Abstract
Coccoliths have dominated the sedimentary archive in the pelagic environment since the Jurassic. The biominerals produced by the coccolithophores are ideally placed to infer sea surface temperatures from their oxygen isotopic composition, as calcification in this photosynthetic algal group only occurs in the sunlit surface waters. In the present study, we dissect the isotopic mechanisms contributing to the “vital effect”, which overprints the oceanic temperatures recorded in coccolith calcite. Applying the passive diffusion model of carbon acquisition by the marine phytoplankton widely used in biogeochemical and palaeoceanographic studies, our results suggest that the oxygen isotope offsets from inorganic calcite in fast dividing species Emiliania huxleyi and Gephyrocapsa oceanica originates from the legacy of assimilated 18O-rich CO2 that induces transient isotopic disequilibrium to the internal dissolved inorganic carbon (DIC) pool. The extent to which this intracellular isotopic disequilibrium is recorded in coccolith calcite (1.5 to +3‰ over a 10 to 25°C temperature range) is set by the degree of isotopic re-equilibration between CO2 and water molecules before intracellular mineralisation. We show that the extent of re-equilibration is, in turn, set by temperature through both physiological (dynamics of the utilisation of the DIC pool) and thermodynamic (completeness of the re-equilibration of the relative 18O-rich CO2 influx) processes. At the highest temperature, less ambient aqueous CO2 is present for algal growth, and the consequence of carbon limitation is exacerbation of the oxygen isotope vital effect, obliterating the temperature signal. This culture dataset further demonstrates that the vital effect is variable for a given species/morphotype, and depends on the intricate relationship between the environment and the physiology of biomineralising algae.
Highlights
Understanding how the physiology of marine organisms affects the fractionation of stable isotopes in biogenic carbonates has geological and biogeochemical importance
Reproduction of more accurate and true oceanic conditions in the laboratory needs to be achieved in order to gain a more geologically-relevant picture of the modulation of the vital effect in coccolithophores. This culture study shows that the magnitude of the vital effect affecting the oxygen isotope composition of cultured coccolith calcite is not constant, and is primarily driven by the dynamics of the internal carbon pool from assimilation to mineralisation, which is controlled by temperature
There is, a slight decrease in the magnitude of this vital effect with increasing temperature, as this isotopic disequilibrium is more extensively erased with increasing temperature
Summary
Understanding how the physiology of marine organisms affects the fractionation of stable isotopes in biogenic carbonates has geological and biogeochemical importance. Such a goal would help the development and validation of geological proxies. Previous culture studies have suggested a control of stable isotope composition ( a vital effect) in coccolith calcite by algal growth rate (Ziveri et al, 2003). The aim of the present study is to deepen our knowledge of the mechanism of oxygen isotope fractionation in coccolith calcite by isolating the vital effect component and determining how its magnitude varies with growth rate, cell size, carbon availability and temperature. Carbon isotope data from cultured coccoliths are not presented in the present study, as they will be published in a subsequent contribution
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