The Mg/Ca-temperature relationship of benthic foraminiferal calcite: New core-top calibrations in the < 4 °C temperature range

Calcite magnesium/calcium ratios were determined for four species of benthic foraminifera (Cibicidoides wuellerstorfi, Cibicidoides mundulus, Oridorsalis umbonatus and Pyrgo murrhina) from core-top samples collected from the Atlantic, Indian, and Pacific Oceans where modern bottom water temperatures are less than 4 °C. Mg/Ca of all four species increase with increasing temperature, and the Mg/Ca:temperature relationship does not appear to be necessarily exponential. Mg/Ca of C. mundulus is consistently higher than C. wuellerstorfi, implying differences in the uptake of Mg by different species within this genus. The observed Mg/Ca for C. wuellerstorfi at temperatures less than 3 °C is lower than those predicted from published Mg/Ca: temperature relationships derived for Cibicidoides spp. at higher temperatures. Previous studies have attributed this change in the Mg/Ca-temperature relationship of C. wuellerstorfi to the effect of decreased carbonate ion saturation (Δ[CO32−]) on Mg/Ca at low temperatures. A relationship between the Δ[CO32−] and Mg/Ca of 0.0083 ± 0.002 (mmol/mol)/(μmol/kg) has been established for C. wuellerstorfi from this study.