(Pages 203-204) Concentration in Aluminium-26 in layers of a ferromanganese crust recovered in the Pacific Ocean

Manganese nodules are generally considered to have accumulated very slowly. Radiometric studies by 230Th, 231Pa, and 234U/238U suggest their growth rates to be a few mm per 10 million yr. This leads to an enigma: how do they escape from burial by associated marine sediments which are accumulating three orders of magnitude faster than the nodules? Though this problem remains open, the postulated slow growth rates might permit manganese nodules to accumulate in a small volume high concentrations of long lived cosmogenic radionuclides 10Be (half tau = 1.5 Myr) and 26Al (half tau = 0.715 Myr) which were formed through the bombardment of atmospheric constituents by galactic cosmic rays. Indeed, Somayajulu4 and Krishnaswami et al.5 have found that three manganese nodules from the Pacific basin concentrate considerable activities of 10Be at their surface (9-68 d.p.m. kg?1). These concentrations are several times higher than those found in pelagic sediments. We report here on a search for 26Al in manganese nodules.