Aluminium-26 in a manganese nodule

Abstract

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 10⁶ yr (refs 1–3). 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 (τ_½ = 1.5 Myr) and ²⁶Al (τ_½ = 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.