Aluminium-26 in a manganese nodule

A search is reported for the long lived cosmogenic radionuclide 26A1, formed through the bombardment of atmospheric constituents by galactic cosmic rays, in the manganese nodule TECHNO-1. The results confirm the postulated concentration of long lived cosmonuclides in the manganese nodule for 26A1, showing an enrichment of the isotope in the nodule relative to that in sediment. It is also shown that the 26A1/10Be ratio in manganese nodules is of the same order as that in sediments, which is expected from similarity of the geochemical behaviours of the two nuclides in seawaters. Assuming an exponential decrease of 26A1 content with depth in the nodule it is estimated that the nodule growth rate is of the order of 1 to 5 mm per 106 yr. (U.K.)