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Controls of eustasy and diagenesis on the ^(238)U/^(235)U of carbonates and evolution of the seawater (^(234)U/^(238)U) during the last 1.4 Myr

Tissot, François L. H. and Chen, Cindy and Go, Benjamin M. and Naziemiec, Magdalena and Healy, Garrett and Bekker, Andrey and Swart, Peter K. and Dauphas, Nicolas (2018) Controls of eustasy and diagenesis on the ^(238)U/^(235)U of carbonates and evolution of the seawater (^(234)U/^(238)U) during the last 1.4 Myr. Geochimica et Cosmochimica Acta, 242 . pp. 233-265. ISSN 0016-7037. doi:10.1016/j.gca.2018.08.022.

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Using a leaching protocol designed for the study of U isotopes in recent carbonates, we measured the U isotope composition, both ^(238)U/^(235)U and ^(234)U/^(238)U, of modern and ancient corals (n = 6), a limestone and a dolostone, as well as 43 shallow-water carbonate sediments from the ODP Leg 166 Site 1009 drill core, on the slope of the Bahamas platform. Although bulk corals record the seawater δ^(238)U value within ±0.02‰, differences of up to 0.30‰ in the δ^(238)U of individual leachates suggest a control of the coral structure and a more positive ^(238)U/^(235)U ratio in the centers of calcification. The drill core δ^(238)U data shows that the ^(238)U/^(235)U ratio of shallow-water carbonates is controlled mainly by (1) variations in sea-level through the mixing of different amounts of platform-derived sediments (with δ^(238)U ∼0.50–0.60‰ heavier than seawater) and pelagic sediments (with seawater-like δ^(238)U values), (2) authigenic U enrichment via pore-watercirculation and U reduction both on the platform and down to ∼5 m below the surface (mbsf) after deposition of the sediment, and, to a lesser extent, by (3) early diagenetic processes (i.e., carbonate dissolution and/or recrystallization) during sediment burial. The global effect of these processes leaves the δ^(238)U values of shallow-water carbonates offset relative to that of seawater by Δ_(Carbonates-SW) = +0.24 ± 0.06‰ (95% CI, including all samples). This shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow-water platform, shelf and slope environments (i.e., most of the carbonate sedimentary record prior to the Mesozoic) to account for the average effect of carbonate diagenesis. Assuming that the ^(238)U/^(235)U ratio of carbonate platform sediments directly records the seawater ^(238)U/^(235)U ratio would underestimate the extent of ocean-seafloor anoxia by at least a factor 10. The rapid fluctuations in δ^(238)U values due to sea-level changes (i) is a factor that should be considered before interpreting δ^(238)U variations as reflecting changes in oceanic paleoredox conditions and (ii) reinforces the need for statistically meaningful data sets. The δ(^(234)U) data suggest that the (^(234)U/^(238)U) ratio of the seawater has remained within ∼20‰ of the modern seawater value during the last 1–1.4 Myr. Furthermore, we find that small-scale (1–15‰) variations in seawater δ(^(234)U) mirror sea-level changes during the penultimate glacial-interglacial period (∼140 to ∼200 ka), thus confirming the record of lower δ(^(234)U)_(SW) during periods of low sea-level stand and expanding it to at least the last two glacial-interglacial events (i.e., ∼0.23 Ma). Such fluctuations in δ(^(234)U)_(initial) values should be taken into account when screening carbonate sediments U-Th ages on the basis of the initial (^(234)U/^(238)U) ratios of the samples.

Item Type:Article
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URLURL TypeDescription
Tissot, François L. H.0000-0001-6622-2907
Dauphas, Nicolas0000-0002-1330-2038
Alternate Title:Controls of eustasy and diagenesis on the 238U/235U of carbonates and evolution of the seawater (234U/238U) during the last 1.4 Myr
Additional Information:© 2018 Elsevier Ltd. Received 19 January 2018, Accepted 10 August 2018, Available online 23 August 2018. FT thanks Anne M. Gothmann for valuable discussions and comments on an earlier version of the manuscript. The Field Museum (Chicago) is thanked for providing several carbonate samples. Constructive criticisms from Morten B. Andersen, Chris Holmden, one anonymous reviewer, and editor Claudine Stirling greatly helped improve the manuscript. This work was supported by grants from ACS (52964-ND2), NSF (EAR1502591 and EAR1444951) and NASA (NNX17AE86G, NNX17AE87G, and NNX15AJ25G) to ND, a Crosby Postdoctoral Fellowship to FT, and funding from NSERC Discovery and Accelerator grant (RGPIN-316500) to AB. Authors Contributions: FT and ND initiated the project. FT designed the research. FT, BMG, MN, GH, CC and PKS performed the research. FT interpreted the data with inputs from ND, CC, AB and PKS. FT wrote the manuscript with contributions from all co-authors.
Funding AgencyGrant Number
American Chemical Society52964-ND2
Crosby Postdoctoral FellowshipUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)RGPIN-316500
Subject Keywords:Shallow-water carbonate; Bahamas; ODP drill core; U isotopes; Sea-level variations; Diagenesis
Record Number:CaltechAUTHORS:20181024-132628441
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Official Citation:François L.H. Tissot, Cindy Chen, Benjamin M. Go, Magdalena Naziemiec, Garrett Healy, Andrey Bekker, Peter K. Swart, Nicolas Dauphas, Controls of eustasy and diagenesis on the 238U/235U of carbonates and evolution of the seawater (234U/238U) during the last 1.4 Myr, Geochimica et Cosmochimica Acta, Volume 242, 2018, Pages 233-265, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90395
Deposited By: Tony Diaz
Deposited On:24 Oct 2018 20:50
Last Modified:16 Nov 2021 03:32

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