  Radiometric dating with 39Ar covers a unique timespan 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 to reduce the sample volume by several
orders of magnitude, compared to conventional techniques. Here we show that the
adaptation of this method to 39Ar is now available for glaciological
applications, by demonstrating the first Argon Trap Trace Analysis (ArTTA)
dating of alpine glacier ice samples. 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 in 3030 m all (Swiss Alps) have been dated by
state-of-the-art micro-radiocarbon analysis in a previous study. The unique
finding of a bark fragment and a larch needle within the ice of Schaufelferner
glacier in 2870 m asl (Stubai Alps, Austria) allows for conventional
radiocarbon dating. At both sites, results of 39Ar dating match the existing
age information based on radiocarbon dating and visual stratigraphy. With our
results, we establish Argon Trap Trace Analysis as the key to decipher so far
untapped glacier archives of the last millennium.
