Radiometric ⁸¹Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica

This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the National Academy of Sciences and can be found at: http://www.pnas.org/.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