Influence of temperature and CO 2 on the strontium and magnesium composition of coccolithophore calcite

Marine calcareous sediments provide a fundamen- tal basis for palaeoceanographic studies aiming to recon- struct past oceanic conditions and understand key biogeo- chemical element cycles. Calcifying unicellular phytoplank- ton (coccolithophores) are a major contributor to both carbon and calcium cycling by photosynthesis and the production of calcite (coccoliths) in the euphotic zone, and the subsequent long-term deposition and burial into marine sediments. Here we present data from controlled laboratory experiments on four coccolithophore species and elucidate the relation be- tween the divalent cation (Sr, Mg and Ca) partitioning in coccoliths and cellular physiology (growth, calcification and photosynthesis). Coccolithophores were cultured under dif- ferent seawater temperature and carbonate chemistry condi- tions. The partition coefficient of strontium ( DSr) was posi- tively correlated with both carbon dioxide (pCO2) and tem- perature but displayed no coherent relation to particulate or- ganic and inorganic carbon production rates. Furthermore, DSr correlated positively with cellular growth rates when driven by temperature but no correlation was present when changes in growth rates were pCO2-induced. Our results demonstrate the complex interaction between environmen- tal forcing and physiological control on the strontium par- titioning in coccolithophore calcite and challenge interpre- tations of the coccolith Sr / Ca ratio from high-pCO2 envi- ronments (e.g. Palaeocene-Eocene thermal maximum). The partition coefficient of magnesium ( DMg) displayed species- specific differences and elevated values under nutrient limi- tation. No conclusive correlation between coccolith DMg and temperature was observed butpCO2 induced a rising trend in coccolith DMg. Interestingly, the best correlation was found between coccolith DMg and chlorophyll a production, sug- gesting that chlorophyll a and calcite associated Mg orig- inate from the same intracellular pool. These and previous findings indicate that Mg is transported into the cell and to the site of calcification via different pathways than Ca and Sr. Consequently, the coccolith Mg / Ca ratio should be de- coupled from the seawater Mg / Ca ratio. This study gives an extended insight into the driving factors influencing the coccolith Mg / Ca ratio and should be considered for future palaeoproxy calibrations.