Stable isotope fractionation of strontium in coccolithophore calcite: Influence of temperature and carbonate chemistry.

Marine calcifying eukaryotic phytoplankton (coccolithophores) is a major contributor to the pelagic production of CaCO3 and plays an important role in the biogeochemical cycles of C, Ca and other divalent cations present in the crystal structure of calcite. The geochemical signature of coccolithophore calcite is used as palaeoproxy to reconstruct past environmental conditions and to understand the underlying physiological mechanisms (vital effects) and precipitation kinetics. Here, we present the stable Sr isotope fractionation between seawater and calcite (Δ88/86 Sr) of laboratory cultured coccolithophores in individual dependence of temperature and seawater carbonate chemistry. Coccolithophores were cultured within a temperature and a pCO2 range from 10 to 25°C and from 175 to 1,240 μatm, respectively. Both environmental drivers induced a significant linear increase in coccolith stable Sr isotope fractionation. The temperature correlation at constant pCO2 for Emiliania huxleyi and Coccolithus braarudii is expressed as Δ88/86 Sr = -7.611 × 10-3 T + 0.0061. The relation of Δ88/86 Sr to pCO2 was tested in Emiliania huxleyi at 10 and 20°C and resulted in Δ88/86 Sr = -5.394 × 10-5 pCO2 - 0.0920 and Δ88/86 Sr = -5.742 × 10-5 pCO2 - 0.1351, respectively. No consistent relationship was found between coccolith Δ88/86 Sr and cellular physiology impeding a direct application of fossil coccolith Δ88/86 Sr as coccolithophore productivity proxy. An overall significant correlation was detected between the elemental distribution coefficient (DSr ) and Δ88/86 Sr similar to inorganic calcite with a physiologically induced offset. Our observations indicate (i) that temperature and pCO2 induce specific effects on coccolith Δ88/86 Sr values and (ii) that strontium elemental ratios and stable isotope fractionation are mainly controlled by precipitation kinetics when embedded into the crystal lattice and subject to vital effects during the transmembrane transport from seawater to the site of calcification. These results provide an important step to develop a coccolith Δ88/86 Sr palaeoproxy complementing the existing toolbox of palaeoceanography.