Thallium stable isotope ratios in naturally Tl-rich soils

Soils on the Erzmatt (Switzerland) formed on hydrothermally mineralized dolomite rock and are naturally Tl-rich. In this study, we investigated if variations in the stable Tl isotope ratios in soil samples from different profiles can be linked to data on the extractability and speciation of soil Tl and whether the isotopic data allow drawing conclusions on the geochemical processes that affected Tl over the course of soil formation. In two soil profiles, we observed a marked accumulation of the heavy Tl-205 isotope in the B horizons, with epsilon Tl-205 values that were up to 7 higher than in the underlying bedrock. This Tl-205 enrichment, however, was neither reflected in the speciation of Tl nor its chemical fractionation. Furthermore, exchangeable soil Tl in the B horizons was found to be much isotopically lighter than the bulk soil Tl. These findings suggest that the observed isotopic shift may be linked to cyclic Tl mobilization and immobilization processes over the period of rock weathering and soil formation. Oxidative Tl uptake by Mn-oxides associated with a Tl-205 enrichment, continuous weathering of the Tl(III)-containing phases, followed by a Tl(I) remobilization (leading to enrichment in Tl-205) are suggested to be responsible for the binding of the heavy Tl isotope fraction into other phases, mainly illite (a dominant Tl host), which is not normally expected. Hence, our results show that the Tl isotopic fractionation data measured in a dynamic multi-phase (soil) system can potentially serve as a proxy for tracing redox-controlled processes, but their use for phase or the sorption process identification is much more complicated.