Explaining discrepant depth profiles of {sup 234}U/{sup 238}U and {sup 230}Th{sub exc} in Mn-crusts
- Heidelberger Akademie der Wissenschaften Im Neuenheimer Feld, Heidelberg (Germany)
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 {sup 234}U/{sup 238}U and of {sup 230}Th{sub exc} with depth, their dating via the measurement of depth profiles of {sup 230}Th and {sup 231}Pa has been recently put into question (Chabaux et al., 1997). In this study the authors 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 {sup 234}U/{sup 238}U and {sup 238}Th{sub exc} deliver very different growth rates of 8.80 {+-} 1.20 and 2.64 {+-} 0.12 mm/Ma, respectively. The authors 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 {times} 10{sup {minus}6} [a{sup {minus}1}]. Application of this model to the data set of Chabaux et al. (1997), reproduces very well their profiles of the {delta}{sup 234}U. The authors conclude that {sup 234}U/{sup 238}U dating of Mn-encrustations is not reliable because of open-system conditions for uranium.
- OSTI ID:
- 687715
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 15 Vol. 63; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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