Applicability of ²⁶Al and ¹⁰Be for depositional age dating in deep-sea records

Reliable depositional age dating is essential for studying million-year-old geologic records. Where methods such as biostratigraphy, magnetostratigraphy, or radiometric dating are insufficient or inapplicable, the atmospheric long-lived cosmogenic radionuclide 10Be (t1/2 = 1.4 million years) has become an important additional dating tool. The cosmogenic radionuclide 26Al (t1/2 = 0.7 million years), which is also produced in the atmosphere, has not been thoroughly investigated as a dating tool. In this study, we measured the concentrations of 10Be and 26Al, along with their stable counterparts 9Be and 27Al, in four deep-sea sediment cores from the Indian Ocean, one Central Pacific ferromanganese crust, and six ferromanganese nodules from the Atlantic Ocean. Our results show that 26Al-based dating is, in principle, similarly applicable to 10Be-based dating but has one major drawback: nucleogenic in-situ 26Al production leads to increasingly unreliable ages in sediments older than approximately 3 million years, with interferences occurring even earlier in ferromanganese deposits. Furthermore, in deep-sea sediments only the 26Al/27Al and 10Be/9Be ratios yield reliable ages, whereas 26Al and 10Be concentrations or the 26Al/10Be ratio are affected by signal dilution due to pelagic carbonates. Thus, we conclude that the primary advantage of measuring 26Al is to corroborate and improve 10Be/9Be ratio-derived ages using 26Al/27Al ratios, but only for deep-sea sediments younger than approximately 3 million years.