1. Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, IN2P3-CNRS-Université de Paris-Sud, Bât. 108, 91405 Orsay, France
2. Laboratoire des Sciences du Climat et de l'Environement/Institut Pierre Simon Laplace, CEA-CNRS-UVSQ, CE Saclay, 91191 Gif-sur-Yvette, France

Correspondence to: G. M. Raisbeck¹ Correspondence and requests for materials should be addressed to G.R. (Email: raisbeck@csnsm.in2p3.fr).

An ice core drilled at Dome C, Antarctica, is the oldest ice core so far retrieved¹. On the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores, the ice at a depth of 3,190 m in the Dome C core is believed to have been deposited around 800,000 years ago², offering a rare opportunity to study climatic and environmental conditions over this time period. However, an independent determination of this age is important because the deuterium profile below a depth of 3,190 m depth does not show the expected correlation with the marine record². Here we present evidence for enhanced ¹⁰Be deposition in the ice at 3,160–3,170 m, which we interpret as a result of the low dipole field strength during the Matuyama–Brunhes geomagnetic reversal, which occurred about 780,000 years ago. If correct, this provides a crucial tie point between ice cores, marine cores and a radiometric timescale.

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