Assessment of a sequential phase extraction procedure for uranium-series isotope analysis of soils and sediments

Abstract

The study of uranium-series (U-series) isotopes in soil and sediment materials has been proposed to quantify rates and timescales of soil production and sediment transport. Previous works have studied bulk soil or sediment material, which is a complex assemblage of primary and secondary minerals and organic compounds. However, the approach relies on the fractionation between U-series isotopes in primary minerals since they were liberated from the parent rock via weathering. In addition, secondary minerals and organic compounds have their own isotopic compositions such that the composition of the bulk material may not reflect that of primary minerals. Hence, there is a need for a sample preparation procedure that allows the isolation of primary minerals in soil or fluvial sediment samples. In this study, a sequential extraction procedure to separate primary minerals from soils and sediments was assessed. The procedure was applied to standard rock sample powders (TML-3 and BCR-2) to test whether it introduced any artefactual radioactive disequilibrium. A new step was introduced to remove the clay-sized fraction (<2 µm). Significant amounts (5-14%) of U and Th were removed from the rock standards during the procedure. No significant alteration in ((234)U/(238)U) and ((230)Th/(238)U) activity ratios of the rock standards occurred during the procedure. Aliquots of soil sample were subjected to the sequential extraction process to test how each step modifies the uranium-series activity ratios and mineralogy. Although no secondary minerals were detected in the unleached soil aliquots, the sequential leaching process removed up to 17% of U and Th and modified their activity ratios by up to 3%. The modification of the activity ratios poses a demand for careful means to avoid redistribution of isotopes back to the residual phase during phase extraction.

Uranium isotopes and dissolved organic carbon in loess permafrost: Modeling the age of ancient ice

“…7b). These data could indicate that U is concentrated in readily dissolved secondary precipitates or loosely sorbed onto particle surfaces due to the liquid water component in permafrost (Osmond and Cowart, 2000), which is consistent with observed U series isotope trends in studies of weathering rinds and soils (Pelt et al, 2008;Suresh et al, 2014). Secondary precipitates might include carbonates, which are known to accumulate U and would be released with application of acid.…”

“…In addition, BET surface area should be routinely assessed as argued in other studies, and while the leaching procedure used here provided a reasonable first step, it could be refined. In particular, use of buffered solutions and particle sizing should be considered (Goldstein et al, 2004;Aciego et al, 2011;Suresh et al, 2013Suresh et al, , 2014. Nonetheless, this work provides evidence that preservation of ground ice through periods of climate warming is a function of landscape processes such as insulation by downslope sediment redistribution, peat accumulation, and plant community succession, as well as the intrinsic properties of syngenetic loess permafrost (high ice content, low hydraulic conductivity upon thaw, location on water-shedding slopes).…”

“…The leach sequence was intended to provide a simple evaluation of the amount and isotopic composition of U retained in secondary coatings on particles (e.g., carbonate, iron oxide, organics), as well as the amount and isotopic composition of U in the residual solid. By measuring ( 234 U/ 238 U) values in all leachates and residues in these samples, we attempted to limit the uncertainties inherent in procedurally defined leaching steps, although in future studies, buffered solutions and particle sizing would be preferred (Lee et al, 2010;Handley et al, 2013;Suresh et al, 2014). Our aim was simply to capture U that might be present in highly exchangeable sites, sorbed tightly onto mineral surfaces, or present in organic complexes.…”

“…If present, weathering rinds may complicate interpretation of ( 234 U/ 238 U) values as a metric of time by adding surface area as well as U adsorbed from the surrounding fluid ( Fig. 1) (Pelt et al, 2008;Suresh et al, 2013Suresh et al, , 2014. While uncertainties of model ages based on ( 234 U/ 238 U) values in both sediment and ice can be substantial, their utility has been demonstrated (Dosseto et al, 2010;Aciego et al, 2011;Handley et al, 2013).…”

Sediment residence time reveals Holocene shift from climatic to vegetation control on catchment erosion in the Balkans

“…DePaolo et al, 2006;Lee et al, 2010;DePaolo et al, 2012;Dosseto and Schaller, 2016) as they are sensitive to changes in the depth of hillslope erosion, the spatial origin o f sediments (erosion of areas with thin versus thick soil covers), and the duration of alluvial transport. Uraniu m-234 beco mes continuously depleted fro m detrital matter by: (a) d irect recoil of 234 Th, an intermediate product between 238 U and 234 U, (b) preferential leaching of 234 U embedded in the reco il tracks, and (c) preferential o xidation of 234 U compared to 238 U (Dosseto et al, 2008a;Ma et al, 2010;Suresh et al, 2013;Suresh et al, 2014a;Suresh et al, 2014b;Gontier et al, 2015). The deplet ion of 234 U becomes significant in the outer rim of detrital grains with large surface-to-volu me rat ios.…”