We use a mass balance approach to calculate long-term weathering rates for three forested watersheds in the southeastern USA. One watershed (Shaver Hollow) is underlain by Precambrian granodiorite, and two (White Oak Run, Deep Run) are underlain by Cambrian metasedimentary units. Each of the study areas receives high levels of acidic precipitation, and each watershed was almost completely defoliated by gypsy moth larvae for at least two consecutive years during the study period. Our analysis uses stream and precipitation chemistry obtained weekly from 1987 to 1993 for Shaver Hollow and from 1980 to 1993 for White Oak Run and Deep Run. Mass balance calculations for the granodiorite watershed indicate that plagioclase feldspar is the dominant reactant and that basalt dikes which comprise a small fraction of the catchment make a significant contribution to the base cation budget. Corresponding calculations for the metasedimentary bedrock watersheds indicate contributions from plagioclase feldspar, muscovite, and biotite. At best, the mass balance models provide an approximation of weathering processes because the soils are immature and contain few stoichiometric clay minerals such as kaolinite. Annual cation release rates from Shaver Hollow suggest soil profile ages of ~4 ka, consistent with the observed surficial geology and the geological history of the area. Base cation efflux patterns during the period of severe defoliation differ markedly between the three watersheds and reflect a continuum of ecosystem sensitivities. The Pedlar granodiorite system is relatively robust to the effects of disturbance: calculated weathering rates and proportions change by <10% during defoliation. The Chilhowee Formation watershed White Oak Run is moderately acid-sensitive and is unstable with respect to defoliation: annual base cation export by roughly 30% during each year of larval infestation. The Deep Run watershed is superficially robust to defoliation, as base cation efflux increased by only 5% during the disturbance. However, seasonal patterns of base cation uptake and release both prior to and during defoliation indicate this system is severely stressed, presumably as a result of chronic acid deposition. Mass balance calculations suggest that base cation availability in this watershed is low enough to inhibit normal biomass aggradation even in the absence of extreme defoliation.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology