Abstract
Abstract Glacial to interglacial environmental changes have a strong impact on coccolithophore assemblage composition. At the same time, glacial terminations are characterised by an increase in atmospheric CO 2 concentration. In order to determine how these two processes influence the calcite production of coccolithophores, we compared coccolith weight estimates obtained with the automated coccolith recognition system SYRACO with SEM assemblage counts covering the penultimate glacial Termination (T II) from two sediment cores in the North Atlantic Ocean. At the temperate Rockall Plateau (ODP Site 980), mean coccolith weight peaks around Heinrich event 11. This is paralleled by a shift within the coccolith assemblage related to the changes of the oceanic frontal system during Termination II. In the tropical Florida Strait, far from the influences of frontal zones, mean Noelaerhabdaceae coccolith weight doubles during Termination II. This is partly due to an assemblage shift towards larger and heavier calcifying morphotypes, but mainly an effect of increasing coccolithophore calcification. This increase is exactly mirroring the rise in atmospheric CO 2 , contradicting previous findings from Termination I. Reconstructions of DIC, alkalinity and calcite saturation at the Florida Strait during Termination II produce higher estimates of these parameters compared to previous studies for which coccolith weight estimates are available, and therefore a change of the carbonate system is the most likely cause for the coccolithophore calcification increase during atmospheric CO 2 rise. Our results illustrate that even during rising atmospheric CO 2 the conditions of the seawater carbonate system can be favourable for coccolithophore calcification. The total CaCO 3 production of a coccolithophore assemblage under increasing CO 2 therefore depends on regional seawater carbonate system characteristics and the local assemblage composition.
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