Iron supply to the Amundsen Sea, Antarctica is dominated by circumpolar deepwater and continental subglacial sources

Glacial melting in West Antarctica has intensified with the increased intrusion of warm ocean water beneath ice shelves, but the processes controlling the export of meltwater-associated micronutrient iron (Fe) to Southern Ocean surface waters remain unclear. Here, we report Fe concentrations and dissolved Fe (dFe) isotope ratios in the inflowing deepwater layer that drives melting of the Dotson Ice Shelf and in the meltwater-enriched outflow to determine meltwater-derived dFe. Isotopic mass balance points to an anoxic Fe-reducing region of the upstream subglacial hydrologic system as the dominant source of meltwater dFe, rather than ice shelf melt itself. Remarkably, total meltwater contributes only ~10% of outflowing dFe, with the majority contributed by inflowing deep water (62%), augmented by inputs from shelf sediments (28%). Outflowing suspended particulate Fe exceeds inflow by 46%, at 100 times the dFe concentration, with 25% in reactive phases. Predictive models of future ecosystem effects should consider that the primary role of ice shelf melting is to provide buoyancy that transports Fe from deep sources to the Fe-limited surface ocean, stimulating phytoplankton growth. Meltwater generated within the Dotson Ice Shelf cavity supplies little dissolved iron. Most meltwater-derived iron found in water flowing out of the cavity comes from subglacial discharge, based on iron concentrations and dissolved iron isotope ratios.