Edited by Philippe Bouyer, Institut d’Optique, Palaiseau, France, and accepted by Editorial Board Member Angel Rubio March 26, 2019 (received for review October 4, 2018)
Alpine summit glaciers have a characteristic age range between 100 and 1,000 years. Reliable dating is the key to access this valuable environmental archive, including the Little Ice Age. Glacier ice contains past air and thus also the rare radioisotope 39Ar, uniquely suitable as an age tracer for this time span. Only argon trap trace analysis (ArTTA), the adaptation of techniques from quantum optics to 39Ar, enables small sample sizes necessary for the application to glacier ice. We present the first dating of glacier ice using less than 2 mL STP of argon from ∼5 kg of ice, finally opening the door for radioargon dating in glaciology.
Radiometric dating with 39Ar covers a unique time span and offers key advances in interpreting environmental archives of the last millennium. Although this tracer has been acknowledged for decades, studies so far have been limited by the low abundance and radioactivity, thus requiring huge sample sizes. Atom trap trace analysis, an application of techniques from quantum physics such as laser cooling and trapping, allows us to reduce the sample volume by several orders of magnitude compared with conventional techniques. Here we show that the adaptation of this method to 39Ar is now available for glaciological applications, by demonstrating the entire process chain for dating of alpine glacier ice by argon trap trace analysis (ArTTA). Ice blocks as small as a few kilograms are sufficient and have been obtained at two artificial glacier caves. Importantly, both sites offer direct access to the stratigraphy at the glacier base and validation against existing age constraints. The ice blocks obtained at Chli Titlis glacier at 3,030 m asl (Swiss Alps) have been dated by state-of-the-art microradiocarbon analysis in a previous study. The unique finding of a bark fragment and a larch needle within the ice of Schaufelferner glacier at 2,870 m asl (Stubai Alps, Austria) allows for conventional radiocarbon dating. At both sites the existing age information based on radiocarbon dating and visual stratigraphy corroborates the 39Ar ages. With our results, we establish argon trap trace analysis as the key to decipher so far untapped glacier archives of the last millennium.
Author contributions: Z.F., P.B., L.R., M.S., A.K., P.H., A.F., W.A., and M.K.O. performed research; Z.F., P.B., S.E., and H.H. analyzed data; and Z.F., P.B., S.E., L.R., M.S., A.K., P.H., H.H., A.F., W.A., and M.K.O. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. P.B. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1816468116/-/DCSupplemental.
Published under the PNAS license.
