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Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments

Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments
Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments
The extraction of a deepwater radiogenic isotope signal from marine sediments is a powerful, though under-exploited, tool for the characterisation of past climates and modes of ocean circulation. The radiogenic and radioactive isotope compositions (Nd, Pb, Th) of ambient deepwater are stored in authigenic Fe–Mn oxyhydroxide coatings in marine sediments, but the unambiguous separation of the isotopic signal in this phase from other sedimentary components is difficult and measures are needed to ensure its seawater origin. Here the extracted Fe–Mn oxyhydroxide phase is investigated geochemically and isotopically in order to constrain the potential and the limitations of the reconstruction of deepwater radiogenic isotope compositions from marine sediments.

Our results show that the isotope compositions of elements such as Sr and Os obtained from the Fe–Mn oxyhydroxide fraction are easily disturbed by detrital contributions originating from the extraction process, whereas the seawater isotope compositions of Nd, Pb and Th can be reliably extracted from marine sediments in the North Atlantic. The main reason is that the Nd, Pb and Th concentrations in the detrital phase of pelagic sediments are much lower than in the Fe–Mn oxyhydroxide fractions. This is reflected in Al/Nd, Al/Pb and Al/Th ratios of the Fe–Mn oxyhydroxide fractions, which are as low as or even lower than those of hydrogenetic ferromanganese crusts. Mass balance calculations illustrate that the use of the 87Sr/86Sr isotope composition to confirm the seawater origin of the extracted Nd, Pb and Th isotope signals is misleading. Even though the 87Sr/86Sr in the Fe–Mn oxyhydroxide fractions is often higher than the seawater Sr isotope composition, the corresponding detrital contribution does not translate into altered seawater Nd, Pb and Th isotope compositions due to mass balance constraints. Overall the rare earth element patterns, elemental ratios, as well as the mass balance calculations presented here highlight the potential of using authigenic Fe–Mn oxyhydroxide coatings as paleoceanographic archives for the analysis of past seawater Nd, Pb and Th isotope compositions.
Neodymium, Lead, Thorium, Fe–Mn oxyhydroxides, Leaching, Mass balance
0009-2541
351-370
Gutjahr, Marcus
5babbbc4-2a1a-48df-a2e3-d87b2483ea9c
Frank, Martin
09ec65ac-f62d-41da-86b2-81908973b8a1
Stirling, Claudine H.
e29154e1-d867-4bfa-bc63-6498d5ec475c
Klemm, Veronika
85ea4073-fb53-4115-a7a4-0c7dbf393a3f
van de Flierdt, Tina
6322ca73-69d3-434e-bf6d-41376bd93b73
Halliday, Alex N.
9d544a5d-f5f0-4bcf-bfdf-210c3abda594
Gutjahr, Marcus
5babbbc4-2a1a-48df-a2e3-d87b2483ea9c
Frank, Martin
09ec65ac-f62d-41da-86b2-81908973b8a1
Stirling, Claudine H.
e29154e1-d867-4bfa-bc63-6498d5ec475c
Klemm, Veronika
85ea4073-fb53-4115-a7a4-0c7dbf393a3f
van de Flierdt, Tina
6322ca73-69d3-434e-bf6d-41376bd93b73
Halliday, Alex N.
9d544a5d-f5f0-4bcf-bfdf-210c3abda594

Gutjahr, Marcus, Frank, Martin, Stirling, Claudine H., Klemm, Veronika, van de Flierdt, Tina and Halliday, Alex N. (2007) Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments. Chemical Geology, 242 (3-4), 351-370. (doi:10.1016/j.chemgeo.2007.03.021).

Record type: Article

Abstract

The extraction of a deepwater radiogenic isotope signal from marine sediments is a powerful, though under-exploited, tool for the characterisation of past climates and modes of ocean circulation. The radiogenic and radioactive isotope compositions (Nd, Pb, Th) of ambient deepwater are stored in authigenic Fe–Mn oxyhydroxide coatings in marine sediments, but the unambiguous separation of the isotopic signal in this phase from other sedimentary components is difficult and measures are needed to ensure its seawater origin. Here the extracted Fe–Mn oxyhydroxide phase is investigated geochemically and isotopically in order to constrain the potential and the limitations of the reconstruction of deepwater radiogenic isotope compositions from marine sediments.

Our results show that the isotope compositions of elements such as Sr and Os obtained from the Fe–Mn oxyhydroxide fraction are easily disturbed by detrital contributions originating from the extraction process, whereas the seawater isotope compositions of Nd, Pb and Th can be reliably extracted from marine sediments in the North Atlantic. The main reason is that the Nd, Pb and Th concentrations in the detrital phase of pelagic sediments are much lower than in the Fe–Mn oxyhydroxide fractions. This is reflected in Al/Nd, Al/Pb and Al/Th ratios of the Fe–Mn oxyhydroxide fractions, which are as low as or even lower than those of hydrogenetic ferromanganese crusts. Mass balance calculations illustrate that the use of the 87Sr/86Sr isotope composition to confirm the seawater origin of the extracted Nd, Pb and Th isotope signals is misleading. Even though the 87Sr/86Sr in the Fe–Mn oxyhydroxide fractions is often higher than the seawater Sr isotope composition, the corresponding detrital contribution does not translate into altered seawater Nd, Pb and Th isotope compositions due to mass balance constraints. Overall the rare earth element patterns, elemental ratios, as well as the mass balance calculations presented here highlight the potential of using authigenic Fe–Mn oxyhydroxide coatings as paleoceanographic archives for the analysis of past seawater Nd, Pb and Th isotope compositions.

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More information

Published date: 15 August 2007
Keywords: Neodymium, Lead, Thorium, Fe–Mn oxyhydroxides, Leaching, Mass balance

Identifiers

Local EPrints ID: 191639
URI: http://eprints.soton.ac.uk/id/eprint/191639
ISSN: 0009-2541
PURE UUID: 9a043ce9-7c0c-4202-8605-d93459935e6c

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Date deposited: 23 Jun 2011 10:51
Last modified: 14 Mar 2024 03:45

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Contributors

Author: Marcus Gutjahr
Author: Martin Frank
Author: Claudine H. Stirling
Author: Veronika Klemm
Author: Tina van de Flierdt
Author: Alex N. Halliday

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