Mg isotopes and Mg/Ca values of coccoliths from cultured specimens of the species Emiliania huxleyi and Gephyrocapsa oceanica

Coccoliths from cultured specimens of two species of coccolithophores (Emiliania huxleyi and Gephyrocapsa oceanica) were sampled during two growth phases (late exponential and stationary), and their Mg isotope values (δ26Mg) as well as Mg/Ca values were measured in order to investigate whether δ26Mg can be used as a temperature proxy. Mg/Ca values were positively related with temperature (~ 0.002 mmol/mol/°C), without statistically significant differences between the two growth phases and the two species. Both species were depleted in heavier Mg isotopes relative to the culture medium, and δ26Mg values were temperature dependent in both growth phases of E. huxleyi, although the δ26Mg values differed in the two growth phases. In G. oceanica, a weak correlation between δ26Mg values and temperature was seen in the late exponential growth phase only, and the δ26Mg values differed between growth phases. The large differences between δ26Mg values as measured in calcite formed during different growth phases indicate that Mg isotopes of coccoliths cannot be simply used as a temperature proxy. Our conclusions are preliminary and more data must be collected in order to fully evaluate the use of Mg isotopes of coccoliths as a temperature proxy.