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Systematic improvement of 39Ar Atom Trap Trace Analysis and its application to derive a millennial palaeotemperature record from groundwater

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2017 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 324116750
 
Final Report Year 2022

Final Report Abstract

With a half-life of 269 years and conservative behaviour as a noble gas, the radioisotope 39Ar is ideally suited to provide age information for water and ice in the centennial age range, which is otherwise not accessible. However, only the recently developed quantum technology Argon Trap Trace Analysis (ArTTA) enables the analysis of 39Ar on reasonably small samples from the environment. The main objectives of this project were to to improve the existing prototype ArTTA machine at the Kirchhoff-Institute for Physics (KIP) in Heidelberg and to apply it for pioneering 39Ar applications, especially in the field of groundwater research, in combination with complementary tracer methods available at the Institute of Environmental Physics (IUP). A particular focus was laid on using 39Ar to constrain age distributions (so called transit time distributions or TTDs) of mixed groundwater and the combination of 39Ar based chronologies with noble gas temperatures (NGTs) to reconstruct climate change signals from the past millennium. Despite a number of setbacks and unexpected turn of events, we ultimatively succeeded in turning the prototype ArTTA machine into a reliable system for routine ArTTA analysis and to implement several ground-breaking applications of the method, which in particular demonstrated the usefulness of 39Ar in TTD determination. The main reasons for significant delays in the progress of the project were laboratory accidents triggered by power failures, which made extensive repairs to the system necessary, as well as interruption of lab work in the first phase of the Covid-19 pandemic. As the scientific highlights of this project we see: Successful implementation of the proposed doubling of the count rate of the ArTTA machine, making a precise dating with 39Ar using water samples as small as 10 L possible. • Realisation of operational procedures and facilities for sampling, extraction and preparation of 39Ar samples from diverse systems such as water, ice, air and gases of unusual composition. • First observation of ocean ventilation by 39Ar in highly resolved depth profiles, constraining local deep ocean TTDs and revealing a higher anthropogenic CO2 storage than so far assumed. • First dating of alpine glacier ice employing the ArTTA technique giving new insights into the chronology of ice formation in the Alps. • First full-scale ArTTA-based groundwater dating study in a highly exploited aquifer in Southern Oman, enabling a demonstration of the potential of 39Ar data to inform state-of-the-art methods for groundwater TTD determination. • Collection of substantial 39Ar and noble gas data sets from groundwaters in Germany and the Netherlands, which still have to be fully exploited regarding the determination TTDs and potentially the compilation of NGT records, although unexpectedly low NGTs may prevent reliable climate reconstruction in some cases. • First application of 39Ar to a lake (Lake Kivu, Democratic Republic of the Congo and Rwanda). The unexpected result of super-modern 39Ar concentrations in this lake prevented the intended deep water dating but instead revealed an input of Ar highly enriched in nucleogenic 39Ar via inflowing groundwater from young basaltic rocks of the volcanic setting of this exceptional lake. The innovative technology developed in this project and its first applications have attracted significant media resonance, e.g., Austrian newspapers and TV reporting on the dating of glacier ice or certain online media reporting on our first study of ocean ventilation.

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