Temperature dependence of δ7Li, δ44Ca and Li/Ca during growth of calcium carbonate

Lithium and calcium isotope ratios, together with the Li/Ca element ratio, have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 °C, and on seasonal growth bands from a coral. These measurements indicate the potential uses of these three environmental proxies. Carbonate Li isotopes are lighter than the solution from which they grow, probably due to equilibrium fractionation. This fractionation is not significantly influenced by temperature and, because Li is not involved in any known biological process, this suggests that carbonates will provide a recorder for the past Li-isotope composition of natural waters and hence for past weathering conditions. As has been observed previously for inorganic aragonite, the Ca-isotopes in inorganic calcite are lighter than the growth solution, and are only weakly dependent on temperature. This confirms that the temperature dependence observed in some species of foraminifera [Geochem. Geophys. Geosyst. 1 (2001)] reflects additional isotope fractionation due to biological processes during mineralization. Ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization. Finally, Li/Ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of ≈4% per °C. This temperature dependence of Li/Ca may prove useful to assess past temperatures in cold environments such as the deep ocean.