VARIATIONS IN WEATHERING INTENSITY ACROSS WESTERN GREENLAND BASED ON RADIOGENIC ISOTOPES

Seawater-derived Pb isotopes in North Atlantic marine sediments display a rapid increase across the last deglacial that is attributed to enhanced chemical weathering of freshly-exposed glacial debris. However, few studies have focused on variations in weathering across the glacial foreland that ultimately impact weathering fluxes to the ocean, and thus provide a long-term record of past chemical weathering. To address this deficiency, we are evaluating the geochemistry of stream waters and bedload sediment across a 170 km transect in western Greenland from the Greenland Ice Sheet (GrIS) to the coast. Stream solute data indicate an increase in weathering intensity toward the coast characterized by a change in the weathering agents (acids) and weathered minerals. The Pb isotopes of stream waters are contaminated by anthropogenic lead and thus cannot be compared with marine records of chemical weathering; however, the difference in Sr isotopic compositions of stream water and bedload ⁸⁷Sr/⁸⁶Sr values (Δ⁸⁷Sr/⁸⁶Sr) decrease toward the coast, consistent with increasing weathering intensity. Unfortunately, the response of Sr isotopes to weathering is a poor analog for Pb isotopic variations because the primary minerals that weather to produce ⁸⁷Sr/⁸⁶Sr variations (micas, carbonate and epidote) are more readily weathered than the heavy minerals that dominate the Pb isotopic weathering flux. Heavy minerals also dominate the Nd isotopic weathering flux, suggesting Nd may be a suitable Pb analog. The ε­_Nd of stream waters increases systematically from the GrIS to the coast as the differences between water and bedload ε­_Nd (Δε­_Nd) decrease; however, the rate of change of Δε­_Nd is more gradual than that of Δ⁸⁷Sr/⁸⁶Sr. The relative difference in weathering intensity defined by Sr and Nd isotopes may provide clues to drivers of chemical weathering in this environment (i.e., exposure age, precipitation, or organic carbon reactions) and determine how radiogenic isotopic weathering fluxes will vary as the GrIS retreats. This information will be critical for interpreting geologic marine Pb isotope records of chemical weathering.