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Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1

Andersen, M. B., Matthews, A., Bar-Matthews, M. and Vance, D. 2020. Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1. Geochemical Perspectives Letters 15 , pp. 10-14. 10.7185/geochemlet.2027

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Abstract

Authigenic uranium isotope compositions of Holocene sapropel S1 (δ238Uauth = +0.10 to +0.52 ‰; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (+0.2 ‰) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high δ238Uauth values are compatible with rapid initial slowdown of thermohaline overturning and the development of an anoxic water column. These conditions would favour U uptake in an organic-rich floccule layer overlying the sediment-water interface. The high δ238Uauth values correlate with low δ98Moauth values (+0.02 to −0.88 ‰), interpreted to reflect weakly euxinic conditions controlled by thiomolybdate–molybdate solution equilibria. The S1 data contrast markedly with published data from last interglacial sapropel S5 from the same core, which show δ238Uauth and δ98Moauth characteristics compatible with a restricted euxinic basin due to progressive slowdown in the thermohaline circulation. The U-Mo isotope data for S1 are similar to a range of published palaeo-settings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Additional Information: This work is distributed under the Creative Commons Attribution Non-Commercial No-Derivatives 4.0 License
Publisher: European Association of Geochemistry
ISSN: 2410-339X
Date of First Compliant Deposit: 3 August 2020
Date of Acceptance: 21 July 2020
Last Modified: 02 Feb 2021 10:08
URI: http://orca.cardiff.ac.uk/id/eprint/133920

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