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Abstract

U and Th isotopic composition have been proven to be a valuable tool in environmental science, oceanography, hydrology, geology, nuclear forensics, and science-based archaeology. These applications require simultaneous detection of both minor and major isotopes at high accuracy. Often, the amount of sample or concentration of U and Th are low and therefore require high measurement sensitivity. The use of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) can yield improved precision for low-abundance isotopes and mass-limited samples by the high absolute sensitivity, simultaneous detection of ions with the multicollector detector block, and higher sample throughput. This can be achieved only with proper optimization of measurement protocol. Measurements with MC-ICP-MS require sample pre-processing in order to reduce matrix interferences and pre-concentrate samples with too low concentrations for direct measurements. These procedures can themselves induce isotope fractionations and consequently yield biased results. Several different protocols have been reported so far, but none with a clear indication of its performance with respect to the mass bias. Therefore, different protocols had been compared in this dissertation and the best one was further optimized in order to ensure reliable, high-quality data on U and Th isotope ratios. The combination of coprecipitation with Ca3(PO4)2 as the preconcentration technique and extraction chromatography with UTEVA resin precleaned in 6 M HNO3 as the separation technique was chosen as an optimal analytical protocol with the highest accuracy. Other techniques had higher column matrix effects, owing to the direct contribution of the organic material stripped from the resin, which was shown to be the biggest influence on precise and accurate U results. The optimal protocol was then used to introduce U and Th as novel environmental tracers to study hydrogeochemical processes in karstic systems with predominantly carbonate lithology. In a dynamic karst river system with the heterogeneous geologic framework, classical hydrogeological approaches are sometimes not enough to properly investigate these complex karstic processes. The combined use of geochemical and physicochemical parameters with U and Th isotopic composition significantly improved the understanding of karst hydrodynamics. The Ljubljanica River catchment and the Krka River in Croatia were the first case studies where U and Th isotope disequilibrium have been used to provide a new perspective to already known data. The 234U/238U activity ratio of dissolved U was found to be a trusted tracer of groundwater and river water sources, reflecting the variability of discharge and lithology of the catchment, and mixing water from different sources. Isotopically lighter U was co-precipitated with carbonate in flowstone and tufa without fractionation, therefore U isotope ratios in terrestrial carbonate formations could reflect the storage of CO2 as authigenic carbonate in tufa. Th and U concentrations and their isotope ratios in carbonate materials were also shown to be a good indicator of carbonate detrital contaminations.