Coupled Mg/Ca and clumped isotope analyses of foraminifera provide consistent water temperatures

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Abstract

The reliable determination of past seawater temperature is fundamental to paleoclimate studies. We test the robustness of two paleotemperature proxies by combining Mg/Ca and clumped isotopes (Δ 47 ) on the same specimens of core top planktonic foraminifera. The strength of this approach is that Mg/Ca and Δ 47 are measured on the same specimens of foraminifera, thereby providing two independent estimates of temperature. This replication constitutes a rigorous test of individual methods with the advantage that the same approach can be applied to fossil specimens. Aliquots for Mg/Ca and clumped analyses are treated in the same manner following a modified cleaning procedure of foraminifera for trace element and isotopic analyses. We analysed eight species of planktonic foraminifera from coretop samples over a wide range of temperatures from 2 to 29°C. We provide a new clumped isotope temperature calibrations using subaqueous cave carbonates, which is consistent with recent studies. Tandem Mg/Ca–Δ 47 results follow an exponential curve as predicted by temperature calibration equations. Observed deviations from the predicted Mg/Ca-Δ 47 relationship are attributed to the effects of Fe-Mn oxide coatings, contamination, or dissolution of foraminiferal tests. This coupled approach provides a high degree of confidence in temperature estimates when Mg/Ca and Δ 47 yield concordant results, and can be used to infer the past δ 18 O of seawater (δ 18 O sw ) for paleoclimate studies.

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Breitenbach, S. F. M., Mleneck-Vautravers, M. J., Grauel, A. L., Lo, L., Bernasconi, S. M., Müller, I. A., … Hodell, D. A. (2018). Coupled Mg/Ca and clumped isotope analyses of foraminifera provide consistent water temperatures. Geochimica et Cosmochimica Acta, 236, 283–296. https://doi.org/10.1016/j.gca.2018.03.010

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