Explaining discrepant depth profiles of U-234/U-238 and Th-230(exc) in Mn-crusts

Manganese encrustations are an important archive for the reconstruction of deep ocean circulation in the past. However, because of discordant growth r ates derived from the decrease of the activity ratios of U-234/U-238 and of Th-230(exc) With depth, their dating via the measurement of depth profiles of Th-230 and Pa-231 has been recently put into question (Chabaux et al., 1997). In this study we present high precision depth profiles of uranium and thori um isotopes (TIMS) of a hydrogenous Mn-crust from the South China Sea. Indeed, the depth profiles of U-234/U-238 and Th-230(exc) deliver very diff erent 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 ura nium 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 e t al. (1997), reproduces very well their profiles of the delta(234)U. We co nclude that U-234/U-238 dating of Mn-encrustations is not reliable because of open-system conditions for uranium. Copyright (C) 1999 Elsevier Science Ltd.