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Behavior of U, Th and Ra isotopes in soils during a land cover change

Behavior of U, Th and Ra isotopes in soils during a land cover change
Behavior of U, Th and Ra isotopes in soils during a land cover change
Over the last decades, the U- and Th-series nuclides were successfully used to determine weathering rates in various environments. The objective of this study is to assess the potential impact of the vegetation change on the U- and Th-series signal recorded in forested soils. This study was carried out from the experimental forest site of Breuil-Chenue (Morvan, France) developed by the INRA-BEF team. The native forest of the site was partially clear-felled in 1976 and replaced by monospecific plantations stands (Oak and Douglas fir). U- and Th-series disequilibria were measured in 2011 in the podzolic soils developed under the native forest, and in the two replanted stands. Separation of primary minerals (biotite, muscovite and perthitic feldspar) and selective extractions of the Fe and Al oxides were performed to investigate the distribution of U and Th among these soil fractions.

The selective extractions suggest that a significant part of U and Th is primarily held by Fe-bearing silicates. Our results suggest that the tree substitution seems to produce a large dissolution of these minerals under the Oaks, resulting to a release of U and Th. However, below 25 cm no impact of this release was observed on U-series disequilibria. A scenario allowing to reconcile the significant mobilization of U and the constancy of U-series disequilibria is proposed. Above 25 cm, additional pedogenic redistribution of U and Th isotopes occurs in all the profiles, inducing some discrepancies between U-series disequilibria. A clear correlation between the (230Th/234U) ratios and the proportions of amorphous and interlayer Al hydroxides has been highlighted. This correlation suggests a mobility of U and Th isotopes strongly associated to Al dynamics in these soils rather than Fe, despite the primary location of U in the Fe-bearing silicates and the overwhelming reported control of UVI by Fe-oxides in oxidized environments. These pedogenic processes make the shallowest horizons of podzolic soils unsuitable for U-series dating. In contrast, a soil production rate can be deduced from the deepest soil layers which do not show such effects on the U-series nuclides. The reproducible U-series disequilibria measured in four whole-profile replicates emphasize the robustness and the significance of the “long-lived” U-series disequilibria in deep soil layers relative to long-term weathering rates, independent of transient perturbations such as land cover changes. Finally, because Ra can strongly accumulates in plants, the (226Ra/230Th) ratios in the different soils were affected by the flux of 226Ra released by litter degradation. The use of this ratio as a long-term chronometer should therefore be performed with caution in such contexts. No direct impact of the vegetation type on the (228Ra/232Th) ratios was identified, due to the short 228Ra half-life.
U-series, Th-series, Soil, Vegetation, Weathering
0009-2541
106-123
Rihs, Sophie
6ea13dfe-5e89-445e-9bd6-d4ba95971bf7
Gontier, Adrien
7ac1f5d1-262d-4391-a869-f078808273a0
Pelt, Eric
441446b6-d0ad-446d-b797-2a2e000d67bf
Fries, David
4de380eb-2770-43b6-a9f7-5bbb742a5b5b
Turpault, Marie-Pierre
1efe0e39-0446-4cd5-8121-7aa2289ff201
Chabaux, François
e0cad09f-e2bf-4ee8-9b8f-40868f181594
Rihs, Sophie
6ea13dfe-5e89-445e-9bd6-d4ba95971bf7
Gontier, Adrien
7ac1f5d1-262d-4391-a869-f078808273a0
Pelt, Eric
441446b6-d0ad-446d-b797-2a2e000d67bf
Fries, David
4de380eb-2770-43b6-a9f7-5bbb742a5b5b
Turpault, Marie-Pierre
1efe0e39-0446-4cd5-8121-7aa2289ff201
Chabaux, François
e0cad09f-e2bf-4ee8-9b8f-40868f181594

Rihs, Sophie, Gontier, Adrien, Pelt, Eric, Fries, David, Turpault, Marie-Pierre and Chabaux, François (2016) Behavior of U, Th and Ra isotopes in soils during a land cover change. Chemical Geology, 441, 106-123. (doi:10.1016/j.chemgeo.2016.08.016).

Record type: Article

Abstract

Over the last decades, the U- and Th-series nuclides were successfully used to determine weathering rates in various environments. The objective of this study is to assess the potential impact of the vegetation change on the U- and Th-series signal recorded in forested soils. This study was carried out from the experimental forest site of Breuil-Chenue (Morvan, France) developed by the INRA-BEF team. The native forest of the site was partially clear-felled in 1976 and replaced by monospecific plantations stands (Oak and Douglas fir). U- and Th-series disequilibria were measured in 2011 in the podzolic soils developed under the native forest, and in the two replanted stands. Separation of primary minerals (biotite, muscovite and perthitic feldspar) and selective extractions of the Fe and Al oxides were performed to investigate the distribution of U and Th among these soil fractions.

The selective extractions suggest that a significant part of U and Th is primarily held by Fe-bearing silicates. Our results suggest that the tree substitution seems to produce a large dissolution of these minerals under the Oaks, resulting to a release of U and Th. However, below 25 cm no impact of this release was observed on U-series disequilibria. A scenario allowing to reconcile the significant mobilization of U and the constancy of U-series disequilibria is proposed. Above 25 cm, additional pedogenic redistribution of U and Th isotopes occurs in all the profiles, inducing some discrepancies between U-series disequilibria. A clear correlation between the (230Th/234U) ratios and the proportions of amorphous and interlayer Al hydroxides has been highlighted. This correlation suggests a mobility of U and Th isotopes strongly associated to Al dynamics in these soils rather than Fe, despite the primary location of U in the Fe-bearing silicates and the overwhelming reported control of UVI by Fe-oxides in oxidized environments. These pedogenic processes make the shallowest horizons of podzolic soils unsuitable for U-series dating. In contrast, a soil production rate can be deduced from the deepest soil layers which do not show such effects on the U-series nuclides. The reproducible U-series disequilibria measured in four whole-profile replicates emphasize the robustness and the significance of the “long-lived” U-series disequilibria in deep soil layers relative to long-term weathering rates, independent of transient perturbations such as land cover changes. Finally, because Ra can strongly accumulates in plants, the (226Ra/230Th) ratios in the different soils were affected by the flux of 226Ra released by litter degradation. The use of this ratio as a long-term chronometer should therefore be performed with caution in such contexts. No direct impact of the vegetation type on the (228Ra/232Th) ratios was identified, due to the short 228Ra half-life.

Full text not available from this repository.

More information

Accepted/In Press date: 8 August 2016
Published date: 21 November 2016
Keywords: U-series, Th-series, Soil, Vegetation, Weathering
Organisations: Geochemistry

Identifiers

Local EPrints ID: 402425
URI: https://eprints.soton.ac.uk/id/eprint/402425
ISSN: 0009-2541
PURE UUID: 0969ef8e-62a3-41c6-b642-61252816cb16

Catalogue record

Date deposited: 07 Nov 2016 13:17
Last modified: 15 Jul 2019 19:55

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