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Post-depositional overprinting of chromium in foraminifera

Research output: Contribution to journalArticle

Original languageEnglish
Pages (from-to)100-111
Number of pages12
JournalEarth and Planetary Science Letters
Volume515
Early online date28 Mar 2019
DOIs
DateAccepted/In press - 1 Mar 2019
DateE-pub ahead of print - 28 Mar 2019
DatePublished (current) - 1 Jun 2019

Abstract

Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical cycling in the oceans. Chromium isotopes are used as a proxy to infer changes in past oceanic redox state. Chromium isotopes in carbonates, including the prime proxy carrier foraminifera, were initially thought to record the seawater composition during crystallisation. However, the uptake of Cr into foraminiferal tests and carbonates is still poorly understood and recent studies question this assumption. We assess whether Cr in foraminiferal calcite is taken up during biomineralisation, has a post depositional origin or is a combination of the two. Laser Ablation-MC-ICP-MS analyses and NanoSIMS imaging of individual tests were used to characterise the distribution of Cr in both planktic and benthic foraminifera. Foraminifera in sediment core-top samples have up to two orders of magnitude more Cr than sediment trap, plankton net, and culture samples. In cultured specimens, Cr is incorporated in foraminiferal tests at low concentrations (0.04 – 0.13 ppm) with a distribution coefficient of ~250 ± 43 (2SE) which is an upper estimate due to substantial loss of dissolved Cr during the experiment. Part of the Cr signal in sedimentary foraminifera may be primary, but this primary signal is likely often overprinted by the uptake of Cr from bottom and pore waters. In sediment samples, there is no significant isotopic offset between individual species and bulk foraminiferal calcite from the same size fraction. The >500 µm fraction has a heavier isotopic composition than the smaller 250 – 500 µm fraction with an offset of -0.3 to -0.5‰ due to an increase in surface area to volume. We propose that Cr in foraminifera is predominantly post-depositional and records bottom/pore water signals. This is contrary to current interpretations of the foraminiferal Cr isotope proxy as a surface seawater redox proxy.

    Research areas

  • Chromium, foraminifera, diagenesis, distribution coefficient, laser ablation, nanoSIMS

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  • Full-text PDF (accepted author manuscript)

    Rights statement: This is the author accepted manuscript (AAM). The final published version (version of record) is available online via Elsevier at https://www.sciencedirect.com/science/article/pii/S0012821X19301426 . Please refer to any applicable terms of use of the publisher.

    Accepted author manuscript, 5 MB, PDF-document

    Embargo ends: 28/03/20

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    Licence: CC BY-NC-ND

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