Radiochronology of Abyssal Sediments with ¹⁰Be and ²⁶Al - Advantages and Obstacles

The long-lived radionuclides ¹⁰Be (t_1/2=1.4 Myr) and ²⁶Al (t_1/2=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.