Chromium isotopic composition of core-top planktonic foraminifera

Xiangli Wang*, N. J. Planavsky, P. M. Hull, A. E. Tripati, H. J. Zou, L. Elder, M. Henehan

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

39 Citations (Scopus)

Abstract

The chromium isotope system (53Cr/52Cr expressed as δ53Cr relative to NIST SRM 979) is potentially a powerful proxy for the redox state of the ocean–atmosphere system, but a lack of temporally continuous, well-calibrated archives has limited its application to date. Marine carbonates could potentially serve as a common and continuous Cr isotope archive. Here, we present the first evaluation of planktonic foraminiferal calcite as an archive of seawater δ53Cr. We show that single foraminiferal species from globally distributed core tops yielded variable δ53Cr, ranging from 0.1‰ to 2.5‰. These values do not match with the existing measurements of seawater δ53Cr. Further, within a single core-top, species with similar water column distributions (i.e., depth habitats) yielded variable δ53Cr values. In addition, mixed layer and thermocline species do not consistently exhibit decreasing trends in δ53Cr as expected based on current understanding of Cr cycling in the ocean. These observations suggest that either seawater δ53Cr is more heterogeneous than previously thought or that there is significant and species-dependent Cr isotope fractionation during foraminiferal calcification. Given that the δ53Cr variability is comparable to that observed in geological samples throughout Earth's history, interpreting planktonic foraminiferal δ53Cr without calibrating modern foraminifera further, and without additional seawater measurements, would lead to erroneous conclusions. Our core-top survey clearly indicates that planktonic foraminifera are not a straightforward δ53Cr archive and should not be used to study marine redox evolution without additional study. It likewise cautions against the use of δ53Cr in bulk carbonate or other biogenic archives pending further work on vital effects and the geographic heterogeneity of the Cr isotope composition of seawater.

Original languageEnglish
Pages (from-to)51-64
Number of pages14
JournalGeobiology
Volume15
Issue number1
DOIs
Publication statusPublished - 1 Jan 2017

Bibliographical note

Funding Information:
This work is funded by Agouron Institute to X.W. A.T received funding (BES grant DE-FG02-13ER16402 and a LabEx International Research Chair) from the ‘Laboratoire d'Excellence’ LabexMER (ANR-10-LABX-19), co-funded by a grant from the French government under the program ‘Investissements d'Avenir’. The authors wish to thank the Ocean Drilling Program, Helen Bostock (NIWA, NZ), Bruce Corliss (Graduate School of Oceanography, University of Rhode Island), and Richard Norris (Scripps Institution of Oceanography, University of California San Diego) for providing core-top sediments.

Publisher Copyright:
© 2016 John Wiley & Sons Ltd

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