Browsing by Subject "Vertisols"
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Item Determining the bimodal infiltration patterns in three playa lakes(Texas Tech University, 1990-08) Evans, Perry WayneNot availableItem Investigating pedogenic carbonate formation by measuring the stable isotope composition of water in Vertisols(2014-05) Okafor, Brandon Jerrod; Breecker, Dan O.; Young, Michael H; Banner, JayThe oxygen isotope compositions of pedogenic carbonates in paleosols are used to reconstruct paleoelevations, paleoatmospheric circulation, paleotemperatures, and paleoprecipitation. The oxygen isotope compositions of pedogenic carbonates are controlled by temperature and the oxygen isotope composition of soil water, which predominately originates from precipitation. In most calcic soils studied, pedogenic carbonates record the oxygen isotope composition of summer precipitation and/or mean annual precipitation subjected to evaporation. However, due to the complex hydrological properties of Vertiols, which are abundant in the rock record, the isotopic composition of soil water could potentially vary and could influence the isotopic composition of pedogenic carbonate. Furthermore, it is well established that soils contain multiple pools of water with different stable isotope compositions but little work has been done to investigate which pools are recorded by pedogenic carbonates. Therefore, the isotopic composition of soil water in modern Vertisols was monitored and compared with the oxygen isotope composition of pedogenic carbonate in the same soils to investigate if the oxygen isotope composition of pedogenic carbonates in Vertiols record mobile or immobile water. The isotope composition of soil water was determined in four ways: 1) measurement of isotope composition of water collected by vacuum distillation of soil samples collected by auger, 2) calculation from measured oxygen isotope compositions of soil CO₂, 3) calculation from measured oxygen isotope compositions of pedogenic carbonate, and 4) measured isotope compositions of water collected under tension in a soil solution sampler. The oxygen isotope compositions of water in equilibrium with CO₂ and water from the solution sampler were indistinguishable at 140cm and were interpreted as mobile water in macropores. The vacuum distilled water (which includes water from a mixture of macropores and micropores) always had lower δ ¹⁸O values than the macropore water and the other sampling methods, implying the presence of water with low δ ¹⁸O values. These oxygen isotope compositions of soil water pools were compared with δ ¹⁸O values of local precipitation (GNIP data from nearby Waco, TX). Below ~100cm, total soil water δ ¹⁸O values converge to -6.3 ± 0.7 %₀ (1σ, n=20), which is isotopically lighter than the δ ¹⁸O of mean annual precipitation (MAP) of Waco, Texas (-3.8 ± 2.7 %₀, 1σ, n=96). This could result from recharge of isotopically light September precipitation (SEPT); (-5.9 ± 2.4 %₀, 1σ, n=8)) replenishing the soil after dry periods and/or the contribution of winter precipitation (WP) (-5.5 ± 2.4 %₀, 1σ, n=25). The δ ¹⁸O values of soil water in equilibrium with soil CO₂ ((-4.1 ± 0.8%₀) are isotopically similar to or heavier than the isotopic composition of MAP. The δ ¹⁸O values of soil water in equilibrium with pedogenic carbonate (-2.7 ± 0.9%₀) are also isotopically similar to the isotopic composition of summer precipitation (SP, including June, July, and August) (-2.0 ± 2.9 %₀, 1σ, n=8). This suggests that, despite the more complex hydrology of Vertisols compared with other soils orders, the δ ¹⁸O values of pedogenic carbonates formed in central Texas Vertisols record SP and/or mean annual precipitation that has been subjected to evaporation, just as they do in other soils. If this holds true for Vertisols formed in other climates, then this facilitates the comparison among δ ¹⁸O values of paleosol carbonates from various soil orders, which is common practice in vertical successions of paleosols. Furthermore, the observation that the σ ¹⁸O values of water in equilibrium with pedogenic carbonate are more similar to the σ ¹⁸O values of macropore than micropore water suggests that pedogenic carbonates in central Texas Vertisols may form in macropores. Formation in macropores is more consistent with CO₂ degassing and/or evaporation, rather than root water uptake, as a proximal driver of calcite precipitation.Item Monitoring Cracking of a Smectitic Vertisol using Three-dimensional Electrical Resistivity Tomography(2013-11-20) Ackerson, Jason PaulUpon desiccation, the matrix of Vertisols and other expansive soils shrinks. Matrix shrinkage results in the formation of cracks that can alter the hydrology of the soil. Despite the importance of cracks, many hydrologic models do not account for cracking due in part to a lack of reliable information on the development and morphology of cracks. Electrical resistivity tomography (ERT) has shown promise as a new, non-destructive method of monitoring cracking in the field. We investigated the use and limitation of ERT for monitoring the spatial degree and extent of cracking in a Texas Vertisol. First, we examined the relationship between soil water content and ERT derived bulk soil electrical resistivity. Results showed that when the soil was cracked, ERT is insensitive to changes in water content with the electrical resistivity of the soil much greater than would be predicted from changes in water content alone. For a direct measurement of the degree and extent of cracking, we filled cracks with cement, excavated the soil, and photographed the exposed cracks. Comparing direct crack measurements with ERT images of the electrical resistivity of the subsoil, we found that a simple linear model could describe the relationship between crack volume and bulk electrical resistivity. Unfortunately, the fit of this model was poor (R^2 from 0.4-0.6) and it showed little promise for accurately estimating crack volume. As a tool for monitoring cracks, it appears that ERT is best suited for identifying probable locations of cracks rather than quantitative evaluation of crack morphology.Item Spatial and Temporal Distribution of Desiccation Cracks in Shrink-Swell Soils(2014-04-17) Neely, Haly LurySoil crack volume estimates, which are important for hydrology models on shrink-swell soils, are currently based on field measurements of vertical shrinkage and an assumption of isotropic shrinkage; however, few studies have validated the resulting crack volume estimates and studies have been limited to soils with very high shrink-swell potentials. In addition, the spatial variability of soil cracking potential is not well understood. First, I was able to improve in situ measurements of soil shrinkage by using a single borehole for all vertical soil movement and water content measurements. Then measurements of soil layer thickness and water content were made for seven soils with varying COLE values, from 0.01 to 0.17 m m^(-1). Soil crack volume was estimated using cement slurry and photographing excavated soil layers at the end of the study. Over drying and wetting cycles, the relationship between soil layer thickness and water content was linear. Modifying an existing crack volume equation with shrink-swell potential and water content was a better fit to cement-estimated crack volume than the unmodified estimates, improving the r^(2) from 0.06 to 0.84. The model over-predicted soil crack volume by a factor of 10 and a minimum shrinkage volume was required to generate visible soil crack volume. Finally, proximally-sensed bulk apparent electrical conductivity was highly correlated to inorganic C, and the depth of maximum sensitivity of the instrument was deeper than suggested by previous research in coarser textured soils. Because inorganic C is related to shrink-swell potential, it may be possible to use proximal sensors to map shrink-swell potential variability.