(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.锰结核（Manganese nodules）通常被认为是极其缓慢地形成的。基于钍-230（230Th）、镤-231（231Pa）以及铀-234/铀-238（234U/238U）的放射测年研究表明，其生长速率约为每1000万年几毫米。这引出了一个未解谜题：锰结核如何逃脱被伴生海洋沉积物掩埋的命运？后者的沉积速率比锰结核的生长速率快三个数量级。尽管该问题迄今仍未解决，但学界推测，缓慢的生长速率或许可使锰结核在小体积内富集大量长寿命宇宙成因放射性核素（cosmogenic radionuclides）：铍-10（10Be，半衰期1.5百万年）与铝-26（26Al，半衰期0.715百万年），这类核素由银河宇宙射线轰击大气成分生成。此前，Somayajulu4与Krishnaswami等人5已发现，取自太平洋盆地的三块锰结核在其表层富集了可观的铍-10活度（9~68 衰变/分钟·千克），该浓度较远洋沉积物中的铍-10浓度高出数倍。本文报道了针对锰结核中铝-26的搜寻研究结果。