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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
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
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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KEYWORDS: weathering, soils, Critical Zone, regolith
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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Importantly, the environmental gradients within the CZ provide energy and resources that sustain humans. Nourishment of the current population (6.7 billion) requires several tens of millions of square kilometers of land in cultivation (Buringh 1989). Today, about 0.23 ha (2.3 x 10-3 km2, equivalent to about 50 m x 50 m) of arable land is available per capita, but roughly twice this area is needed to provide the diverse diet essential for human health (Lal 1989). If we extrapolate the current erosion rate of about 12 million metric tonnes per year over the next 40 years to the point when the population will reach 9 billion, only 20% of the arable land needed for adequate nutrition will be available. Furthermore, desertification has already caused severe degradation of land in up to 10-20% of drylands worldwide through a variety of factors, including climate change and human activity (Millenium Ecosystem Assessment 2005). Such observations and calculations have led many policy makers to conclude that loss of soil and declining soil fertility threaten sustainable development.
How might we treat and understand the CZ as a sustainable system—a system "that lasts" (Chesworth 2002)? At the longest timescales, geologists conceptualize the CZ as a weathering engine or reactor (FIG. 2), in which rocks at depth are fractured, ground, dissolved, and bioturbated into transportable materials (Anderson et al. 2007 this issue). At the shortest timescales, geochemists investigate the rates and mechanisms of reactions at mineral-water and organism-water interfaces (Chorover et al. 2007 this issue). All of these important and coupled chemical, physical, and biological processes are combined in our definition of the weathering process. Quantitative models of these coupled processes and how they respond to tectonic, climatic, and anthropogenic forcings are needed to truly understand the evolution of the CZ (FIG. 3).
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The processes within the weathering engine occurring over this variety of timescales combine to produce patterns at the grain (millimeter), clast (centimeter), pedon (meter), landscape (kilometer), and global (thousands of kilometers) scales. Although often geochemical in nature, interpretation of these processes requires the input of researchers from many scientific disciplines. Despite our 10,000 years of association with soil, we still lack the conceptual and quantitative models needed to predict the outcome of the coupled processes within the CZ, precisely because of their interdisciplinary nature and the variety of scales that are represented.
The challenge to cross disciplines and scales in understanding the CZ is a growing focus for scientists from geology, soil science, hydrology, environmental engineering, chemistry, and ecology (Anderson et al. 2004; Brantley et al. 2006). By way of introduction to the CZ, we discuss in this paper the geochemical story written in the regolith, defined here as the weathered rock material overlying pristine bedrock, as documented by chemical gradients at the pedon scale. We then describe the flux of materials through the CZ, and we conclude with a discussion of issues that transect disciplines and scales of time and space, as we think about CZ sustainability. In the rest of this issue, our colleagues discuss how geochemical patterns are established through processes occurring at the mineral-water interface (Chorover et al. 2007), as well as how they are affected by biota (Amundson et al. 2007), dust (Derry and Chadwick 2007), and erosion (Anderson et al. 2007).
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CHEMICAL GRADIENTS IN THE REGOLITH |
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TOPABSTRACTINTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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Using such a conceptualization to consider profiles of concentrations normalized to the parent rock as a function of depth, one can identify five end-member categories of elemental regolith profiles (FIG. 4): (1) immobile profiles exhibit parent concentrations at all depths; (2) depletion profiles exhibit depletion at the top, grading to parent concentration at depth; (3) depletion-enrichment profiles exhibit depletion at the top, enrichment at depth resulting from precipitation or translocation, and a return to parent concentration at greater depth; (4) addition profiles show enrichment from external input at the top grading to parent concentration at depth; and (5) biogenic profiles exhibit enrichment at the top and a depleted zone that grades downward to parent concentration at depth. These profile types are end-members; any individual profile may be some mixture of end-members. These end-member models can often be used to interpret the story of the CZ.
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To see these characteristic types of profiles requires that the elemental concentrations of regolith are first normalized to the concentration of an immobile element (Brimhall and Dietrich 1987; White 1995). Normalization corrects for contraction or expansion of soil and for apparent dilution or enrichment effects that occur when multiple elements in a system react (FIG. 4). For example, relatively unreactive elements, such as Zr (found in zircon), Ti (rutile, anatase), and Nb (columbite, tantalite), commonly become enriched in regolith as more soluble constituents are depleted. When the concentration of an immobile element is normalized to that of another immobile element, the concentration-depth curve exhibits the characteristics of an immobile profile (FIG. 4A).
In contrast, depletion profiles (FIG. 4B) document loss of elements from the surface, often from mineral dissolution. For example, FIGURE 4B shows a reaction front caused by leaching of Cu from Rose Hill Shale (central Pennsylvania) during soil development. Almost 50% of the Cu has been lost from this soil at the top of the profile. The high affinity of Cu for organic ligands may explain Cu leaching from this soil developed on the relatively non-reactive shale parent. As another example, geochemists often analyze the depletion profile of Na to interpret the dissolution of plagioclase (White 1995).
An element that is leached at the surface may reprecipitate at depth in the regolith, resulting in a depletion-enrichment profile (FIG. 4C). An example is the soil sequence E-Bs, E-Bt commonly observed in forests where conditions at the top of the profile favor Al dissolution, and less acidic conditions with lower organic ligand concentration deeper in the profile favor formation of secondary Al precipitates. Depletion-enrichment profiles may also be exhibited by Fe, as well as by other elements, including those that are affected by the chemistry of Fe and Al.
Input to soils through dust deposition and fixation from the atmosphere can create distinctive addition profiles. FIGURE 4D shows profiles for Mn and C developed on weathered Rose Hill Shale. Such profiles commonly document the effect of wet or dry deposition (e.g. Mn in FIG. 4D) or input of elements resulting from biological fixation (C in FIG. 4D). Addition profiles demonstrate both addition at the surface and redistribution by downward transport or bioturbation. These profiles are therefore characterized by concentrations that are highest at the surface, reaching parent concentrations at depth. Prediction or modeling of concentration-depth profiles for biologically fixed elements such as C are especially difficult because of their dependence on many biological and non-biological factors (Schimel et al. 1994).
Biogenic profiles are of great importance from the perspective of soil sustainability. They characterize elements such as K that, although released from parent material, are redistributed in the CZ because of their role as nutrients (FIG. 4E). In biogenic profiles, elements characteristically show depletion at depth and enrichment at the surface. These profiles are much more difficult to predict quantitatively than the profiles of non-nutrient elements because of the need for understanding biological fluxes (Amundson et al. 2007). Biogenic profiles document the secretion of organic acids by plants or fungi through roots at depth and the consequent dissolution of primary minerals. If the released nutrients are taken up by roots, transferred to biomass, and recycled in the upper layers of the soil, then a biogenic profile results (Jobbágy and Jackson 2001).
Time Evolution of Profiles
Our discussion so far has largely ignored the evolution through time of
these concentration-depth profiles. In regolith not experiencing significant
erosion, a reaction front such as that shown in
FIGURE 4B
moves downward with time at a rate we term the weathering advance
rate (FIG. 2).
Once the normalized elemental or mineralogical concentrations at the top of
the profile no longer change with time (perhaps because of total depletion of
a mineral or the formation of a stable mineral assemblage), the profile
becomes quasi-stationary (Lichtner
1988). At this time, the shape of the reaction front remains
constant while the entire profile moves downward. In contrast, in systems
where erosion is significant, a true steady-state profile can develop where
the concentration-depth curve remains constant with time. In such a system,
the rate of erosion equals the rate of weathering advance.
In a steady-state profile, movement of parent material through the weathering engine is ongoing. Such a profile can support ecosystems with constant nutrient delivery. In contrast, in a quasi-stationary profile that moves downward with time and is not significantly eroded, regolith at the surface may become nutrient-poor. In such regimes, dust addition (e.g. FIG. 4D) may be essential for nutrients required to support biota (Derry and Chadwick 2007). In such cases, erosion can rejuvenate the weathering-derived nutrient supply (Porder et al. 2005). Currently, many researchers are attempting to predict such processes at the scale of the mineral interface, clast, pedon, catchment, and globe (e.g. Lichtner 1988; Sak et al. 2004; Goddéris et al. 2006; Navarre-Sitchler and Brantley 2007; Chorover et al. 2007). At present, however, it is impossible to predict these processes in the full context of biological uptake and erosional loss over time and space. The development of fully coupled models will require interdisciplinary teams of CZ scientists.
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EROSION AND WEATHERING ADVANCE RATES |
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How can CZ scientists quantify these rates to characterize the weathering engine? The erosion rate is easier to measure than the weathering advance rate. For example, the erosion rate can be estimated by measuring the riverine sediment + dissolved load or by estimating soil loss at a given location (TABLE 1). In addition, interpretation of cosmogenic isotope concentrations in soil or sediment can reveal the duration of exposure to cosmic rays (e.g. Bierman and Steig 1996). Erosion rates are inferred from these exposure ages using model calculations based on penetration depths of cosmic rays and the assumption that layers penetrated by cosmic radiation maintain steady-state thickness.
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Erosion estimates made using these various techniques do not necessarily agree. Riverine estimates are lower by a factor of about 5 than estimates based on soil loss, at least partly because eroded soils are redeposited and stored on land before transport to the ocean (e.g. Lerman and Wu 2007). Furthermore, sediment studies commonly report suspended loads and neglect bedloads. In addition, the temporal and spatial scales for measuring erosion affect the estimate of erosive flux (e.g. von Blanckenburg 2005).
While many researchers have estimated rates of erosive loss, W (TABLE 1), our understanding of regolith formation processes and rates, i.e. the value of w, is limited. We need to understand how to compare weathering advance rate estimates, such as those in TABLE 2 for transformation of bedrock to regolith (ranging from 0.05 to 10 mm per 100 years), to erosion rate estimates, such as those in TABLE 1, reported at the continental and global scales (ranging from 7 to 80 mm per 100 years). Weathering advance rates have been derived from approaches based on mass balance, cosmogenic isotopes (10Be), slope curvature, and dissolved-solute loss from catchments. In fact, although it would be very interesting to compare w to W quantitatively to determine if steady state exists, often the estimates of regolith production are based on the assumption that regolith thickness is at steady state. Estimates of weathering advance rates have been shown to vary over 7 orders of magnitude when compared across laboratory to watershed spatial scales (Navarre-Sitchler and Brantley 2007). Clearly, understanding the CZ as a weathering engine will require many observations, new conceptual models, and powerful numerical models.
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SCALING TO PREDICT CZ PROCESSES |
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No less troublesome than temporal scales, spatial scales within the CZ vary over 16 or more orders of magnitude, from the atomic to the global scale. To characterize the important chemical and physical properties (e.g. from the scale of an individual soil grain up to regional lithologic variations) and processes (e.g. fluid movement along a grain boundary up to large-scale flow in major hydrologic basins), observations must be made ranging in scale from that of individual atomic layers, where atomic force microscopy is used, to that of entire continents, where satellite remote-sensing tools are required.
A useful approach to these scaling issues is to study processes from both ends of the time and distance spectra. For example, measurement of the rates of weathering in the laboratory can be compared to rates measured in the field. Thus, the study of chronosequences—regolith systems developed within the same climate on the same lithology but characterized by variations in duration of weathering exposure—yields rates for CZ processes over timescales of 103 to 106 years. When such comparisons are made (Fig. 2, Anderson et al. 2007 this issue), we generally observe that field rates are slower than laboratory rates by up to five orders of magnitude and that apparent weathering rates decrease with duration of exposure to weathering (White and Brantley 2003). Variations in weathering rates as a function of the duration of measurement may be related to how we conceptualize and parameterize the reactive surface area over time and the approach to equilibrium of weathering systems. However, weathering rates have also been observed to change with anthropogenic impact (Raymond and Cole 2003).
While silicate dissolution kinetics have been observed to vary by up to five orders of magnitude across temporal scales, up to seven orders of magnitude variation has been seen in these rates across spatial scales (Navarre-Sitchler and Brantley 2007). Thus, patterns in chemical signals documented at the scale depicted in FIGURE 4 (pedon scale) must be renormalized for comparison to patterns developed at larger spatial scales (FIG. 5). Soil geochemical surveys have been conducted globally to delineate geochemical and textural patterns at scales of approximately 1:10,000,000 (national scale), 1:1,000,000 (state or regional scale), and 1:1000 (local scale) (Jobbágy and Jackson 2001; Reimann 2005). In some cases, for example for isotopic signatures of nitrogen in soils and plants, global patterns can be related to regional variations in mean annual precipitation and temperature (Amundson et al. 2003). In other cases, regional patterns can be related to the distribution of the underlying lithology; for example, chromium in surface soil is related to the distribution of ultramafic rocks (FIG. 5A). We also observe for elements such as mercury that land use variations related to industrialization, urbanization, waste disposal, mining, and agriculture influence continental-scale chemical variations in the regolith (FIG. 5B). Elevated concentrations of mercury in soils of the eastern United States are related to the location of coal-fired power plants. Documenting, understanding, and predicting such regional- to global-scale patterns must be accomplished in the next decade by researchers within all the disciplines of CZ science, as we attempt to make policy decisions to safe-guard our planet's ecosystems.
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Despite the importance of such studies, few direct estimates of carbon loss or gain in soils over large areas and long timescales are available. Perhaps the best example of direct evidence of large-scale carbon loss from soils is the data from almost 6000 soil sample sites throughout England and Wales. These sites were sampled in 1978 and resampled in 2003. The data document systematic carbon loss of 0.6% per year (relative to the existing soil carbon content) from the upper 15 cm regardless of soil properties and land use (Bellamy et al. 2005). The authors conclude that this loss is linked to climate change. These dramatic data may indicate an important positive feedback between release of CO2 from microbial metabolism in soils and average global changes in temperature, soil moisture, and nutrients. Of course, in contrast to this dataset from the UK, soil also has the ability to sequester carbon and aid in removing atmospheric CO2 (Lal 2004). Many more studies at small and large spatial scales and short and long timescales are needed to document carbon loss and carbon sequestration in soils.
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NETWORKS OF PEOPLE AND SITES |
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TOPABSTRACTINTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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Researchers also identified the need for a global network of observatories to investigate CZ processes across gradients in environmental variables (FIG. 6). Scientists envisioned that these observatories would generally be in operation for about five years and that "deployments" of instrumentation and researchers would investigate these sites to answer process-oriented research questions. CZ researchers around the world have begun to build this network and the cyber infrastructure that will provide access to the data (czen.org). These huge endeavours will generate intellectual debate for years to come. Aspects of each of these questions are addressed in the papers presented in this issue.
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IS THE CRITICAL ZONE SUSTAINABLE? |
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In fact, Carter and Dale (1974) noted that the sites of the earliest civilizations in Mesopotamia and in the Nile and Indus valleys were all characterized by fertile soil, dependable water supplies, low relief, and limited soil erosion. These properties enabled surplus crop production, which supported the development of non-agricultural pursuits. While the fertile valleys and waters of the Tigris and Euphrates rivers in Mesopotamia made this area a cradle for civilization, deforestation and grazing in the Armenian hills to the northwest have now caused massive downstream silt transport and destruction of the irrigation systems that supported early Mesopotamian agriculture (Carter and Dale 1974).
This example is not an anomaly. Human impact on soils has occurred in three main waves. The first started 4000 years ago, when civilizations occupying river basins began moving upslope from the valleys, exposing virgin soils to seasonal rains (McNeill and Winiwarter 2004). The second phase of impact occurred in the 16th to 19th centuries, with the implementation of improved ploughshares to break up soil. The third phase is occurring now, with population pressure leading to the destruction of tropical rainforests. Humans are now a geological force transforming the Earth's surface. Forty percent of the Earth's ice-free land is currently cultivated or used as permanent pasture (Richter and Markewitz 2002). This agriculture induces transport of 70% of Holocene sediment (Wilkinson 2005). With industrialization came mining operations, which have also negatively impacted the landscape. For example, as of 1977, when stricter regulations were implemented, the mining industry alone had disturbed 2.3 Mha in the United States, an area roughly the size of the state of Vermont (Lal et al. 2004). As a result of human activities, the current global rate of erosion is about an order of magnitude higher than the natural erosion rate (Hooke 2000; Wilkinson 2005).
The extent of the planetary transformation brought about by humans has led to the new term "Anthropocene," which has been defined as "a new geological epoch in which humankind has emerged as a globally significant, and potentially intelligent, force capable of reshaping the face of the planet" (Clark et al. 2004). While much current interest has focused on the impact of humans on the chemistry of Earth's envelope of air and the effects of this chemical change on climate (IPCC 2007), our species is rapidly changing the chemistry of the hydrosphere and the regolith as well. Conceptualizing the complex interplay of chemistry, biology, geology, and physics within the skin of the Earth as a system—the Critical Zone—forces scientists to work together across disciplines and scales. In so doing, scientists will learn how to interpret recurrent patterns observed in the CZ and how to protect the CZ for all life. Importantly, heterogeneities in the CZ at all scales require advances in our ability to model the time evolution of the CZ. The list of tools and techniques for moving forward in this area is growing rapidly, enabling strides forward. Whether our species can be truly intelligent, as opposed to just geologically significant with respect to our impact on the planet, remains to be seen.
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GLOSSARY |
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TOPABSTRACTINTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONCHEMICAL GRADIENTS IN THE...EROSION AND WEATHERING ADVANCE...SCALING TO PREDICT CZ...NETWORKS OF PEOPLE AND...IS THE CRITICAL ZONE...GLOSSARYREFERENCES |
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Amundson R, Austin AT, Schuur EAG, Yoo K, Matzek V, Kendall C, Uebersax A, Brenner D, Baisden WT (2003) Global patterns of the isotopic composition of soil and plant nitrogen. Global Biogeochemical Cycles 17: 1031, doi10129/2002GB001903[CrossRef]
Amundson R, Richter DD, Humphreys GS, Jobbágy EG, Gaillardet
J (2007) Coupling between biota and Earth materials in the
Critical Zone. Elements 3:327
-332
Anderson SP, Blum J, Brantley SL, Chadwick O, Chorover J, Derry LA, Drever JI, Hering JG, Kirchner JW, Kump LR, Richter D, White AF (2004) Proposed initiative would study Earth's weathering engine.EOS Transactions of the American Geophysical Union85 : 265-269
Anderson SP, von Blanckenburg F, White AF (2007)
Physical and chemical controls on the Critical Zone. Elements3
: 315-319
Bellamy PH, Loveland PJ, Bradley RI, Lark RM, Kirk GJD (2005) Carbon losses from all soils across England and Wales 1978-2003. Nature 437:245 -248[CrossRef][Medline]
Bierman PR, Steig E (1996) Estimating rates of denudation and sediment transport using cosmogenic isotope abundances in sediment. Earth Surface Processes and Landforms21 : 125-139[CrossRef][ISI]
Brantley SL, White TS, White AF, Sparks D, Richter D, Pregitzer K, Derry L, Chorover J, Chadwick O, April R, Anderson S, Amundson R (2006) Frontiers in Exploration of the Critical Zone, An NSF-sponsored Workshop. National Science Foundation,30 pp
Brimhall GH, Dietrich WE (1987) Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metosomatic hydrochemical systems: Results on weathering and pedogenesis. Geochimica et Cosmochimica Acta51 : 567-587[CrossRef][ISI][GeoRef]
Buringh P (1989) Availability of agricultural land for crops and livestock production. In: Pimental LD, Hall CW (eds) Food and Natural Resources. Academic Press, San Diego, pp69 -83
Carter VG, Dale T (1974) Topsoil and Civilization, revised edition. University of Oklahoma Press, Norman,292 pp
Chesworth W (2002) Sustainability and the end of history. Geotimes 47: 52. Accessed at: www.agiweb.org/geotimes/oct02/comment.html
Chorover J, Kretzschmar R, Garcia-Pichel F, Sparks DL
(2007) Soil biogeochemical processes within the Critical Zone.Elements
3:321
-326
Clark WC, Crutzen PJ, Schellnhuber HJ (2004) Science for global sustainability: toward a new paradigm. In: Schellnhuber HJ, Crutzen PJ, Clark WC, Claussen M, Held H (eds) Earth System Analysis for Sustainability. The MIT Press, Cambridge, pp1 -28
Derry LA, Chadwick OA (2007) Contributions from
Earth's atmosphere to soil. Elements 3:333
-338
Fletcher RC, Buss HL, Brantley SL (2006) A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation. Earth and Planetary Science Letters244 : 444-457[CrossRef][ISI][GeoRef]
Goddéris Y, François LM, Probst A, Schott J, Moncoulon D, Labat D, Viville D (2006) Modelling weathering processes at the catchment scale: The WITCH numerical model. Geochimica et Cosmochimica Acta 70:1128 -1147[CrossRef][ISI][GeoRef]
Goldhaber MB (2004) Sulfur rich sediments. In: Mackenzie FT (ed) Sediments, Diagenesis, and Sedimentary Rocks. Treatise on Geochemistry 7 (Holland HD, Turekian KK Series editors), Pergamon Press, Amsterdam, pp 257-288
Gustavsson N, Bolviken B, Smith DB, Severson RC (2001)Geochemical Landscapes of the Conterminous United States - New Map Presentations for 22 Elements . U.S. Geological Survey Professional Paper 1648
Heimsath AM, Dietrich WE, Nishiizumi K, Finkel RC (1997) The soil production function and landscape equilibrium.Nature 288:358 -361
Hooke R LeB (2000) On the history of humans as
geomorphic agent. Geology 28:843
-846
IPCC (Intergovernmental Panel on Climate Change) (2007) Working Group 1: The Physical Basis of Climate Change, http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
Jobbagy EG, Jackson RB (2001) The distribution of soil nutrients with depth: Global patterns and the imprint of plants.Biogeochemistry 53:51 -77
Lal R (1989) Land degradation and its impact on food and other resources. In: Pimentel D (ed) Food and Natural Resources. Academic Press, San Diego, pp85 -140
Lal R (2004) Soil carbon sequestration: Impacts on
global climate change and food security. Science304
: 1623-1627
Lal R, Sobecki TM, Livari T, Kimble JM (2004)Soil Degradation in the United States: Extent, Severity, and Trends . CRC Press, Boca Raton, Florida, 204 pp
Leopold A (1933) The conservation ethic.Journal of Forestry 31:634 -643
Lerman A, Wu L (2007) Kinetics of global geochemical cycles. In: Brantley SL, Kubicki JD, White AF (eds) Kinetics of Water-Rock Interaction. Springer-Kluwer Publishers, pp649 -730
Lichtner PC (1988) The quasi-stationary state approximation to coupled mass transport and fluid-rock interaction in a porous medium. Geochimica et Cosmochimica Acta52 : 143-165[CrossRef][ISI][GeoRef]
Liu S, Bliss N, Sundquist E, Huntington TG (2003) Modeling carbon dynamics in vegetation and soil under the impact of soil erosion and deposition. Global Biogeochemical Cycles17 : 1074, doi10.1029/2002GB002010[CrossRef]
McNeill JRA, Winiwarter V (2004) Breaking the sod:
Humankind, history, and soil. Science.304
: 1627-1629
Millenium Ecosystem Assessment (2005)Ecosystems and Human Well-Being: Desertification Synthesis . World Resources Institute, Washington DC
Miller PJ, Van Atten C (2004) North American Power Plant Air Emissions. Commission for Environmental Cooperation of North America, Montreal, 87 pp (accessed on 2/15/07, www.cec.org/files/pdf/POLLUTANTS/PowerPlant_AirEmission_en.pdf)
Murphy SF, Brantley SL, Blum AE, White AF, Dong H (1998) Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: II. Rate and mechanism of biotite weathering.Geochimica et Cosmochimica Acta 62:227 -243[CrossRef][ISI][GeoRef]
Navarre-Sitchler A, Brantley S (2007) Basalt weathering across scales. Earth and Planetary Science Letters261 : 321-334[GeoRef]
Porder S, Paytan A, Vitousek PM (2005) Erosion and landscape development affect plant nutrient status in the Hawaiian islands.Oecologia 142:440 -449[CrossRef][ISI][Medline]
Raymond PA, Cole JJ (2003) Increase in the export of alkalinity from North America's largest river. Science301 : 88-91[CrossRef][ISI][GeoRef]
Reimann C (2005) Sub-continental-scale geochemical
mapping: sampling, quality control, and data analysis issues.Geochemistry: Exploration, Environment, Analysis5
: 311-323
Richter DD, Markewitz D (2002) UnderstandingSoil Change . Cambridge University Press, Cambridge, UK,255 p
Sak PB, Fisher DM, Gardner TW, Murphy K, Brantley SL (2004) Rates of weathering rind formation on Costa Rican basalt.Geochimica et Cosmochimica Acta 68:1453 -1472[CrossRef][ISI][GeoRef]
Schimel DS, Braswell BH, Holland EA, McKeown R, Ojima DS, Painter TH, Parton WJ, Townsend AR (1994) Climatic, edaphic, and biotic controls over storage and turnover of carbon in soils. Global Biogeochemical Cycles 8:279 -294[CrossRef][ISI][GeoRef]
Smith DB, Cannon WF, Woodruff LG, Garrett RG, Kalssen R, Kilburn JE, Horton JD, King HG, Goldhaber MB, Morrison JM (2005) Major- and Trace-Element Concentrations in Soils from Two Continental-Scale Transects of the United States and Canada. USGS Open-File Report 2005-1253 (http://pubs.usgs.gov/of/2005/1253/pdf/OFR1253.pdf)
Troeh FR, Thompson LM (1993) Soils and Soil Fertility. Oxford University Press, Oxford,470 pp
Velbel MA (1986) The mathematical basis for determining rates of geochemical and geomorphic processes in small forested watersheds by mass balance: Examples and implications. In: Colman SM, Dethier DP (eds) Rates of Chemical Weathering of Rocks and Minerals. Academic Press, pp. 439-449
von Blanckenburg F (2005) The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment. Earth and Planetary Science Letters237 : 462-479[CrossRef][ISI][GeoRef]
von Blanckenburg F, Hewawasam T, Kubik PW (2004) Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka. Journal of Geophysical Research 109: 1-22, F03008, doi: 10.1029/2003JF000049
White AF (1995) Chemical weathering rates of silicate minerals in soils. In: White AF, Brantley SL (eds) Chemical Weathering Rates of Silicate Minerals. Mineralogical Society of America Reviews in Mineralogy 31, pp 407-458
White AF, Blum AE, Schulz MS, Vivit DV, Stonestrom DA, Larsen M, Murphy SF, Eberl D (1998) Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: I. Long-term versus short-term weathering fluxes. Geochimica et Cosmochimica Acta62 : 209-226[CrossRef][ISI][GeoRef]
White AF, Brantley SL (eds) (1995) Chemical Weathering Rates of Silicate Minerals. Mineralogical Society of America Reviews in Mineralogy 31, 584 pp
White AF, Brantley SL (2003) The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chemical Geology202 : 479-506[CrossRef][ISI][GeoRef]
Wilkinson BH (2005) Humans as geologic agents: A
deep-time perspective. Geology 33:161
-164
Wilkinson BH, McElroy BJ (2007) The impact of humans
on continental erosion and sedimentation. Geological Society of America
Bulletin 119:140
-156
Yang D, Kanae S, Oki T, Koike T, Musiake K (2003) Global potential soil erosion with reference to land use and climate changes.Hydrological Processes 17:2913 -2928[CrossRef][ISI][GeoRef]
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, 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
