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

Sea surface temperature (SST) is a critical variable in the Earth's climate system since it influences atmospheric circulation, the hydrological cycle and, via ocean surface density, drives ocean circulation. A detailed reconstruction of past SST's 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 delta Ca-44 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 delta Ca-44 and delta O-18-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 delta Ca-44 temperature dependence of similar to 0.034 parts per thousand degrees C-1 similar to inorganic calcite (0.015 parts per thousand degrees C-1) and cultured O. universa (0.019 parts per thousand degrees C-1). The gradient resembles the global sediment delta Ca-44 compilation, but it is significantly smaller than the temperature calibration of 0.22 +/- 0.02 parts per thousand degrees C-1 for cultured G. sacculifer. The modern data set shows a general correlation between 644 Ca and depth habitat reflecting a similar temperature gradient to the Eocene sample set. In contrast, the lower absolute delta Ca-44 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 delta Ca-44 in foraminiferal calcite that may overprint a potential T-relationship. (C) 2008 Elsevier B.V. All rights reserved.