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Rates and time scales of clay-mineral formation by weathering in saprolitic regoliths of the southern Appalachians from geochemical mass balance

Jason R. Price^,1, Michael A. Velbel¹ and Lina C. Patino¹

¹ Department of Geological Sciences, 206 Natural Sciences Building, Michigan State University, East Lansing, Michigan 48824-1115, USA

Rates of clay formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina, have been determined from solute flux-based mass balance methods. A system of mass balance equations with enough equations and unknowns to allow calculation of secondary-mineral formation rates as well as the more commonly determined primary-mineral dissolution rates was achieved by including rare earth elements (REE) in the mass balance.

Rates of clay-mineral formation determined by mass balance methods have been used to calculate the time needed for a 5% (50 g kg^–1) change in relative clay abundance in the saprolite at Coweeta; this corresponds to the "response time" of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using X-ray diffraction (XRD). Response times range from tens of thousands to hundreds of thousands of years. Extrapolating the Coweeta clay formation rates to other southern Appalachian regoliths, the time required to form measured clay abundances ("production times") in eastern Blue Ridge and Inner Piedmont regolith have been calculated. The production times of clay-mineral assemblages range from 2 k.y. to 2 m.y., with mean values ranging from 50 k.y. to 1 m.y. The results of this study are consistent with the arguments of Thiry (2000) that the best resolution of the paleoclimatic record in marine clay-rich sediments and mudrocks is 1 or 2 m.y.

Key Words: clays • rates • mass balance • Appalachians • regolith

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