The Effect of Temperature and Precipitation on Sodium Depletion Fronts in Soils Developed on Peoria Loess
Open Access
- Author:
- Williams, Jennifer Zan
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 22, 2008
- Committee Members:
- Susan Louise Brantley, Thesis Advisor/Co-Advisor
- Keywords:
- mathematical modeling
sodium depletion
activation energy of dissolution
albite weathering
climosequence
soils
Global Climate Model - Abstract:
- A north-south transect along the Mississippi River valley provides an opportune environmental gradient across which to investigate chemical weathering. Soil profiles along this transect are interpreted to have developed from a uniform parent material, the Peoria Loess, with pedogenesis commencing between 13 – 10 14C ka. We examined mineral evolution in these soils using X-ray fluorescence (XRF) and X-ray powder diffraction (XRD) mineralogical analysis. Results indicated quartz, feldspars, and clays dominate the loess and soils. In each pedon the Na concentrations generally decrease from the deepest collected sample to the surface, defining depletion profiles. These depletion profiles document the chemical weathering of plagioclase. Consistent with this interpretation, kaolinite and illite concentrations are greatest in the surface horizons and decrease with depth. In addition, the montmorillonite concentrations are observed greatest with depth, consistent with primary mineralogy inherited from the source region. In order to interpret the concentration versus depth profiles as a function of climate variation along this transect, we simulated climate at 10 ka and 6 ka, as well as for modern day using the GENESIS v2 Global Climate Model (GCM). The GCM calculated annual averages for surface air temperature, precipitation, and downward flux of soil pore water. Uniform soil texture was assumed to be constant throughout this transect from 28º to 50º N latitude and 94º to 88º W longitude along the Mississippi River valley. The Na profiles were fit to mathematical model equations for profile development using the GCM outputs to determine values of a kinetic parameter for each pedon. The fraction of albite dissolved (f) varied from approximately 0% in the north to 26% in the south as determined through integration of the model equations. Variation in f is observed to largely depend upon the variation in annual rainfall from north (0.5 m / y) to south (1.2 m / y). The kinetic parameters were corrected for porefluid advection along the transect and plotted against 1/T. From these plots, the best estimate of the apparent activation energy for Na-plagioclase dissolution is observed to be 95 ± 24 kJ/mol. Such quantitative interpretations of soil profiles can provide ways to understand and predict the effects of climate on soil chemistry.