Progress 10/01/07 to 09/30/12
Outputs
OUTPUTS: Overall outputs for the University of Arizona portion of this project included: (i) monitoring of soil moisture and temperature sensors across the environmental gradient that represents the southern Arizona node of this project; (ii) detailed soil physical, mineralogical, and geochemical characterization across the gradient and for various landscape positions; (iii) quantification of long-term erosion rates using cosmogenic nuclides measured in the saprock; (iv) measurement of soil organic carbon mean residence time using radiocarbon analyses; (v) numerical modeling that couples erosion, climate, and topography to predict soil depth and landscape evolution; and (iv) inclusion of these data in global synthesis of climate and tectonic controls on chemical weathering and denudation. We also began high resolution mapping of soil physical and chemical properties for specific catchments across the environmental gradient - this work is currently ongoing and in preparation. These results have been disseminated through a number of peer reviewed publications and presented at national and international meetings such as the Soil Science Society of America, the American Geophysical Union, Goldschmidt, European Geosciences Union, and meetings of the Critical Zone Observatory investigators and students. PARTICIPANTS: Collaborators on this project at the University of Arizona included Jon Pelletier, Jon Chorover, Paul Brooks, and Peter Troch. Graduate students trained through this project included Katherine Heckman, Rebecca Lybrand, Molly Holleran, and Matthew Levi. Research staff involved in this project included Stephen Meding, Assistant Research Scientist, as well as over ten undergraduate students. This project received partial support from the National Science Foundation Critical Zone Observatory Program and the Research Experience for Undergraduates program. TARGET AUDIENCES: The target audiences for this work include the research community in general as well as regional land managers and management agencies. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Arid and semiarid lands cover roughly 36% to 40% of global land area, highlighting the importance these ecosystems play in the global carbon cycle. The controls of arid and semiarid ecosystem carbon cycle processes, such as soil organic matter turnover and mineral weathering, remain poorly understood. To address this knowledge gap, we established a set of long-term soil monitoring sites across a gradient of semiarid ecosystems in the Sonoran Desert of the Southwestern USA. These sites were established as part of the Western Regional Pedology Benchmark Soilscapes project and as part of the newly emergent Critical Zone Exploration Network (CZEN), sponsored in part by the US National Science Foundation. The primary objectives of the Sonoran Desert Environmental Gradient (SDEG) include: (i) characterizing climate forcing controls of soil physical, chemical and biological processes, and the flux of chemical species from soils to surface waters; and (ii) developing predictive models of carbon cycle response to climate and climate change. Linkages among climate, erosion and mineral weathering are central to pedogenesis and critical zone evolution. We approach these linkages through synthesis of climate, erosion and regolith geochemical data for upland terrain, coupled with detailed studies on climate and landscape position controls on pedon-scale regolith weathering patterns across the steep semiarid climate gradient encompassed by the SDEG in southern Arizona, USA. Climate forcing was quantified in terms of effective energy and mass transfer (EEMT), that includes energy flux to the subsurface critical zone in the form of primary production and effective precipitation, whereas chemical depletion and mineral transformation were quantified using a combination of geochemical, isotopic and mineralogical analyses. The regional synthesis indicated regolith chemical depletion increased exponentially with water availability and EEMT for sites with annual temperature greater than 5*C and erosion rates greater than 10 g/m2/yr, suggesting first order control of climate on chemical depletion, and second order control of temperature and erosion. SDEG geochemical and mineralogical data indicated strong linkages among EEMT, physical erosion, regolith depth, chemical depletion and mineral assemblage. Specifically, divergent landscape positions demonstrated a pattern similar to that in the regional synthesis of increasing chemical depletion with increasing EEMT. In contrast, convergent landscape positions demonstrated minimal mineral mass loss and relatively greater content of neogenic secondary mineral phases. Solution chemistry data suggest the convergent positions concentrate soluble weathering products from adjacent divergent positions, thus resulting in locally reduced mineral-solution weathering gradients and promotion of neogenic mineral precipitation. The coupled datasets indicate that timing and amount of available water is a central control on regolith weathering with strong local-scale modification related to landscape position.
Publications
- Rasmussen, C. (2008) Mass balance of carbon cycling and mineral weathering across a semiarid environmental gradient, Geochimica et Cosmochimica Acta 72: A778-A778.
- Pelletier, J.D. and C. Rasmussen. (2009) Geomorphically-based predictive mapping of soil thickness in upland watersheds. Water Resources Research 45: W09417.
- Pelletier J.D. and Rasmussen C. (2009) Quantifying the climatic and tectonic controls on hillslope steepness and erosion rates. Lithosphere 1(2): 73-80
- Rasmussen C., Troch P.A., Chorover J., Brooks P., Pelletier J., and Huxman T. (2011) An open system framework for integrating critical zone structure and function. Biogeochemistry 102(1-3): 15-29.
- Chorover J., Troch P.A., Rasmussen C., Brooks P., Pelletier J., Breshears D.D., Huxman T., Lohse K., McIntosh J., Meixner T., Papuga S., Schaap M., Litvak M., Perdrial J. Harpold A., and Durcik M. (2011) How Water, Carbon, and Energy Drive Critical Zone Evolution: The Jemez-Santa Catalina Critical Zone Observatory. Vadose Zone Journal, 10(3): 884-899, doi: 10.2136/vzj2010.0132.
- Lybrand R., Rasmussen C., Jardine A., Troch P.A., and Chorover J., (2011) The effects of climate and landscape position on chemical denudation and mineral transformation in the Santa Catalina mountain critical zone observatory. Applied Geochemistry, 26(S): S80-S84, doi: 10.1016/j.apgeochem.2011.03.036.
- Rasmussen C., Brantley S., Richter D.D., Blum A., Dixon J., and White A.F. (2011) Strong climate and tectonic control on plagioclase weathering in granitic terrain. Earth and Planetary Science Letters, 301(3-4): 521-530, doi: 10.1016/j.epsl.2010.11.037.
- Pelletier J.D., Barron-Gafford G.A., Breshears D.D., Brooks P.D., Chorover J., Durcik M., Harman C.J., Huxman T.E., Lohse K.A., Lybrand R., Meixner T., McIntosh J.C., Papuga S.A., Rasmussen C., Schaap M., Swetnam T.L., and Troch P.A. (2013) Coevolution of nonlinear trends in vegetation, soils, and topography with elevation and slope aspect: A case study in the sky islands of southern Arizona. Journal of Geophysical Research: Earth Surface, doi: 10.1002/jgrf.20046
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