MUSCOVITE WEATHERING IN THE BRUBAKER RUN WATERSHED, SOUTHEASTERN PENNSYLVANIA: LONG-TERM WEATHERING RATES, WEATHERING PRODUCT CHARACTERIZATION, AND SOIL/ATMOSPHERIC CO2 CONSUMPTION DURING APPALACHIAN PIEDMONT REGOLITH FORMATION

Silicate mineral weathering during landscape evolution is a natural mechanism by which atmospheric CO₂ is consumed over geologic time, and has a significant impact on global climate change. In studies of silicate mineral weathering, the weathering of muscovite is often ignored because its weathering is typically concomitant to that of biotite. However, in the Brubaker Run watershed (BRW) of southeastern Pennsylvania, biotite is absent and muscovite weathers relatively rapidly at a long-term (10³-10⁵ years) average rate of approximately 700 mol/ha/yr. The long-term BRW muscovite weathering rate was determined using elemental flux loss values calculated from ¹⁰Be-derived total denudation rate data, and the bulk chemistry of bedrock and soil. Long-term weathering rates may better represent chemical weathering on geologic timescales by averaging chemical weathering during glacial and interglacial periods, and by not being influenced by short-term changes in biomass. Proper determination of the transformational secondary muscovite weathering product chemistry at BRW is essential in order to accurately quantify the weathering rate of the parent muscovite. The weathered muscovite at BRW contains more K⁺ and less Na⁺ than does the parent muscovite, and Fe is completely oxidized. The muscovite weathering rate is significantly underestimated when the weathering products are assumed to be solely kaolinite and/or gibbsite. If it is assumed that the only acid involved in chemical weathering is carbonic, then the quantity of soil/atmospheric CO₂ consumed by muscovite weathering at BRW is 130 mol/ha/yr. The present-day global average soil/atmospheric CO₂ consumption by silicate weathering is estimated to be ~1,000 mol/ha/yr. Because present-day chemical weathering rates reflect a relatively warm interglacial period, they are likely higher than the long-term average. Therefore, the soil/atmospheric CO₂ consumption at BRW that is attributable to muscovite weathering (~13% of the present-day global average) may be treated as a minimum value. These results indicate that muscovite weathering may be more important in global geochemical cycles than previously believed.