Western Indian Ocean bottom water temperature calibration – are benthic foraminiferal Mg/Ca ratios a reliable palaeothermometry proxy?

Abstract. Mg/Ca ratios measured in benthic foraminifera have been explored as a potential palaeothermometry proxy for bottom water temperatures (BWT). Mg/Ca-BWT calibrations from the Indian Ocean are rare and comprise conflicting results. Inconsistencies between studies suggest that calibrations may need to be region specific. The aim of this study was to develop benthic foraminifera (Uvigerina peregrina, Cibicidoides wuellerstorfi and Cibicidoides mundulus) based Mg/Ca – BWT calibrations in the tropical western Indian Ocean. Testing variations of existing analytical protocols, aimed at optimising cleaning of the foraminifera while avoiding sample loss in the process, entailed that a previously established protocol by Barker et al. (2003) was the most suitable for our study. The majority of samples of Cibicidoides mundulus and Uvigerina peregrina, however, remained contaminated, rendering those data unusable for Mg/Ca core-top calibrations. Only Mg/Ca ratios in Cibicidoides wuellerstorfi allowed a tentative Mg/Ca - BWT calibration with the relationship being: Mg/Ca = 0.19 ± 0.02 ∗ BWT + 1.07 ± 0.03, 𝑟² = 0.87. While this result differs to some degree from previous studies it principally suggests that existing core-top calibrations from the wider Indian Ocean can be applied to core-tops in the western Indian Ocean. The agreement of Mg/Ca ratios at lower temperatures in Cibicidoides wuellerstorfi, Cibicidoides mundulus and Uvigerina peregrina with Mg/Ca ratios reported for these species at low temperatures in other studies supports this conclusion. Many uncertainties surrounding the Mg/Ca proxy exist and more calibration studies are required to improve this method.