Reviews and syntheses: Rivers are major sources of dissolved iron to the ocean

Abstract. Iron (Fe) is a bio-essential micronutrient for all known life. Due to sparse solubility in seawater under oxic conditions, Fe availability often limits, or co-limits, primary production in the ocean. Extensive research in recent decades has therefore aimed to constrain the key Fe sources and sinks in the global ocean. Accordingly, Fe is now recognised as a key nutrient in ocean biogeochemical models, and determining how Fe availability is changing in response to shifts in source dynamics alongside ocean acidification, deoxygenation, and warming remains a major research challenge. A long-standing hypothesis is that rivers make little direct contribution to Fe availability in the ocean on annual-to-interannual timescales. Strong emphasis has instead conventionally been placed on atmospheric deposition, and to a lesser extent shelf sediments and hydrothermal vents, as the major new Fe sources relevant to offshore marine environments. As widely demonstrated in many estuaries worldwide, dissolved Fe (dFe) rapidly flocculates across salinity gradients which markedly attenuates riverine dFe fluxes to the ocean. However, recent work in both the Congo and the Transpolar Drift has challenged the notion that estuarine removal precludes a significant role for rivers in offshore Fe budgets. Multiple analytical approaches have consistently implied dominant contributions of riverine dFe to basin-scale budgets for both the Arctic and the South Atlantic Oceans. In both cases, plumes enriched in river-derived dFe are found to extend >1000 km off-shelf and exceed atmospheric deposition as the major regional dFe source. Yet these dFe plumes, and the associated Fe fluxes, appear to be consistently absent from ocean biogeochemical models. These observations question the long-standing paradigm that rivers are only of limited importance to the oceanic Fe cycle and indicate a refinement of budget and model representations is needed.