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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Breitenbach, Sebastian, Mleneck-Vautravers, Maryline J., Grauel, Anna-Lena, Lo, Li, Bernasconi, Stefano M., Müller, Inigo A., Rolfe, James, Gázquez, Fernando, Greaves, Mervyn, Hodell, David A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2018
• Subjects: F600 Geology; F700 Ocean Sciences; F800 Physical and Terrestrial Geographical and Environmental Sciences; F900 Others in Physical Sciences; Planktonic foraminifera
• Online Access: https://nrl.northumbria.ac.uk/id/eprint/42160/; https://doi.org/10.1016/j.gca.2018.03.010; https://nrl.northumbria.ac.uk/id/eprint/42160/1/1-s2.0-S0016703718301613-main.pdf

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 (Δ47) on the same specimens of core top planktonic foraminifera. The strength of this approach is that Mg/Ca and Δ47 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–Δ47 results follow an exponential curve as predicted by temperature calibration equations. Observed deviations from the predicted Mg/Ca-Δ47 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 Δ47 yield concordant results, and can be used to infer the past δ18O of seawater (δ18Osw) for paleoclimate studies.