The δ⁴⁴Ca-temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures

We report direct δ⁴⁴Ca-temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a ⁴³Ca-⁴⁸Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species-dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ⁴⁴Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ⁴⁴Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ⁴⁴Ca difference between marine isotope stage 1 (MIS-1) and MIS-2 is 0.71 ± 0.24. According to the current δ⁴⁴Ca-temperature calibration this value corresponds to a temperature difference between MIS-1 and MIS-2 of ∼3.0 ± 1.0°C.