Uranium-series Geochemistry in Residual Soils: Examples from Central Pennsylvania
Open Access
- Author:
- Carte, Jared L
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 07, 2019
- Committee Members:
- Matthew S Fantle, Thesis Advisor/Co-Advisor
Julie Genevieve Cosmidis, Committee Member
Herschel Adams Elliott, Committee Member - Keywords:
- Uranium
Weathering
Geochemistry
Soils
Soil Uranium - Abstract:
- A significant volume of research utilizing soil uranium activity ratios as tracers of fluid-mineral interactions, chronometers of weathering processes, and records of paleohydrology and paleoclimate has been conducted over the past ~30 years. However, there is a need to better constrain the underlying mechanisms leading to uranium activity ratio [(234U/238U), where parentheses indicate activity ratio] variations in soils in order to refine models and allow for more rigorous data interpretation. This is especially true for unique soil environments, such as the old (~10 Ma) residual soils formed on carbonate bedrock in the Nittany Valley of Central Pennsylvania investigated in the present study. The goal of this study is to interpret variations in soil (234U/238U) using a suite of measurements including particle size, major element composition, trace element chemistry, mineralogy, and sequential chemical extractions. This allows for the relative importance of various physical and chemical properties of the soil in controlling (234U/238U) values to be determined. The combination of major element, mineralogical, and sequential extraction datasets indicate that the majority of uranium is found in the clay mineral fraction of these soils, with relatively small fractions associated with iron (oxy)hydroxides, organic matter, and adsorbed on mineral surfaces. Particle size measurements combined with α-recoil models indicate that the general depth trends in (234U/238U) secular disequilibria are primarily controlled by changes in particle geometry in different depth intervals. However, the magnitude of (234U/238U) secular disequilibria based on particle size and α-recoil models [(234U/238U) ranges from 0.47 to 0.76] is much greater than actually measured in these soils [(234U/238U) ranges from 0.875 to 0.987]. Combining sequential extraction data with mass balance calculations suggests that the incorporation of relatively high (234U/238U) uranium from soil pore waters and fertilizer can partially explain this difference, but this process alone is not sufficient to fully explain the discrepancy between modeled and measured soil (234U/238U). Therefore, bulk mineral dissolution must be the dominant factor controlling (234U/238U) values in these soils. Mineral dissolution is shown to increase measured (234U/238U) values by as much as 0.45 above steady state α-recoil based predictions. Independent evidence for extensive dissolution in these soils is provided by negative U mass transfer coefficient values and chemical index of alteration values as high as 94. The results of this study have important implications for the application of U-series nuclides to soils and weathering profiles. This work provides evidence that mineral dissolution can be a first-order factor controlling soil (234U/238U), contributes a unique dataset for old residual soils formed on carbonate bedrock, and systematically evaluates the role that incorporation of pore fluid and fertilizer derived U can play on soil (234U/238U) values.