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

Abstract

Reconstruction of sea surface temperatures from the oxygen isotope composition (δ18O) of calcite biominerals synthesised in the mesopelagic zone of the oceans requires knowledge of the δ18O of seawater and constraints on the magnitude of biological 18O∕16O fractionation (the so-called vital effect). In the palaeoceanography community, seawater δ18O and salinity are unduly treated as a common parameter owing to their strong covariation both geographically and in the geological register. If the former parameter has arguably no notable influence on the biogeochemistry of marine calcifiers, 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 δ18O constant, and measured basic physiological parameters (growth rate and cell size), and the isotope composition (18O∕16O and 13C∕12C) of coccoliths grown under a range of salinity, between 29 and 39. Ultimately, the overarching aim of this biogeochemical study is to refine the accuracy of palaeotemperature estimates using fossil coccoliths. We found that despite significant physiological changes in the coccolithophores, varying salinity does not modulate biological fractionation of oxygen isotopes. This observation contrasts with previous in vitro manipulations of temperature and carbonate chemistry that led to substantial changes in the expression of the vital effect. As such, salinity does not affect temperature estimation from coccolith-bearing 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.