Ice half a million years old at the base of the Skytrain Ice Rise ice core

Skytrain Ice Rise is a separate ice flow centre at the inland edge of the Ronne Ice Shelf, on the periphery of the West Antarctic Ice Sheet. An ice core drilled through to the base of the ice at 651 m was dated as far as 126 ka before present, which is found at 627 m depth. This ice has been used so far to investigate the climate and the ice sheet stability of the Holocene and the last interglacial. Here we investigate the ice between 627 and 651 m depth. Three methods for dating old ice have been applied to samples within this depth range. Analysis using the ATTA method of ⁸¹Kr, with a half-life of 229 kyr, has been carried out on three samples between 635 and 648 m, as well as on one younger sample of known age. ⁴⁰Ar in the atmosphere is increasing with time, and therefore the deficit compared to modern of the derived quantity ⁴⁰Ar_atm can be used to date ice. Two samples of deep ice have been analysed for this measure. Finally the ratio of ³⁶Cl/¹⁰Be should be independent of production rate changes, and has an apparent half-life of 384 kyr. Five samples were analysed between 633 and 650 m. We first compare the findings from the three methods to establish their consistency. The combination of data from the three methods suggests that, despite flow disturbances that are apparent around the last interglacial (LIG) ice, the ages are monotonically increasing with depth. Ice just above the bottom is around half a million years old, suggesting that the ice at Skytrain Ice Rise has been present since before Marine Isotope Stage 11. The climate record will be shown, but has to be interpreted very carefully because we can assume that flow disturbances, similar to those in the LIG, have affected ice at the interfaces between cold and warm periods, leading to missing sections of the record.