Al and Be isotope composition of two ferromanganese crusts from the Pacific Ocean

The technique of accelerator mass spectrometry (A.H.S.) has opened up a number of new potential geochronological applications of long lived cosmogenic nuclei- However the absence of any obvious stage leading to equilibrium between the radioactive and corresponding stable isotope(s) in the geochemical cycle of many of these species, complicates considerably their potential dating applications. The radioisotope 10Be (half-life 1.5 My) is formed by spallation reactions between cosmic rays and 14N, 16O in the atmosphere. It is transferred to the oceans in soluble form by precipitation and dry deposition. The 10Be et 16Al having similar chemical behaviours and being influenced by the same geophysical and geochemical phenomena before being incorporated into the marine sediments, their ratio is tested in order to date those sediments. This approach is also applied to manganese nodules and ferromanganese crusts.