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Effective use of cerium anomalies as a redox proxy in carbonate-dominated marine settings

Tostevin, R; Shields, GA; Tarbuck, GM; He, T; Clarkson, MO; Wood, RA; (2016) Effective use of cerium anomalies as a redox proxy in carbonate-dominated marine settings. Chemical Geology , 438 (C) pp. 146-162. 10.1016/j.chemgeo.2016.06.027. Green open access

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Abstract

Rare earth elements and yttrium (REY) have a distinct distribution pattern in seawater, and this pattern may be faithfully preserved in carbonate sediments and rocks. Anomalous concentrations of redox-sensitive cerium (Ce) compared with neighbouring REY originate in oxic water column conditions, and as such, Ce anomalies can provide a potentially useful redox proxy in carbonate-dominated marine settings. The methods used to extract REY from carbonates vary widely, and may suffer from widespread leaching of REY from accessory non-carbonate minerals and organic matter, limiting the application of Ce anomalies for palaeo-redox reconstruction. We have systematically compared different methods on 195 carbonate samples with varying purity (% carbonate) from both modern and ancient environments. We used sequential leaching experiments in nitric acid to identify the most ‘seawater-like’ portion of the carbonate sample where contributions from non-carbonate minerals and organic matter are minimised. We also compared the results of sample dissolution in different types and strengths of acid. Our results confirm that REY concentrations can be inadvertently contaminated by partial leaching of clays and Fe (oxyhydr)oxides during a single-step digestion, and we suggest a pre-leach of 20% of the sample, followed by a partial leach of 40% of the sample to selectively dissolve carbonate. We suggest that REY studies are optimised in carbonates with > 85% CaCO3, and show that dolomites behave differently during the leaching process and must be treated separately. We present REY patterns for modern carbonate-rich sediments from a range of environments, and show that seawater REY are faithfully preserved in some non-skeletal carbonate, but modified leaching procedures are necessary for impure, unlithified or organic rich carbonate sediments. We combine REY with Fe-speciation data to identify how Fe oxides and clays contribute to the REY signal and explore how the two proxies can be used together to provide a complex and high-resolution redox reconstruction in carbonate-dominated marine environments.

Type: Article
Title: Effective use of cerium anomalies as a redox proxy in carbonate-dominated marine settings
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.chemgeo.2016.06.027
Publisher version: http://dx.doi.org/10.1016/j.chemgeo.2016.06.027
Language: English
Additional information: © 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Keywords: Rare earth elements; Ce anomalies; Acid leaching; Method development; Redox proxies; Fe-speciation; Carbonate geochemistry
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1517864
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