Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis

Benthic foraminiferal magnesium/calcium ratios were determined on one hundred and forty core-top samples from the Atlantic Ocean, the Norwegian Sea, the Indian Ocean, the Arabian Sea and the Pacific Ocean, mostly at sites with bottom water temperatures below 5 °C. Mg/Ca ratios are consistently lower, by ∼ 0.2 mmol/mol, in samples cleaned using oxidative and reductive steps than using oxidative cleaning. Differences between Cibicidoides species have been identified: Mg/Ca of Cibicidoides robertsonianus > Cibicidoides kullenbergi > Cibicidoides wuellerstorfi. Comparison with bottom water temperatures support observations of lowered Mg/Ca of C. wuellerstorfi at temperature below ∼ 3 °C compared with values predicted by published calibrations and from other Cibicidoides species. Hydrographic data shows that carbonate ion saturation (Δ[CO₃^2-]) decreases rapidly below this temperature. An empirical sensitivity of Δ[CO₃^2-] on Mg/Ca has been established for C. wuellerstorfi of 0.0086 ± 0.0006 mmol/mol/μmol/kg. A novel application using modern temperatures and Last Glacial Maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on Mg incorporation. This may significantly affect calculated δ¹⁸O_seawater obtained from foraminiferal δ¹⁸O and Mg/Ca temperature.