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Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera

Staudigel, Philip T., John, Eleanor H., Buse, Ben and Lear, Caroline H. ORCID: 2022. Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera. Geology 50 (7) , pp. 760-764. 10.1130/G49984.1

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Trace element and δ18O values of foraminifera are widely used to reconstruct oceanic temperatures throughout the Cenozoic and beyond. Previous work evaluating the geochemistry of foraminifera with differing degrees of physical preservation have shown that Mg/Ca and δ18O paleothermometers give discrepant values in recrystallized tests, with planktonic oxygen isotopes often yielding significantly lower temperatures than Mg/Ca ratios. To study the mobility of elements during diagenesis, we performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is readily identifiable and preserved, to an extent, in texturally recrystallized tests. A reaction-diffusion model was used to test whether the preservation of Mg-banding and the decoupling of δ18O and Mg/Ca values could be the result of diffusively limited “closed-system” recrystallization. Results show that, in a closed system, internal features (such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca composition, while the bulk δ18O value will typically change faster than Mg/Ca. This is observed regardless of what partitioning coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy is more diagenetically robust than the δ18O proxy. Thus, this model can explain the observed decoupling of these two proxies. Furthermore, the preservation of intra-test Mg-banding shows potential for use in evaluating the preservation of primary Mg/Ca values and hence the accuracy of paleotemperature reconstructions.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Additional Information: This paper is published under the terms of the CC-BY license.
Publisher: Geological Society of America
ISSN: 1943-2682
Funders: NERC
Date of First Compliant Deposit: 10 February 2022
Date of Acceptance: 24 January 2022
Last Modified: 28 Nov 2023 10:01

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