Biological and ecological insights into Ca isotopes in planktic foraminifers as a palaeotemperature proxy

Sea surface temperature (SST) is a critical variable in the Earthʹs climate system since it influences atmospheric circulation, the hydrological cycle and, via ocean surface density, drives ocean circulation. A detailed reconstruction of past SSTʹs is therefore a central goal of palaeoceanographic studies. Although calcium isotopes in foraminiferal carbonate have been introduced as a new proxy for SST reconstruction, there is considerable debate about their robustness and general applicability. To resolve some of these questions, we have investigated the extent to which other environmental parameters e.g. the calcification temperature, depth stratification, growth rates and/or environmental adaptation may influence the δ44Ca values of planktic foraminifers in modern and Eocene samples. Geographically distributed data sets are affected by the exchange of cryptic species, i.e. morphologically similar but genetically distinct species, and by a mixing of optimal versus less optimal adaptation. Thus, we have compared species within individual samples to evaluate whether the well documented depth stratification of foraminifers is reflected in their calcium isotopes. The Eocene data set shows a general agreement between δ44Ca and δ18O-derived calcification temperatures which supports a temperature effect on Ca isotope incorporation. The vertical temperature gradient using the different depth habitats of several foraminiferal species indicates a δ44Ca temperature dependence of ~ 0.034‰ °C− 1 similar to inorganic calcite (0.015‰ °C− 1) and cultured O. universa (0.019‰ °C− 1). The gradient resembles the global sediment δ44Ca compilation, but it is significantly smaller than the temperature calibration of 0.22 ± 0.02‰ °C− 1 for cultured G. sacculifer. The modern data set shows a general correlation between δ44Ca and depth habitat reflecting a similar temperature gradient to the Eocene sample set. In contrast, the lower absolute δ44Ca values for the Eocene foraminifers suggest a lower seawater isotope composition. In situ analyses of individual calcite layers reveal large isotopic differences between the different calcite layers of the foraminifers highlighting the strong biological control on δ44Ca in foraminiferal calcite that may overprint a potential T-relationship.