Oxygen isotope analyses of coccoliths from eight coccolithophore species grown in laboratory culture indicate that biological fractionation occurs during the formation of calcium carbonate. Cultures of coccolithophores, including species not previously grown in the laboratory were maintained at various temperatures ranging from 12-28°C. Over this range of temperature, oxygen isotope values for Emiliania huxleyi, Gephyrocapsa oceanica, and Crenalithus sessilis are found to be 1‰ enriched in 18O relative to calcium carbonate precipitated under equilibrium conditions. Coccoliths of Calcidiscus leptoporus, Umbilicosphaera sibogae, U. hulburtiana, Syracosphaera pulchra, and Cricosphaera carterae were found to be 2.5‰ depleted in 18O relative to equilibrium. These findings help clarify previously reported equivocal oxygen isotope data derived from sedimentary calcareous nannofossils and reveal the importance of species-specific isotopic fractionation.
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