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Elements; October 2007; v. 3; no. 5; p. 307-314; DOI: 10.2113/gselements.3.5.307
© 2007 Mineralogical Society of America
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Crossing Disciplines and Scales to Understand the Critical Zone

Susan L. Brantley1, Martin B. Goldhaber2 and K. Vala Ragnarsdottir3

1 Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute, Pennsylvania State University
University Park PA 16802, USA
E-mail: brantley{at}essc.psu.edu
2 USGS Crustal Team, MS 973, Denver Federal Center
Denver CO 80225, USA
E-mail: mgold{at}usgs.gov
3 Department of Earth Sciences
University of Bristol, Bristol BS8 1RJ, UK
E-mail: Vala.Ragnarsdottir{at}bristol.ac.uk


Figure 2
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  		FIGURE 1 Schematic of fluxes and processes in the Critical Zone. The CZ is defined as the volume extending from the upper limit of vegetation down to the lower limit of groundwater. Heterogeneities within the CZ present a challenge to researchers developing models to predict these fluxes and processes. Understanding the CZ requires that scientists cross disciplines and work at all scales (AFTER ANDERSON ET AL. 2004).

 

Figure 3
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  		FIGURE 2 Left: a road cut through soil and saprolite developed on the Rio Blanco quartz diorite (Luquillo Mountains, Puerto Rico). Unweathered rock is fractured spheroidally into corestones within the weathering engine at this site. Two corestones appear in the photo, right of the ladder. Once corestones are produced, further fracturing and disintegration produces saprolite grains (Fletcher et al. 2006). At this site, large corestones transform relatively quickly compared to the regolith residence time; therefore, regolith consists mostly of large corestones and saprolite grains. Conceptually, rock flows through the weathering engine at the weathering advance rate, w, and is removed at the top at the erosion rate, W. If W = w, then the thickness of the weathering layer remains constant. For such a steady state, all layers within the engine—incipiently weathered bedrock, saprolite, and soil—must be maintained at constant thickness. This one-dimensional model, which describes the weathering engine operative at ridgetops, is being extended to two- and three-dimensions for describing hillslope processes (ANDERSON ET AL. 2007 THIS ISSUE). PHOTOGRAPH BY ERIC PELT

 

Figure 4
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  		FIGURE 3 Coupled chemical, physical, and biological weathering processes in the Critical Zone are affected by climate and by anthropogenic and tectonic forcing over vastly different timescales. The output from the CZ weathering engine is documented in the response of the atmosphere, rivers and oceans, and soils and sediments, which can be read in the geological record.

 

Figure 5
Figure 5
Figure 5
Figure 5
Figure 5
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  		FIGURE 4 Plots of normalized concentration, {tau}, versus depth for a soil developed in the Shale Hills watershed on the Rose Hill Shale (A-D) and on the Gettysburg Diabase (E), both in central Pennsylvania, USA. The parameter {tau}i,j represents elemental concentration normalized to account for volume changes and relative loss or gain of elements (Brimhall and Dietrich 1987).

Formula

Where C represents concentration (mol/m3) of immobile (i) or mobile (j) elements in weathered (w) or parent (p) material. When {tau} = 0, concentration is identical to that of the parent; when {tau} < 0 or > 0, there is elemental loss or gain, respectively; where {tau} = -1 for 100% loss of the element. (A) {tau} for Nb and {tau} for Zr, plotted versus depth in regolith, document immobile profiles. (B) {tau} for Cu is an example of a depletion profile. Cu-organic complexes formed at the surface enhance the loss of Cu from this soil. (C) {tau} for Al documents an addition-depletion profile. Al-organic complexes at the surface deplete Al, releasing it for reprecipitation at depth. (D) {tau} for Mn and {tau} for C portray addition profiles. Mn and C have been added to the soil as dust and through biological fixation, respectively. (E) {tau} calculated for K within regolith overlying the Gettysburg diabase documents a biogenic profile. K has been removed by roots at depth, then enriched at the surface by biological processes.

 

Figure 6
Figure 6
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  		FIGURE 5 (A) Chromium in A-horizon soils across the United States. Vertical bars are proportional to Cr content (maximum = 5030 ppm). The base map depicts gridded Cr content based on 1323 soil samples, where warmer colors indicate higher Cr contents (Gustavsson et al. 2001). Note the elevated Cr in the west (California; data from Smith et al. 2005). The inset shows Cr content in A-horizon soil samples in northern California (Goldhaber M, unpublished data; larger circles represent higher Cr). The highest Cr concentrations occur over ultramafic rocks (shown in green). (B) Mercury in A-horizon soils across the United States (Smith et al. 2005). Vertical bars are proportional to Hg content (maximum = 0.18 ppm). Inset represents Hg emissions from coal-fired power plants for 2002 (Miller and Van Atten 2004). The base map is contoured for precipitation. Note the correspondence between Hg concentration in eastern United States with higher power plant Hg emissions in this part of the country.

 

Figure 7
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  		FIGURE 6 Conceptual diagram outlining a model for a network of observatories to investigate Earth surface processes. Each box represents a CZ observatory site or sites. These sites are chosen to span environmental gradients as shown, but other gradients could also be explored. For example, network legs could investigate gradients in pH, water composition, uplift rate, or deformation rate. CZ observatory sites for such a network are envisioned to be instrumented for relatively short periods. Sites (noted in green) located on more than one gradient should be instrumented for longer durations. FIGURE FROM BRANTLEY ET AL. 2006

 




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