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Reconstruction of sea surface temperatures from the oxygen isotope composition (δ 18 O) of calcite biominerals synthesised in the mesopelagic zone of the oceans requires knowledge of the δ 18 O of seawater and constraints on the magnitude of biological 18 O/ 16 O fractionation (the so-called vital effect). In the palaeoceanography community, seawater δ 18 O and salinity are unduly treated as a common parameter owing to their strong co-variation both geographically and in the geological register. If the former parameter has arguably no notable influence on the biogeochemistry of marine calcifiers, 10 salinity potentially does. However how salinity per se and the effect of osmotic adjustment can modulate the biogeochemistry, and in turn, the expression of the vital effect in calcite biomineral such as the coccoliths remains undocumented. In this culture-based study of coccolithophores (Haptophyta) belonging to the Noelaerhabdaceae family, we kept temperature and seawater δ 18 O constant, and measured basic physiological parameters (growth rate and cell size), and the isotope composition ( 18 O/ 16 O and 13 C/ 12 C) of coccoliths grown under a range of salinity comprised between 29% and 1539%. The overarching aim of this biogeochemical work has a geological finality and aims to refine the accuracy of palaeotemperature estimates using fossil coccoliths. We found that, although entailing large physiological changes in coccolithophores, salinity does not modulate biological fractionation in the oxygen isotope system. This is a contrasting observation with previous in vitro manipulations of temperature and carbonate chemistry that led to substantial changes in expression of the vital effect. As such, salinity is not a complicating factor to derive temperatures from coccolith-bearing 20 pelagic sequences deposited during periods of change in ice volume, especially at the highest latitudes, or in coastal regions.By contrast, the carbon isotope composition of the coccoliths is influenced by a growth rate-mediated control of salinity with implications for deriving productivity indices from pelagic carbonate.

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