Accumulation of ferromanganese crusts derived from carrier-free 10Be/9Be

The occurrence of Be-10 in natural archives is commonly used to date their formation and growth on time scales of million years. Accelerator Mass Spectrometry (AMS) can perform a direct measurement of the Be-10/Be-9 ratio. The carrier-free method, in which no Be-9 carrier is added to the original sample, is especially suitable for Be-10/Be-9 ratio determination in the marine environment. By normalizing the Be-10 content to Be-9, temporal variations of Be uptake processes into the archive are eliminated. Here, we present a simple method for the chemical extraction of beryllium from ferromanganese (FeMn) crusts or nodules, the measurement procedure, and the first carrier-free Be-10/Be-9 measurements at the 3 MV AMS facility VERA. Several tests of chemical methods are discussed including different options to short-cut and accelerate the procedure for special cases. Results from FeMn crust 237KD from cruise VA13/2 in the Pacific ocean show the known Be-10/Be-9 distribution with depth that is commonly related to a changing growth rate of the archive. In this context we discuss the potential influence of diffusion and adsorption processes on the age models of FeMn crusts that are based on radioactive nuclides such as Be-10 and Th-230. Including an open-system behavior for these isotopes in the description of their profiles allows interpreting the accumulation of crusts with a constant growth rate over millions of years and does not require the assumption of abrupt growth changes.