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

摘要

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 ~(26)Al (t_(1/2) = 0.717 Ma) to ~(10)Be (t_(1/2) = 1.386 Ma) measured with accelerator mass spectrometry (AMS), and its potential as a chronometer for dating old ice. The ~(26)Al/~(10)Be ratio decreases with an effective half-life of t_(1/2)(~(26)Al/~(10)Be) = 1.49 Ma. For its application as a chronometer, the atmospheric ~(26)Al/~(10)Be ratio has to be well characterized. However, the properties of atmospheric ~(26)Al 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 ~(26)Al/~(10)Be ratio in the atmosphere and in surface firn has been carried out, and pilot measurements of the ~(26)Al/~(10)Be 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 × 10~(- 3). The data also suggest that non-atmospheric sources of ~(26)Al, such as extraterrestrial, in situ produced or re-suspended ~(26)Al, do not contribute significantly to the observed ~(26)Al/~(10)Be ratio. In addition, atmospheric mixing seems to exert only a minor influence. In a first application of the method, ~(26)Al/~(10)Be 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 ~(26)Al/~(10)Be 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 ~(26)Al/~(10)Be ratio. Thus, at an ice depth of 2760 m an approximate date of (6.7 ± 2.6) × 10~5 years was established.