『Abstract
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.
Keywords: clays; rates; mass balance; Appalachians; regolith』
Introduction
Background
Study area
Geomorphic considerations
Clay formation rates
Methods
Field work
Petrography
X-ray diffraction
Scanning electron microscopy (SEM): Secondary and backscattered
electron imaging (BSE)
Electron microprobe phase analyses (EMPA)
Rare earth element analyses by inductively coupled plasma mass
spectrometry (ICP-MS)
Watershed mass balance methods
Results
Mineral compositions
Watershed mass balance and rare earth elements
Atmospheric inputs of rare earth elements
Mineralogic sources and sinks of rare earth elements
Aluminum and rare earth element stream fluxes
Primary mineral weathering rates and secondary mineral formation
rates
Discussion
Response times for watersheds at Coweeta Hydrologic Laboratory
Clay formation rates in the southern Appalachians
Conclusions
Acknowledgments
References cited