninsula and that organic complexation of Fe by humic substances here may be strongly influenced by competition for binding sites by other cations and/or with other Fe‐binding organic ligands.