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

Abstract

We present successful 81 Kr-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 81 Kr 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 (i) 85 Kr and 39 Ar analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the 81 Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 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 81 Kr analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, 81 Kr dating of ice cores is a future possibility.geochronology | paleoclimatology | glaciology

Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle

“…Finally, the very low δ 18 O atm values of −0.4 ‰ close to the glacier terminus are a clear indication of interglacial origin, in this case most likely Eemian (∼ 125 ka BP). The Eemian age of this part of the glacier has already been independently confirmed by Buizert et al (2014) using Kr-81 dating.…”

“…We can therefore use these data to assign ages to the different locations by comparing the gas composition to other ice cores. We restrict the age range to the last glacial cycle, a well-justified assumption given that these sites are 10+ km upstream from a site that has been dated to 120 ka BP (before present = before 1950 AD) and close to two other sites that were dated to 12 and 50 ka BP by Buizert et al (2014) using Kr-81 radioisotope dating. The gas data then suggest unique ages for cores D2 and D3, and two possible ages for D1 (Fig.…”

“…All samples originate from 4.0 to 5.5 m depth and were carefully examined to be free of fractures that occasionally exist at this depth. The methane and δ 18 O atm data from the along-flow profile were presented in Buizert et al (2014), but they are discussed here in much greater detail. In addition, three cores were drilled to 12 m depth in different locations (Fig.…”

“…Most previous work has relied on radiometrically dated ash layers (Dunbar et al, 2008) or stable water isotopes (Aciego et al, 2007;Spaulding et al, 2013) to establish ages for the outcropping ice. In recent years advances in measuring rare noble gas isotopes have led to new radiometric dating techniques applied to the trapped gas in the ice (Bender et al, 2008;Buizert et al, 2014;Yau et al, 2016). Common to all radiometric dating techniques are relatively high uncertainties which exclude these methods from high-resolution dating.…”

Spatial pattern of accumulation at Taylor Dome during the last glacial inception: stratigraphic constraints from Taylor Glacier

“…An along-flow transect of water stable isotopes from just below the equilibrium line to the terminus of the glacier revealed ice from the last glacial period in several sporadic intervals (Aciego et al, 2007); the sporadic nature of the occurrences was later shown to be an artifact of sampling nearly parallel to isochrones such that they were occasionally crossed (Baggenstos et al, 2017). More recent across-flow profiles dated with gases revealed ice that varies continuously in age from the Holocene to the last ice age 20 (Schilt et al, 2014;Bauska et al, 2016;Baggenstos et al, 2017), with ice of last interglacial or older age found near the terminus of the glacier (Baggenstos et al, 2017;Buizert et al, 2014). The most utilized archive to date is a 500 m section (called the 'Main Transect') oriented perpendicularly to isochrones across a syncline/anticline pair containing ice spanning ~50 ka before present (BP) to the mid Holocene (7 ka).…”