Allanite weathering and rare earth elements in mass balance calculations of clay genesis rates at the Coweeta Hydrologic Laboratory, western North Carolina, United States: The response times of changes in clay mineral assemblages to fluctuations in climate.

摘要

Flux-based mass balance methods have been used to calculate rates of clay formation/dissolution in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina. The mass balance calculations of this study include rare earth elements (REE), which is fundamentally different from any study performed at Coweeta or elsewhere. The primary advantage of using additional elements such as the rare earths is that a larger number of equations can be constructed, allowing determination of a larger number of unknowns. This situation does not always exist in mass balance calculations.;The REE in Coweeta stream waters are strongly influenced by the weathering of previously unrecognized allanite which is a very important source of Ca in stream waters. Due to the inclusion of allanite that also contains and releases Na during weathering, the primary mineral weathering rates calculated as part of this study differ from those of previous studies (i.e., Taylor and Velbel 1991). All new plagioclase dissolution rates are lower but still within 20% of those reported by Taylor and Velbel (1991). Similarly, the biotite weathering rates reported herein are within 40% of those reported by Taylor and Velbel (1991). The garnet weathering rates of this study are up to approximately 75% slower than those calculated by Taylor and Velbel (1991). Without allanite, large weathering rates were needed to balance the Ca in stream waters.;The results of this study demonstrate that kaolin precipitation/formation is favored in warmer and/or drier climates, and gibbsite is favored in cooler and/or wetter climates. Vermiculite appears to be influenced by lithology and climate.;The clay genesis/dissolution rates 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; i.e., the "response time" of the clay mineral to a change in climate. Response times occur on time scales of tens of thousand to hundreds of thousands of years. The results of this study concur with the arguments of Thiry (2000) that the best resolution of the paleoclimatic record in clay-rich sediments and mudrocks is 1 or 2 Ma.