Regional conditions cause contrasting behaviour in U-isotope fractionation in black shales: Constraints for global ocean palaeo-redox reconstructions

Gangl, S.K., Stirling, C.H., Jenkyns, H.C., Preston, W.J., Clarkson, M.O., Moy, C.M., Dickson, A.J. and Porcelli, D. 2023. Regional conditions cause contrasting behaviour in U-isotope fractionation in black shales: Constraints for global ocean palaeo-redox reconstructions. Chemical Geology 623 , 121411. 10.1016/j.chemgeo.2023.121411

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Abstract

The U-isotope system is a well-established palaeo-redox proxy that potentially constrains the global extent of marine anoxia during average as well as extreme redox events throughout Earth's history. A typical archive that forms underneath a reducing water column and acts as an intense U sink is organic-rich black shale. However, the degree to which black shale archives reflect the marine U-isotope signature is not well understood because U-isotope fractionation between U(VI)-bearing seawater and U(IV)-bearing black shales may vary as a function of local environmental conditions. Here, we present a combination of U-isotope and elemental concentration datasets, supported by a complementary Mo-isotope record, for the Furlo sedimentary section in Marche–Umbria, Italy and interrogate the combined systematics to unravel the mechanisms controlling the U-isotope fractionation factor between black shales and ambient seawater. We examine black shales deposited before and during Oceanic Anoxic Event 2 (OAE 2), which was one of the most extreme climatic perturbations of the Mesozoic Era that took place around the Cenomanian–Turonian boundary (Late Cretaceous, c. 94 Ma). The results of this study show that the U-isotope signature in the black shales deposited before OAE 2 was controlled by different mechanisms than the U-isotope ratios recorded in black shales deposited during OAE 2, with both stratigraphic intervals likely influenced by local environmental conditions. Probable local environmental changes include increased U reduction associated with biomass at or above the sediment–water interface and varying dissolved hydrogen sulphide concentrations in the water column and sediment. The overall results of this study confirm that black shales are a highly complex archive for U-isotope studies of past oceanic redox conditions, due to the sensitivity of the U-isotope fractionation mechanism to local environmental conditions, which are difficult to constrain. We propose the application of a ∆238Ushale-seawater of 0.6 ± 0.1 ‰ to black shale records deposited under locally constant euxinic conditions at non-restricted settings.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	Elsevier
ISSN:	0009-2541
Date of First Compliant Deposit:	27 October 2023
Date of Acceptance:	1 March 2023
Last Modified:	27 Oct 2023 14:45
URI:	https://orca.cardiff.ac.uk/id/eprint/163530

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