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The uranium isotopic record of shales and carbonates through geologic time

Chen, X. and Tissot, F. L. H. and Jansen, M. F. and Bekker, A. and Liu, C. X. and Nie, N. X. and Halverson, G. P. and Veizer, J. and Dauphas, N. (2021) The uranium isotopic record of shales and carbonates through geologic time. Geochimica et Cosmochimica Acta, 300 . pp. 164-191. ISSN 0016-7037. doi:10.1016/j.gca.2021.01.040.

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In the modern ocean, U reduction and incorporation into anoxic sediments imparts a large isotopic fractionation of approximately +0.6‰ that shifts the seawater δ²³⁸U value (²³⁸U/²³⁵U, expressed as δ²³⁸U per mil deviation relative to CRM-112a) relative to continental runoff. Given the long residence time of U in the modern oceans (∼400 kyr), the isotopic composition of carbonates (taken as a proxy for seawater) reflects the global balance between anoxic and other sinks. The U isotopic composition of open-marine carbonates has thus emerged as a proxy for reconstructing past changes in the redox state of the global ocean. A tenet of this approach is that the δ²³⁸U values of seawater and anoxic sediments should always be fractionated by the same amount. In order to test this hypothesis, we have measured the U concentrations and isotopic compositions of carbonates spanning ages from 3250 Ma to present. A first-order expectation for the Archean and possibly Proterozoic is that near-quantitative U removal to extensive anoxic sediments should have shifted the uranium isotopic composition of seawater and carbonates towards lower values. Instead, the measurements reveal that many Archean and Proterozoic carbonates have unfractionated δ²³⁸U values similar to those of continents and riverine runoff. These results are inconsistent with the view that the U isotopic composition of seawater simply reflects the areal extent of anoxic sediments in the past. We consider two plausible explanations for why the U isotopic composition of Archean and Proterozoic carbonates is not fractionated from the crustal and riverine composition: (1) the residence time of U could have been much shorter in the Precambrian oceans when anoxic settings were much more extensive, and (2) the process of incorporation of U into anoxic sediments in the Precambrian imparted a smaller U isotopic fractionation than in the modern because of differences in the efficiency or mechanism of uranium removal. This study highlights the challenges inherent to applying knowledge of the modern marine U isotopic cycle to periods of Earth’s history when ocean-floor anoxia was much more extended, anoxic basins were ferruginous, and atmospheric oxygen content was significantly lower than present.

Item Type:Article
Related URLs:
URLURL TypeDescription
Tissot, F. L. H.0000-0001-6622-2907
Bekker, A.0000-0002-1154-0585
Dauphas, N.0000-0002-1330-2038
Additional Information:© 2021 Elsevier Ltd. Received 17 October 2019, Revised 21 January 2021, Accepted 25 January 2021, Available online 3 February 2021. This work was supported by NASA grants NNX17AE86G (LARS), NNX17AE87G and 80NSSC20K0821 (Emerging Worlds), and 80NSSC17K0744 (Habitable Worlds), and NSF grant EAR-2001098 (CSEDI) to ND; ACS Petroleum Research Fund grant 52964 to FT and ND; NSERC Discovery and Accelerator grants to AB; National Science Foundation (NSF OCE-1846821) to MFJ. Discussions with Andrew Heard, Clara Blättler and Jacob Waldbauer are greatly appreciated. We thank C. Stirling for her editorial handling of the manuscript and T.W. Dahl and two anonymous reviewers for their constructive comments that helped improve the quality of the manuscript. Author contributions: Dauphas, Tissot, Chen, and Bekker conceived the project. Tissot, Bekker, Halverson, and Veizer made the sample selection. Chen performed the U isotopic analyses. Liu, Chen, and Dauphas modeled the influence of anoxia on U residence time. Nie modeled U speciation in seawater. Jansen evaluated the effects of the different forcing factors on the ocean mixing timescale in the Archean. Chen, Dauphas, and Jansen wrote the first draft of the manuscript, which was edited by all the co-authors. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
American Chemical Society Petroleum Research Fund52964
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Subject Keywords:Uranium; Isotopes; Carbonate; Shale; Residence time; Ocean mixing time
Record Number:CaltechAUTHORS:20210203-153225881
Persistent URL:
Official Citation:X. Chen, F.L.H. Tissot, M.F. Jansen, A. Bekker, C.X. Liu, N.X. Nie, G.P. Halverson, J. Veizer, N. Dauphas, The uranium isotopic record of shales and carbonates through geologic time, Geochimica et Cosmochimica Acta, Volume 300, 2021, Pages 164-191, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107894
Deposited By: Tony Diaz
Deposited On:04 Feb 2021 00:12
Last Modified:16 Nov 2021 19:07

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