Abteilung Wasserressourcen und Trinkwasser

Lake Kivu is a 485 m deep tropical rift lake in East-Africa and well-known for its very high concentrations of dissolved carbon dioxide and methane in the stratified deep waters. In view of future large-scale methane extraction for power production, there is a need for predicting the evolution of gas concentrations and lake stability using numerical modelling. However, knowledge about the geochemical origin and transport processes affecting dissolved gases in the lake is still partially missing. Due to their inert nature, the analysis of dissolved noble gases can help to shed light on such questions. To learn more about transport processes in Lake Kivu, we extended a well-established sampling method for dissolved noble gases to work in the lake's high gas pressure waters. The results of our analysis show a distinct non-atmospheric isotopic signal in the deep waters (below 250 m) with ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios ~250% and ~20% higher than air saturated water (ASW). Moreover, the gas concentration profiles reveal a striking lack of atmospheric noble gases in the deep waters with respect to ASW. While Ne is depleted by ~45%, the more soluble ³⁶Ar and Kr even decrease by ~70%. In contrast, ⁴He concentrations increase with depth by a factor of up to ~600. We attribute this excess He and the increases in ³He/⁴He and ⁴⁰Ar/³⁶Ar to the inflow of magmatic gases into Lake Kivu, along with a significant contribution of radiogenic ⁴He. To explain the depletion of atmospheric noble gases, we present and discuss three different scenarios, namely continuous outgassing, the inflow of depleted groundwater and a large past outgassing event. Due to the best agreement with our observations, we conclude that the inflow of depleted groundwater is likely responsible for the observed atmospheric noble gas depletions.

X-ray tomographic microscopy is a well-established analysis technique in different fields of the Earth Sciences to access volumetric information of the internal microstructure of a large variety of opaque materials with high-spatial resolution and in a non-destructive manner. Synchrotron radiation, with its coherence and high flux, is required for pushing the temporal resolution into the second and sub-second regime and beyond, and therefore moving from the investigation of static samples to the study of fast dynamic processes as they happen in 3D. Over the past few years, several hardware and software developments at the TOMCAT beamline at the Swiss Light Source contributed to establishing its highly flexible and user-friendly fast tomography endstation, making a large variety of new dynamic in situ and operando investigations possible. Here we present an overview of the different devices, including an in-house developed detector, a new highly efficient macroscope and a programmable fast rotation stage. Their tight interplay and synchronization are key for lifting experimental design compromises and follow dynamic processes with high spatial and temporal resolution unfolding over prolonged periods of time, as often required by many applications. We showcase these new capabilities for the Earth Sciences community by presenting three different geological studies, which make use of different sample environments. With a tri-axial deformation rig, chemo-mechanical-hydraulic feedbacks between gypsum dehydration and halite deformation have been studied, while the spatio-temporal evolution of a solute plume has been investigated for the first time in 3D with a flow cell. A laser-based heating system available at the beamline provides access to the high temperatures required to address bubble growth and collapse as well as bubble-bubble interaction and coalescence in volcanological material. With the integration of a rheometer, information on bubble deformation could also be gained. In the near future, upgrades of most large-scale synchrotron radiation facilities to diffraction-limited storage rings will create new opportunities, for instance through sub-second tomographic imaging capabilities at sub-micron length scales.

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 ²⁰⁵Tl isotope in the B horizons, with ε²⁰⁵Tl values that were up to 7 higher than in the underlying bedrock. This ²⁰⁵Tl 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 ²⁰⁵Tl enrichment, continuous weathering of the Tl(III)-containing phases, followed by a Tl(I) remobilization (leading to enrichment in ²⁰⁵Tl) 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.