Marine palaeotemperature studies are increasingly using oxygen isotope analyses of the minute calcium carbonate structures produced by a group of marine phytoplankton, the coccolithophores1-6. To provide a sound experimental basis for palaeotemperature calculations using the isotopic data from analyses of coccoliths, we have grown coccolithophores in laboratory batch culture in controlled environmental conditions, and determined the oxygen isotopic compositions of the coccoliths produced at known temperatures. The results reported here indicate that the oxygen isotopic composition of the coccoliths of all the species studied is strongly temperature dependent. A 'vital effect' was observed in all the species, with the isotopic values of different culture samples falling into two definite groups containing separate taxa. The difference in vital effect between different species suggests that calcification processes may vary among different taxa and indicates that a re-evaluation of coccolith oxygen isotope palaeoclimatic interpretations may be in order.
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