Manganese encrustations are an important archive for the reconstruction of deep ocean circulation in the past. However, because of discordant growth rates derived from the decrease of the activity ratios of 234U/238U and of 230Thexc with depth, their dating via the measurement of depth profiles of 230Th and 231Pa has been recently put into question (Chabaux et al., 1997).In this study we present high precision depth profiles of uranium and thorium isotopes (TIMS) of a hydrogenous Mn-crust from the South China Sea.Indeed, the depth profiles of 234U/238U and 230Thexc deliver very different growth rates of 8.80 ± 1.20 and 2.64 ± 0.12 mm/Ma, respectively. We solve this discrepancy with a simple model which assumes exchange of uranium adsorbed in the Mn-crust with uranium dissolved in the pore water. The best agreement to the data is obtained applying an exchange coefficient of 5 x 10−6 [a−1]. Application of this model to the data set of Chabaux et al. (1997), reproduces very well their profiles of the δ234U. We conclude that 234U/238U dating of Mn-encrustations is not reliable because of open-system conditions for uranium.