DFG project ArTTA-ICE: Final report

The aim of this project was to explore the potential of Atom Trap Trace Analysis (ATTA) for age dating of glacier ice using the radioactive noble gas isotope 39Ar. The first targets for the application of this novel dating approach were cold-based summit glaciers in the Austrian Alps, which offer excellent preservation of stratified ice layers and climatic signals but have been difficult to use due to limitations of existing dating techniques. 39Ar, with a half-life of 268 years, fills a critical gap in ice chronologies between about 100 and 1000 years, a range poor-ly covered by traditional radiometric dating tracers such as 210Pb, 3H, and 14C. The analysis of 39Ar in reasonably small samples of glacier ice, however, has only become feasible by the recent development of the ATTA technique, which uses laser cooling and trapping to isolate and count single atoms, hugely increasing sensitivity compared to radioactive decay counting. The ATTA laboratory at Heidelberg University – one of only two labs worldwide capable of performing 39Ar ATTA (or ArTTA) measurements – has previously demonstrated 39Ar analy-sis on ice samples of a few kilograms. This project established and optimized a complete 39Ar dating workflow for ice, from sampling strategies and contamination control to gas extraction and measurement, and validated re-sults through comparison with independent methods and glaciological modeling. Comparison campaigns on Austrian glaciers such as Jamtalferner and Weißseespitze confirmed the method’s reliability and robustness across sampling techniques, including the use of ice core samples. As these glaciers are rapidly losing their most recent layers in times of global warm-ing, the unique ability of 39Ar to detect surface ages on the centennial timescale, extending beyond the reach of previously used industrial-era tracers, was found to be of high relevance. Another great success was the seamless integration of 39Ar ages with 14C dating of deeper, older ice layers for two long (6 to 12 ka) Alpine ice core records from Weißseespitze (Austria) and Dôme du Goûter (France). The combination of 39Ar with 14C and other dating methods also enabled the establishment of consistent chronologies of a cave ice deposit in Italy an ice cap on Svalbard (Norway). The approach of ArTTA analysis of small samples was further extended to a first demonstra-tion of the feasibility of 39Ar exposure dating of moraine boulders. Collectively, these devel-opments show the wide and reliable applicability of the 39Ar dating tracer in glaciology and demonstrate its critical value for the understanding of endangered glacial climate records.