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Radiochronology of Abyssal Sediments with 10Be and 26Al - Advantages and Obstacles
Feige, J.; Wallner, A.; Bourlès, D.; Fifield, L. K.; Golser, R.; Rugel, G.; Merchel, S.; Steier, P.; Winkler, S. R.
The long-lived radionuclides 10Be (t1/2=1.4 Myr) and 26Al (t1/2=0.7 Myr) are widely used for dating applications, e.g. determination of surface exposure durations of terrestrial quartz or meteorites to cosmic rays. Stratigraphic chronologies of marine reservoirs such as sediments, deep-sea crusts and nodules are often obtained via 10Be /9Be ratios, while 26Al/27Al ratios are only scarcely used.
The deep-ocean floor is shielded from cosmic rays by several km of water, preventing direct cosmogenic production. In marine reservoirs, inputs of 10Be and 26Al occur due to their production in the atmosphere, in which 10Be is however ~500 times more abundant than 26Al. The freshly produced radionuclides settle onto the open ocean floor within ~100 (26Al) and ~1000 (10Be) years. An additional source may be the production via the 23Na(alpha,n)26Al reaction induced by alpha-particles originating from decay of U and Th.
We analyzed 85 samples from four deep-sea sediment cores from the Indian Ocean and 21 from two manganese nodules from the Atlantic Ocean. The isotope ratios of 26Al/27Al and 10Be/9Be were measured by AMS at the VERA facility (University of Vienna, Austria) and at the DREAMS facility (HZDR, Dresden, Germany). Due to the low 26Al/27Al ratios, i.e. ~10-14 vs. 10Be/9Be: ~10-11, the measurement of 26Al is much more time consuming. However, as stable 27Al is naturally enough abundant in sediments (in contrast to stable 9Be), the work without addition of a carrier and stable isotope measurements reduces potential error sources.
Our results indicate that 10Be/9Be and 26Al/27Al are both valuable tools for dating deep-sea sediments up to 3-5 Myr. Beyond these limits in-situ production of 26Al becomes significant. The nodule in-situ production of 26Al is clearly dominating the atmospheric production.
Keywords: AMS; deep-sea
14th International Conference on Accelerator Mass Spectrometry (AMS-14), 14.-18.08.2017, Ottawa, Canada