Talk at Theme 7, Sesssion 07i at 2009 Goldschmidt -- Comparing Sites Across CZEN

The following abstract is for a talk presented at the 2009 Goldschmidt in Davos, Switzerland.

This will be a part of Theme 7, Sesssion 07i: Rates and Mechanisms of Erosion and Weathering Processes: From Experiments to Models

Climatic controls of regolith weathering and mass flux in granitic terrain - a synthesis of Critical Zone Exploration Network data

C. RASMUSSEN*(a), J.Z. WILLIAMS(b), S. BRANTLEY(b), D. RICHTER(c), A. WHITE(d), AND R. APRIL(e)
a. Department of Soil, Water and Environmental Science, The University of Arizona (*crasmuss@cals.arizona.edu)
b. Earth and Environmental Systems Institute, The Pennsylvania State University
c. Nicholas School of the Environment and Earth Sciences, Duke University
d. U. S. Geological Survey
e. Department of Geology, Colgate University

What controls the depth and chemistry of the Earth’s regolith? Answering this question is key to understanding landscape evolution, soil forming processes, and the feedback between mineral weathering and atmospheric carbon dioxide levels. We present a synthesis of regolith weathering data for granitic terrain associated with the Critical Zone Exploration Network. Data for regolith and rock geochemistry, climate and landscape age were compiled and a regolith geochemical mass balance calculated relative to bedrock using an immobile element approach [1]. A unitless mass transfer coefficient (?, g g-1) and mass flux (?, g m-3) were calculated for the entire regolith profile at each site [2]. The ? depeltion profiles indicated greater depth of weathering and near complete loss of cations such as Na with increasing precipitation. Regolith integrated ? and ? values demonstrated threshold type functions relative to mean annual precipitation, with a dramatic increase of Si and cation loss above a mean annual precipitation (MAP) of 1,000 mm yr-1. We fit a range of functions to depth profiles of bedrock normalized Na concentrations (mol m-3 regolith) to derive a lumped kinetic parameter (K), assuming albite weathering dominated the observed Na mass loss [3]. The regolith K values followed an exponential decay function relative to MAP, with the greatest K values at low precipitation, i.e., less than 500 mm yr-1, that decreased asymptotically with increasing MAP. The data suggest an interaction of climate, weathering rate and mass flux in that low MAP sites exhibited minimal mass loss but potentially rapid kinetics. This pattern reversed in a threshold-like fashion to large regolith mass flux and slow weathering kinetics with increasing precipitation.