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Abstract. Reconstruction of sea surface temperatures from the oxygen isotopecomposition ( δ 18O ) of calcite biominerals synthesised in themesopelagic zone of the oceans requires knowledge of the δ 18O of seawater and constraints on the magnitude ofbiological 18O∕16O fractionation (the so-called vital effect).In the palaeoceanography community, seawater δ 18O andsalinity are unduly treated as a common parameter owing to their strongcovariation both geographically and in the geological register. If the formerparameter has arguably no notable influence on the biogeochemistry of marinecalcifiers, salinity potentially does. However, how salinity per se and theeffect of osmotic adjustment can modulate the biogeochemistry, and in turn,the expression of the vital effect in calcite biomineral such as thecoccoliths remains undocumented. In this culture-based study ofcoccolithophores (Haptophyta) belonging to the Noelaerhabdaceae family, wekept temperature and seawater δ 18O constant, and measuredbasic physiological parameters (growth rate and cell size), and the isotopecomposition ( 18O∕16O and 13C∕12C ) of coccolithsgrown under a range of salinity, between 29 and 39. Ultimately, theoverarching aim of this biogeochemical study is to refine the accuracy ofpalaeotemperature estimates using fossil coccoliths. We found that despitesignificant physiological changes in the coccolithophores, varying salinitydoes not modulate biological fractionation of oxygen isotopes. Thisobservation contrasts with previous in vitro manipulations of temperature andcarbonate chemistry that led to substantial changes in the expression of thevital effect. As such, salinity does not affect temperature estimation fromcoccolith-bearing pelagic sequences deposited during periods of change in icevolume, especially at the highest latitudes, or in coastal regions. Bycontrast, the carbon isotope composition of the coccoliths is influenced by agrowth rate mediated control of salinity with implications for derivingproductivity indices from pelagic carbonate.

The effect of salinity on the biogeochemistry of the coccolithophores with implications for coccolith-based isotopic proxies