Cosmogenic 26Al in the atmosphere and the prospect of a 26Al/10Be chronometer to date old ice

Cosmogenic radionuclides in the one-million-year half-life range offer unique possibilities for age determinations in geophysics. In measurements where the radioactive decay is being utilized as a clock, uncertainties in age determinations may be reduced if the ratio of two radioisotopes with different half-lives can be used as a chronometer. In this work we investigate the atomic ratio of atmospheric Al-26 (t(1/2) = 0.717 Ma) to Be-10 (t(1/2) = 1.386 Ma) measured with accelerator mass spectrometry (AMS), and its potential as a chronometer for dating old ice. The Al-26/Be-10 ratio decreases with an effective half-life of t(1/2)(Al-26/Be-10) = 1.49 Ma. For its application as a chronometer, the atmospheric Al-26/Be-10 ratio has to be well characterized. However, the properties of atmospheric Al-26 have been understood only poorly so far. At the VERA AMS facility of the University of Vienna, a first systematic study of the global variations of the Al-26/Be-10 ratio in the atmosphere and in surface firn has been carried out, and pilot measurements of the Al-26/Be-10 ratio in deep Antarctic ice have been performed. Our results indicate that this ratio is globally constant to within 5% in the atmosphere and in surface firn with a mean value of 1.89 x 10(-3). The data also suggest that non-atmospheric sources of Al-26, such as extraterrestrial, in situ produced or re-suspended Al-26 do not contribute significantly to the observed Al-26/Be-10 ratio. in addition, atmospheric mixing seems to exert only a minor influence. In a first application of the method, Al-26/Be-10 ratios were measured in chips collected in connection with the drilling of the lowest part of an ice core (2250 to 2760 m) in Dronning Maud Land, Antarctica. Surprisingly, variable Al-26/Be-10 ratios ranging between 0.5 and up to 2 times the atmospheric ratio were found at different locations in this deep ice core. While the cause for the ratios higher than atmospheric remains unexplained so far, the ratios lower than atmospheric may be caused by radioactive decay, allowing a first dating attempt using the Al-26/Be-10 ratio. Thus, at an ice depth of 2760 m an approximate date of (6.7 +/- 2.6) x 10(5) years was established. (C) 2009 Elsevier B.V. All rights reserved.