  We present the first successful 81Kr-Kr radiometric dating of ancient polar
ice. Krypton was extracted from the air bubbles in four ~350 kg polar ice
samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated
using Atom Trap Trace Analysis (ATTA). The 81Kr radiometric ages agree with
independent age estimates obtained from stratigraphic dating techniques with a
mean absolute age offset of 6 +/- 2.5 ka. Our experimental methods and sampling
strategy are validated by 1) 85Kr and 39Ar analyses that show the samples to be
free of modern air contamination, and 2) air content measurements that show the
ice did not experience gas loss. We estimate the error in the 81Kr ages due to
past geomagnetic variability to be below 3 ka. We show that ice from the
previous interglacial period (MIS 5e, 130-115 ka before present) can be found
in abundance near the surface of Taylor Glacier. Our study paves the way for
reliable radiometric dating of ancient ice in blue ice areas and margin sites
where large samples are available, greatly enhancing their scientific value as
archives of old ice and meteorites. At present, ATTA 81Kr analysis requires a
40-80 kg ice sample; as sample requirements continue to decrease 81Kr dating of
ice cores is a future possibility.
