CONTROLS ON LONG-TERM AND PRESENT-DAY CHEMICAL WEATHERING RATES: A UNIFYING CONCEPTUAL MODEL FOR APPALACHIAN WATERSHEDS

Controls on chemical weathering are poorly understood. Previous studies of long-term (geologic timescale) chemical weathering rates have found tectonics to be the sole influence. Investigations of present-day chemical weathering rates report climate as the control. Studies combining long-term and present-day chemical weathering rate data yield models that include both climate and tectonic parameters.

Present-day saprolitization rates (rates at which the weathering front penetrates fresh bedrock) have been calculated for northern Appalachian Piedmont and southern Appalachian Blue Ridge watersheds using stream solute-based geochemical mass-balance methods. For the northern Piedmont and southern Blue Ridge present-day saprolitization rates far exceed present-day physical erosion rates. Long-term saprolitization rates have also been determined for the northern Piedmont using soil and bedrock chemistry, and total denudation rates derived from 10-beryllium analyses of stream sediment. Northern Piedmont long-term saprolitization rates (~4.5 m/My) are approximately a factor of two lower than those of the present-day (~8 m/My), consistent with theoretical models.

Based on the models of controls on chemical weathering from the literature, and the long-term and present-day saprolitization rates for Appalachian watersheds, a unifying conceptual model of the relationship between chemical weathering, climate, and tectonics may be developed. The term “chemically-dominated weathering regime” (CDWR) is established to define present-day, interglacial conditions whereby chemical weathering is controlled by climate. During a CDWR the physical erosion rate is concomitant to the saprolitization rate and the regolith is thickening. In contrast, long-term average chemical weathering conditions are dominated by relatively physically-erosive glacial/periglacial periods and may be termed “physically-dominated weathering regimes” (PDWR). Based on the timing of recent glacial-interglacial cycles, CDWR and PDWR operate approximately 20% and 80% of the time, respectively, yielding a regolith of constant thickness on geologic timescales.