HIGH LATITUDE HYDROLOGY: WATER IN A CHANGING WORLD

Since the Last Glacial Maximum, continental ice sheets have retreated from approximately 15% of the terrestrial earth surface. This retreat exposed watersheds underlain by fine-grained sediments that are susceptible to chemical weathering. These landscapes are comprised of either proglacial watersheds or “deglaciated” watersheds, from which “non-glacial” streams drain. In western Greenland, non-glacial and proglacial streams have similar specific discharge even though individual proglacial streams can have large annual discharges. The similarity in specific discharge suggests that water composition, rather than water volume, is the primary control of gas, solute, and isotopic fluxes to the ocean and atmosphere. Stream compositions vary as weathering reactions in watersheds shift towards completion with increasing exposure age and precipitation. However, processes driving these changes have rarely been evaluated in the foreland of an actively retreating continental ice sheet. To evaluate these processes, we have analyzed chemical and isotopic compositions of gas, water, and sediments from deglaciated watersheds extending from recently exposed arid watersheds at the ice edge to humid watersheds with longer exposure ages at the coast in western and southern Greenland. These samples show that dissolved 87/86Sr ratios are >0.003 greater than bedload values in recently exposed ice-proximal proglacial watersheds and the difference decreases to near zero in coastal watersheds. Mass balance models of major element concentrations indicate weathering reactions shift from predominately carbonic acid weathering of carbonate minerals in ice-proximal watersheds to predominately sulfuric acid weathering of silicate minerals in coastal watersheds. These different weathering reactions contribute to distinct CO2 fluxes from the landscape. Measured gas concentrations at subglacial outflows suggest CO2 and CH4 sources depend on both the amount of organic matter sequestered beneath the ice sheet and the extent of mineral weathering in subglacial environments. Proglacial stream compositions are affected by extensive hyporheic exchange in sandurs, while ecosystem development decreases P and Fe fluxes. Differences in gas, solute, and isotopic compositions indicate that ice sheet retreat during glacial terminations changes riverine oceanic and atmospheric fluxes; understanding these differences could provide predictive capability for continued changes in fluxes as ice sheets retreat in a future warmer world.