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Physical and Chemical Controls on the Critical Zone

¹ INSTAAR and Dept. of Geography UCB-450
University of Colorado, Boulder, CO 80309, USA
E-mail: suzanne.anderson{at}colorado.edu
² Institute of Mineralogy, University of Hannover
Callinstrase 1, 30167 Hannover, Germany
³ U.S. Geological Survey, Menlo Park, CA 94025, USA

View larger version (46K): [in this window] [in a new window]   FIGURE 1 Vertical layering within the Critical Zone, showing rock parcel trajectory (black dashed line) and water flow path (blue dashed line) in a system with minimal addition of solid material at the top. Not all layers are present in all systems. Chemical weathering flux varies with depth and is controlled by mineral and weathering solution reactivity and water flow rate. In depositional systems, such as loess, volcanic ash falls, floodplains, and footslopes, particle trajectory is downwards relative to the surface and chemical flux can be highest at the top of the reactor where minerals are freshest.

View larger version (18K): [in this window] [in a new window]   FIGURE 2 K-feldspar weathering rate as a function of time. Orders-of-magnitude lower field-based rates suggest external controls (e.g. pore-fluid saturation), which are in turn determined by physical hillslope processes (AFTER WHITE AND BRANTLEY 2003; REPRODUCED WITH PERMISSION FROM ELSEVIER).

View larger version (21K): [in this window] [in a new window]   FIGURE 3 Comparison of flux rates determined from material in rivers (Millot et al. 2002) and from the residual CZ material (Riebe et al. 2004). Flux in the CZ was determined from solid-phase analysis using immobile elements and cosmogenic nuclides. Error bars for both data sets were taken from the published data sources.

View larger version (38K): [in this window] [in a new window]   FIGURE 4 (A) Scanning electron microscope image of weathered granite, sampled 7.5 m below the surface at Panola, Georgia, USA. It shows altered plagioclase with pristine K-feldspar and biotite. (B and C) Soil water (crosses) and groundwater (circles) are far from saturation (dotted line) for albite, but groundwater is saturated with respect to K-feldspar (FROM WHITE 2005).