Diffusion and supply rates of 10Be and 230Th radioisotopes in two manganese encrustations from the South China Sea

Ages of two Mn encrustations estimated by their 230Th and 10Be distributions are compared with K-Ar ages and micropaleontological datings of their nuclei to discuss possible diffusion and supply effects of the radioisotope distribution and their influence on the reliability of age determinations. Based on comparable results obtained by the different methods the effective diffusion coefficient of 10Be can be calculated as D* <= 1.0 x 10-8 cm2/y. This coefficient is 3 to 8 times smaller than the best estimates available at present. In both nodules we observe lower 10Be concentrations in the uppermost 2 to 3 mm (1.3 m.y.), which suggests that 10Be uptake has been reduced since the middle Pleistocene. The 2.7-fold increase of the growth rate starting 3.2 m.y. ago coincides with the initiation of the northern hemisphere glaciation. (author)