| Radiogenic 87Sr and 206,207,208Pb isotopes and non-radiogenic 144Nd are preferentially leached during initial incongruent weathering of fresh bedrock and sediments in glacial terrains. With increased duration and intensity of weathering, isotopic compositions of runoff approach parent-rock values. Therefore, magnitudes of offsets between Sr, Nd and Pb isotope values of water and bedrock provide information about the extent of weathering. Average weathering conditions and intensities have been reconstructed for the last deglacial and over Pleistocene and longer time scales from records in deep sea sediments based on total fluxes of radiogenic isotopes. Weathering intensity varies among drainage systems beneath continental glaciers, pro-glacial streams, and de-glaciated watersheds unconnected to the glaciers, however, suggesting marine records may be complicated by differing fluxes for each type of watershed. Yet no studies have evaluated fluxes of these isotopes from the different types of watersheds to the oceans, and thus their relative contributions to marine records are unknown. This project addresses the hypothesis that as the Greenland Ice Sheet (GrIS) retreats, increased exposure and weathering will alter fluxes of radiogenic isotopes to the oceans. Specific hypotheses to be tested are (1) more radiogenic Sr and Pb and less radiogenic Nd discharge in water originating from the GrIS than from de-glaciated watersheds without connections to the ice sheet, (2) values of isotope ratios depend on the exposure age of the de-glaciated watersheds and the intensity of weathering in each environment, and (3) the total flux of radiogenic isotopes from continental glaciers depend on the relative aerial extents of glaciated and de-glaciated watersheds. These hypotheses will be tested in western Greenland near Kangerlussuaq, where retreat of the GrIS since the Last Glacial Maximum has exposed an approximately 175 km wide strip of land. |
