Radiochemical studies of deep-sea manganese nodules

Five manganese nodules collected from the Northern Mid-Pacific have been analysed for their U and Th isotopes.It is found that the enrichment of thorium compared to uranium is universal in deep-sea manganese, nodules and in the top sides of the nodules there are large excesses of ²³⁰Th and ²³¹Pa decreasing exponentially with depth. For each of the nodules, the concordant growth rates are obtained from three different methods:²³⁰Th_ex ²³⁰Th_ex/²³²Th and ²³¹Pa_ex.The growth rates of manganese nodules are closely related to the chemical compositions of the nodules and the types of the underlying sediments. The growth rates of five nodules are determined to be in the range of 0.79-7.4 mm/10⁶a, in agreement with those predicted from the chemical compositions of the nodules. By the comparison of the extrapolated ²³⁰Th_ex,²³⁰Th_ex/²³²Th and ²³¹Pa_ex data from the top and bottom surfaces of the nodule from Site M₂₁ yields, the nodule turnover time is (9.83-13.7)×10⁴a.