Geochemical mass balances and weathering rates in forested watersheds of the southern Blue Ridge 论文

摘要

Weathering rates of rock-forming silicate minerals in natural forested watersheds can be calculated using a system of geochemical mass balance equations constructed and constrained by petrologic, mineralogic, hydrologic, botanical, and aqueous geo- chemical data. Solving the systems of equations for the weathering rates of biotite mica, almandine garnet, and oligoclase—andesine plagioclase feldspar in deeply weathered (saprolitized) schists and gneisses of a study area (the USDAFS Coweeta Hydrologic Labora- tory, near Otto, N.C.) suggests that: (1) The rate at which the weathering front penetrates into the fresh rock (3.8 cm/1000 yrs) agrees well with the average denudation rate for the southern Appalachians (4 cm/1000 yrs) determined by a variety of different methods. (2) When normalized to the estimated mineral surface area in the watersheds, rates of garnet and plagioclase weathering are approximately one to two orders of magnitude slower than rates determined in laboratory experiments under similar hydrogeochem- ical conditions. Two possible sources of this discrepancy are (A) the character of artificially-treated mineral surfaces, which renders them more reactive than their natural counterparts, and/or (B) diffi- culties in estimating reactive mineral surface area in natural systems. (3) Present rates of primary mineral weathering and secondary mineral neof ormation at Coweeta appear to be sufficient to neutral- ize present levels of atmospheric hydrogen ion input. The relatively good accord between field and laboratory geochemical and geo- morphologic results suggests that present sources of error may not be large, that mass-balance models provide quantitative estimates of rates of important environmental processes, and that refinement of laboratory kinetic data and field surface area determinations should rank high among future research priorities.