Manganese crusts are growing very slowly (few mm/Myr) on the ground of the oceans. They are absorb material which has been diluted in the ocean water, i.e. they preserve the ocean water's isotopic composition for millions of years. Therefore, manganese crusts are an important reservoir for paleoceanographic, paleoclimatic and paleoastronomic studies. Usually, crusts are dated by measuring a depth profile of the cosmogenic radionuclide 10Be. In deeper (i.e. older) layers a reduced concentration is expected due to radioactive decay. However, one has to assume a constant flux of 10Be into the crust as well as a low mobility of beryllium in the crust. For the radionuclide 26Al the production is substantially different. The main fraction is not absorbed from the ocean water, but produced in-situ via the reaction 23Na(α,n)26Al. A significant fraction of the α particles is from the decay of the short lived 230Th. Therefore, 26Al is produced mainly at the crust's surface and an exponential decrease of the 26Al with the depth is expected. A profile of 26Al covering the last 10 Myr has been measured and will be compared with the corresponding 10Be profile.
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