Measurement of meteoric Al-26 in terrestrial archives is difficult even using currently well-developed AMS systems. Al-26 concentrations in ice samples were, therefore, scarcely published. Nevertheless, successful analyses of those promise us the following benefits for ice core studies. First, we can examine the solar-geomagnetic modulation of cosmogenic nuclide production using both Be-10 and Al-26 in the same ice core. Second, radiometric Al-26/Be-10 dating can be utilized for the deep part of long ice cores. Third, we may have an interesting tool to evaluate the atmospheric changes, even if the atmospheric Al-26/Be-10 ratio at the station where an ice core was retrieved had changed so much that we can not apply the Al-26/Be-10 dating method.
In this study, we have attempted to determine Al-26 concentrations in an ice core from Dome Fuji station, Eastern Antarctica. About 200 g of ice samples was melted by using a microwave oven, followed by adding 0.1 mL of HNO3, 0.3 mg of Be-9 carrier, and 0.1 mg of Al-27 carrier. Cation exchange separation was then performed to isolate the aluminum fraction. The fractions were pooled for each ten samples (each 5-m intervals) bringing the final weight of an ice sample and Al-27 carrier to ~2 kg and 1 mg, respectively. Al-26 measurements were performed by using MALT-AMS system at the University of Tokyo. Three target samples were made from one pooled fraction to gain enough Cs-beam exposure time (total 4.5 hours for a pooled sample) at the ion source of the AMS system. Typical atomic Al-26/Al-27 ratios of the sample and chemical blank were 2 x 10-14 and 1 x 10-15, respectively. Fully propagated error of the measurements was ~10%.
The Al-26 concentrations that we determined in the three samples (dated as 1541-1800 yr A.D.) are 160-210 atoms g-1. The Al-26/Be-10 ratios of the samples are ~2 x 10-3. The ratios are nearly the same with the previously published values on Antarctic ices (Middleton and Klein, 1987), but much lower than the known atmospheric Al-26l/Be-10 ratio (3.8 ± 0.6 x 10-3: Raisbeck et al., 1983). Although the value ~2 x 10-3 may be able to consider the initial Al-26/Be-10 ratio for the radiometric dating on Antarctic ices, further studies are necessary for prooving this assumption.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)