IMPLICATIONS FOR THE EXTENT OF WEATHERING IN GREENLAND MORAINES BASED ON PB AND SR ISOTOPES

Seawater Pb isotopes preserved in North Atlantic marine sediments increase rapidly across the last deglaciation (20-8 ka). This increase is believed to reflect incongruent chemical weathering of freshly exposed, glacially comminuted sediment, whereby radiogenic Pb isotopes are preferentially released from accessory phases. These glacially derived sediments weather faster than the global average and produce fluxes of radiogenic isotopes that reflect weathering rate and intensity. Melting of the Greenland Ice Sheet (GrIS) progressively exposes deglaciated watersheds on the continent, but little work has been done to characterize weathering in these regions. We collected moraine samples from a transect of deglaciated watersheds in western Greenland spanning 150 km from the edge of the modern GrIS to the coast. Exposure ages across this transect range from 10 ky to ~200 yrs, and there is a transition from a positive water balance at the coast to a negative water balance near the ice sheet. Samples were collected from trenches in 12 moraines and leached with 1 N HCl. Leachates and leached solids were then analyzed for Pb and Sr isotopes. Leachate Pb isotopic ratios were consistently more radiogenic than ratios of associated leached solids. The offset between ²⁰⁸Pb/²⁰⁴Pb ratios of leachates and paired solids (D²⁰⁸Pb/²⁰⁴Pb) decreases across the transect from the Little Ice Age moraine (200-300 yrs) near the GrIS (D²⁰⁸Pb/²⁰⁴Pb ~ 22), to the ~7 ky moraines (~17), to the ~10 ky moraines near the coast (~5). This offset is presumably due to preferentially weathering of Th-rich rather than U-rich accessory phases since the offsets for ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios are relatively constant. Stream waters were contaminated by anthropogenic Pb, but the offset between ⁸⁷Sr/⁸⁶Sr of stream waters versus bedload also decreases from the GrIS to the coast. These trends in the weathering flux of radiogenic isotopes across the deglacial watershed imply an increase in the intensity of silicate weathering with increased exposure age and water balance from inland to the coast. These trends also suggest that elemental, isotopic, and nutrient fluxes to the oceans, as well as CO₂ draw-down due to silicate weathering, are likely to vary in response to retreat of the GrIS.