GLACIAL FORELAND WEATHERING IN WESTERN GREENLAND: IMPLICATIONS FOR PAST AND FUTURE WEATHERING CONDITIONS

Variations in the extent and type of chemical weathering across glacial forelands should impact the global carbon cycle and solute and radiogenic isotope fluxes to the ocean as continental ice sheets retreat. However, little is known of the relationships between weathering and gradients of exposure age and climate as watersheds shift from draining ice sheet melt in proglacial watersheds to runoff from nonglacial sources in deglacial watersheds. In this study we identify trends in carbonate and silicate mineral weathering by carbonic (H₂CO₃) and sulfuric (H₂SO₄) acids based on elemental mass balances and Sr isotopic composition of stream waters and bedload sediments. We evaluate these chemical weathering trends in one proglacial and four deglacial watersheds across a ~170 km transect in western Greenland between the coast at Sisimiut and the Greenland Ice Sheet (GrIS) at Kangerlussuaq. H₂CO₃ silicate weathering and H₂SO₄ carbonate weathering increase while H₂CO₃ carbonate weathering decreases toward the coast. The increase in silicate weathering in coastal deglacial watersheds indicates a more intense weathering environment compared to inland deglacial watersheds that are dominated by weathering of trace reactive minerals. More extensive coastal weathering is also shown by a decrease in the difference between stream water and bedload ⁸⁷Sr/⁸⁶Sr values (Δ⁸⁷Sr/⁸⁶Sr) from 0.029 inland to 0.000 by the coast. High Δ⁸⁷Sr/⁸⁶Sr (0.025) and silicate weathering in the proglacial system likely records enhanced biotite weathering in this young watershed. At present, weathering is a net sink of CO₂ and inland deglacial watersheds provide greater CO₂ sequestration per liter of water than coastal deglacial watersheds due to high solute concentrations and H₂CO₃-driven weathering. Weathering in coastal deglacial watersheds with elevated H₂SO₄-driven weathering and proglacial watersheds with low solute concentrations have similar magnitudes of CO₂ drawdown per liter of water. As the GrIS retreats the proportion of deglacial watersheds will grow. Related changes in precipitation patterns may affect the weathering intensity in the foreland, and therefore CO₂ sequestration and oceanic fluxes of solutes and radiogenic isotopes.