Mg/Ca-temperature calibration for costate Bulimina species (B. costata, B. inflata, B. mexicana): A paleothermometer for hypoxic environments

Costate species of Bulimina are cosmopolitan, infaunal benthic foraminifers which are common in the fossil record since the Paleogene. In the present study, we evaluate the temperature dependency of Mg/Ca ratios in Bulimina inflata, B. mexicana and B. costata from an extensive set of core-top samples from the Atlantic, Pacific and Indian Oceans. The results show no significant offset in Mg/Ca values between costate morphospecies when present in the same sample. The apparent lack of significant inter-specific/inter-morphotype differences amongst the analyzed costate buliminids allows for the combined use of their data-sets for our core-top calibration. Over a bottom-water temperature (BWT) range of 3–13 °C, the Bulimina species show a sensitivity of ∼0.12 mmol/mol/°C which is comparable to that of epifaunal Cibicidoides species and higher than that of the shallow infaunal Uvigerina spp., the most commonly used taxon in Mg/Ca-based palaeotemperature reconstruction. The reliability and accuracy of the new Mg/Ca-temperature calibration is corroborated in the fossil record by a case study in the Timor Sea which demonstrates the presence of southern-sourced waters at intermediate depths for the past 26,000 years. Costate species of Bulimina might thus provide a valuable alternative for BWT reconstruction in mesotrophic to eutrophic settings where many of the commonly used (more oligotrophic) species are rare or absent, and be particularly useful in hypoxic settings such as permanent upwelling zones where costate buliminids often dominate foraminiferal assemblages. The evaluation further reveals a mean positive offset of ∼0.2 mmol/mol of the Atlantic data-set over the Indo-Pacific data-set which contributes to the scatter in our calibration. Although an explanation for this offset is not straightforward and further research is necessary, we hypothesize that different levels of export production and carbonate ion concentrations in pore waters are likely reasons.