A 16.5 KA POST-GLACIAL LAKE SEDIMENT RECORD OF APATITE WEATHERING AND DEPLETION FROM SARGENT MOUNTAIN POND, MAINE (USA): IMPLICATIONS FOR CARBON FLUX AND NUTRIENT BUDGETS ON RECENTLY DEGLACIATED LANDSCAPES

A 16.5 ka lake sediment record of stable lead (Pb) isotopic ratios and phosphorus (P) flux indicates an approximately 4,000-year period of apatite-dominated chemical weathering in the Sargent Mountain Pond (SMP) catchment in Maine (USA). During depletion of apatite, the Pb isotopic signal shifted to one dominated by slower primary silicate mineral weathering. Early, low ²⁰⁷Pb/²⁰⁶Pb values (²⁰⁷Pb/²⁰⁶Pb=0.799) suggest a sedimentation and weathering regime dominated by strongly urogenic accessory minerals (e.g., apatite). Following this period, non-urogenic primary minerals dominate (²⁰⁷Pb/²⁰⁶Pb=0.814). From approximately 1850 AD to present, atmospheric Pb pollution (²⁰⁷Pb/²⁰⁶Pb=0.840) controls the signal. Core data indicate a shift from high apatite-derived P flux from the catchment to lower values, indicating a reduction in lake productivity from its early eutrophic status to meso-oligotrophic conditions in the lake's first 4,000 years after deglaciation. Laboratory chemical weathering experiments involving similar parent material to the SMP catchment till seek to verify at the mineral scale the weathering trajectories inferred at the catchment scale from the core. These results, coupled with numerical modeling of catchment- to continental-scale evolution of apatite (and associated P) availability, enable predictive modeling of the effects of future deglaciation on global carbon flux and nutrient budgets.