Explaining discrepant depth profiles of 234U/238U and 230Thexc in Mn-crusts

被引：19

作者：

Neff, U ^{[1
]}

Bollhöfer, A ^{[1
]}

Frank, N ^{[1
]}

Mangini, A ^{[1
]}

机构：

[1] Heidelberger Akad Wissensch, D-69120 Heidelberg, Germany

来源：

GEOCHIMICA ET COSMOCHIMICA ACTA | 1999年 / 63卷 / 15期

关键词：

D O I：

10.1016/S0016-7037(99)00135-0

中图分类号：

P3 [地球物理学]; P59 [地球化学];

学科分类号：

0708 ; 070902 ;

摘要：

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 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 thorium 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 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 delta(234)U. We conclude 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.

引用

页码：2211 / 2218

页数：8

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