Raimund Muscheler

Dating the bottommost section of an ice core is often complicated by strong layer thinning and possible disturbances in the stratigraphy. The radioactive decay of atmospherically produced ³⁶Cl and ¹⁰Be can provide age estimates, where traditional methods can no longer be used. In this study, we investigated ice from the bottom of the Skytrain ice core, which was drilled in West Antarctica next to the Ronne Ice Shelf and has previously been dated to 126 kyr BP about 24 m above bedrock.

Apart from decay, radionuclide concentrations in ice can be influenced by production rate variations, atmospheric transport and deposition variations, and, at low accumulations sites, by chlorine loss through hydrogen chloride outgassing. Using the ³⁶Cl/¹⁰Be ratio largely removes production related variations and we were able to confirm that no ³⁶Cl loss occurs at Skytrain Ice Rise, as the nuclear weapon test caused peak in 36Cl concentrations was found at the expected depth corresponding to the 1950s and 1960s. An analysis of samples with known age showed that individual radionuclide concentrations and the ³⁶Cl/ ¹⁰Be ratio are negatively correlated to the _δ¹⁸O signal, which was used to apply a climate correction that enabled a higher precision for age estimates of previously undated samples by reducing the uncertainty of the initial ratio from 14 % to 9 %. The deepest analysed section of the Skytrain ice core was found to be 561 (+110 -109) kyr old.