Abstract
The controls on stable Sr isotope fractionation into the calcite produced by the coccolithophore species Emiliania huxleyi, Coccolithus pelagicus spp. braarudii and Gephyrocapsa oceanica are investigated. Each species has been cultured under controlled laboratory conditions at a range of temperatures (10–25°C) to test the potential of δ88/86Sr as a proxy for growth rate and/or sea surface temperature. Coccolithophores are one of the most abundant pelagic calcifiers; since Sr substitutes to some degree for Ca in their calcite coccoliths, coccolithophores represent a significant output of Sr from seawater, potentially influencing the δ88/86Sr mass balance in the modern oceans. The coccoliths are investigated for their δ88/86Sr, Sr/Ca, and δ18O measured as function of temperature. As temperature is increased all species show a negative stable Sr isotopic fractionation, related to a temperature controlled growth rate. We infer the shift of δ88/86Sr to lighter values as indicative of a kinetic control on the isotope fractionation, revealing the potential of δ88/86Sr as a proxy for growth rate, which in these experiments is primarily dictated by temperature. The results from these experiments indicate that coccolithophore calcite incorporates Sr with a very light δ88/86Sr isotope composition as low as ∼0‰.
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