Temperature dependence of δ44/40Ca, δ18O, Mg/Ca, Sr/Ca fractionation in calcite tests of foraminifera from a sediment trap

Foraminifera are useful geological archives used for reconstructing paleo-temperature and paleo-ocean chemistry. Oxygen isotope (δ¹⁸O) and Mg/Ca have been used in the calcite tests of foraminifera to successfully derive a paleo-thermometer in order to reconstruct the paleo- temperatures. Use of Ca isotope (δ^44/40Ca) as a temperature proxy attracts interest since Ca isotopic fractionation shows temperature dependency in foraminifera in culture experiments. Higher temperature sensitivity of Ca stable isotope ( 0.24 ‰/°C) was observed for laboratory-cultured foraminifera samples while negligible temperature sensitivity was reported for foraminifera collected from core top samples. This discrepancy between studies suggests a complex biomineralization pathway in foraminifera as well as the effects of additional factors on the δ^44/40Ca composition of forams. To understand the applicability of Ca isotopic fractionation in foraminifera, δ^44/40Ca measurements were performed in benthic (Cibicidoides wuellerstorfi), thermocline planktic species (G. truncatulinoides and G. inflata) and other planktic species (G. ruber, G. bulloides, O universa, G. inflata, G. siphonifera) collected from the North Atlantic Ocean using a sediment trap. Some of these samples were earlier analysed for Mg/Ca and ^δ18O measurements and used for the Mg/Ca thermometry calibration by Anand et al. (2003). The calibration for δ^44/40Ca-T was done using calcification temperatures estimated from both δ¹⁸O and Mg/Ca using published δ¹⁸O-T and Mg/Ca-T relationships. When the species G. ruber and those with estimated habitat temperatures less than 3 °C are excluded, δ^44/40Ca values show a significant (r² = 0.76) relationship (δ^44/40Ca (‰) = 0.35 Exp (0.055 × T (°C)) with temperature estimated from Mg/Ca ratio of the foram. Based on this relationship involving multiple foraminifera species, the temperature sensitivity of δ^44/40Ca in foraminifera is 0.055 ‰/°C which can be useful to reconstruct paleotemperatures with a precision of ± 1.5 °C.

Anand P., Elderfield H. and Conte M. H. (2003) Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series. Paleoceanography 18.