Abstract: Weathering produces porosity that influences rocks’ hydrologic properties and thus affects ecosystem processes. The objective of this research was to quantify three-dimensional geometrical pore parameters and characterize microstructure differences as rock weathers. Three granodiorite rock fragments (clasts) and two soil clods sampled from a moraine chronosequence in the eastern Sierra Nevada, California, were evaluated. Surface ages of the parent moraines, ranging from 78 to 120 ka, were used to approximate clast weathering. Samples were scanned at a volume element (voxel) size of 6.9 × 104 μm3 at the High-Resolution X-ray computed tomography (CT) facility at the University of Texas, Austin. Images were analyzed with the Three-dimensional Medial Axis Rock (3DMA) software (CT-3DMA). Porosity estimated by bulk density and CT methods indicated excellent agreement between the two methods. Characteristic coordination numbers increased with weathering, and the soil had the highest value; means for 78-ka rock and 100-ka rock and soil were 2.49, 2.68, and 4.32. The difference between rock treatments was not significant, but tended to increase with age. The difference between soil and rock was significant. The characteristic path length varied between 1.20 and 1.46 but was not different among rock age. The mean value for the soil was 0.86, and the difference between rock and soil was significant. Pore path tortuosity ranged from a high of 1.35 in the 78-ka rock to a low of 1.31 in the soil, with significant differences between rock and soil. Results suggest that rocks imaged with X-ray CT and analyzed in 3DMA will help quantify microstructural differences in pore geometry related to rock weathering.
1Center for Agroforestry and
2Department of Soil, Environmental and Atmospheric Sciences, School of Natural Resources, University of Missouri, Columbia, MO;
3Department of Environmental Science and Technology, University of Maryland, College Park, MD;
4Soil & Water Sciences Program, Department of Environmental Sciences, University of California, Riverside, CA;
5Jackson School of Geosciences, University of Texas, Austin, TX.
Address for correspondence: Ranjith P. Udawatta, Department of Soil, Environmental and Atmospheric Sciences, 302 ABNR, University of Missouri, Columbia, MO 65211; E-mail: UdawattaR@missouri.edu
Financial Disclosures/Conflicts of Interest: The work was funded by a grant from the University of California Kearney Foundation of Soil Science awarded to R. C. Graham.
Received October 4, 2011.
Accepted for publication May 7, 2012.