Seasonally-evolving hydrological controls on dissolved iron exported in Greenland glacial meltwater streams

Glacial runoff has the potential to deliver large amounts of reactive dissolved and particulate iron to surrounding marine environments. Here we investigate a glacial system along the western margin of the Greenland ice sheet where multiple land-terminating glaciers feed a large proglacial lake which then drains into a nearby fjord. We explore the temporal dynamics of iron export in this system using a suite of samples from a single meltwater-derived stream near the ice-sheet edge characterizing the early (May) and late (July) melt-season. We find that dissolved (<0.2 μm) iron concentrations in May are variable, but high (~3-9 μM), and that these concentrations decrease in July (~2-4 μM). Building on our previous work at this site, we hypothesize that the seasonal evolution of the subglacial drainage structure from a delayed-flow distributed network to an efficient channelized system alters the subglacial biogeochemical weathering regime and associated iron export. We also probe the potential for further biogeochemical processing in the proglacial environment to enhance glacially-derived dissolved iron concentrations prior to export to the marine environment. Using a suite of point samples we find that reactions both during floodplain transport and within the proglacial lake serve to increase dissolved iron concentrations to ~10 μM and ~18 μM, respectively. Given that our field site is representative of many land-terminating glacial systems, these findings may be broadly representative of dissolved iron concentrations in Greenland meltwater runoff that transits through a proglacial environment. Meltwater input to the ocean is highest during the summer meltseason, coincident with depleted marine nutrient concentrations associated with the spring phytoplankton bloom. Consequently, glacially-derived runoff may serve as a significant iron source in coastal ecosystems surrounding Greenland.