Seasonal trends, mineralogy, and speciation of particulate and dissolved iron in West Greenland meltwater outflows

Geochemical and physical weathering processes can affect the chemical evolution of glacial waters and release particulate and dissolved iron via glacial runoff; however, the control of these processes by the lithology and the glacial environment is poorly understood. This study characterizes the reactivity and potential bioavailability of particulate bound Fe from two glacial outflows of the Greenland Ice Sheet (GrIS) to determine the influence of glacial setting (cold-based versus polythermal) and lithology on iron fluxes. Samples were collected over two entire melt seasons from supraglacial and subglacial waters, and sequentially extracted Fe from suspended sediment was analyzed. Results show seasonal patterns of dissolved Fe, higher concentrations of reactive Fe in the north (15.8 +/- 6.8 versus 6.7 +/- 4.0 mg/g) in a predominantly sedimentary catchment, and a higher ratio of bioavailable iron in the south, mostly contributed through subglacial weathering. This suggests that, in order to accurately determine Fe fluxes for the GrIS, Fe concentrations must be measured over an entire melt season due to the variability of highly reactive Fe concentrations measured in different glacial settings and lithologies. The bioavailable particulate Fe flux estimated for the entire GrIS from these data is 0.74 Tg/yr. With increasing melt duration and freshwater runoff of GrIS, it is likely this amount will increase in future years.