Novel Dating of Alpine Glaciers by 39Ar-ATTA

Alpine summit glaciers preserve valuable information on past climate conditions, because their internal layer architecture can indicate their response to climate fluctuations as periods of glacier growth, hiatus or mass loss. This information has not yet been recovered, but the glacier archive is under immediate pressure by ongoing warming conditions with local climate extremes. In particular, at present it is not clear to what extent climate fluctuations over the last 1000 years, such as the so-called "Little Ice Age", have affected ice- covered summit regions. Knowledge of the internal age distribution is key to decipher the past accumulation history of the glacier. Because annual layer counting is not possible at these sites, age information has to come from radiometric dating. In the age range of 100- 1000 years, only the noble gas radioisotope 39Ar has a suitable half-life of 269 years to serve as a radiometric dating tool for water and ice. However, the abundance of 39Ar is extremely low, with only about 10.000 39Ar atoms contained in 1 kg of modern ice. Recent major technical advances in measuring 39Ar by Atom Trap Trace Analysis (ArTTA) have paved the way to make 39Ar dating of glacier ice feasible by reducing the required sample size from tons to a few kilograms. This project aims at developing, validating and ultimately applying the ArTTA dating method to retrieve untapped climate records. A unique opportunity for the proposed research is generated by ongoing research activities at the Austrian Academy of Sciences and Heidelberg University. A collaborative pilot study has already successfully demonstrated the feasibility of 39Ar glacier ice dating. The proposed activities comprise full method development, i.e. to go beyond mere demonstration of the feasibility to a systematic analysis of the potential and limitations. The validation of the 39Ar dating makes use of glacier sites with already existing age information and additional radiometric dating, e.g. 14C. Then, the 39Ar dating is applied exemplarily to recover the unique climate record comprised in the internal age architecture of Alpine summit glaciers. The transfer from today`s mass and energy balance of the glaciers allows to infer climate fluctuations corresponding to the past accumulation change. The infrastructure and the already existing density of knowledge make summit glaciers in the European Alps a unique target for this purpose. Ultimately, the project will introduce glacier ice dating by 39Ar in paleo-climate research, thereby promising similar advances as through the application of 14C to glacier ice. Accordingly, this project will also help to reveal the impact of past climate changes on alpine glaciers, which is useful to substantiate projections of future glacier retreat and immediately relevant for the societal response to climate change.