Carbon Isotopic Fractionation of Alkenones and Gephyrocapsa Coccoliths Over the Late Quaternary (Marine Isotope Stages 12-9) Glacial-Interglacial Cycles at the Western Tropical Atlantic

The sensitivity of coccolithophores to changing CO2 and its role modulating cellular photosynthetic carbon isotopic fractionation (epsilon(p)) is crucial to understand the future adaptation of these organisms to higher CO2 world and to assess the reliability of epsilon(p) for past CO2 estimation. Here, we present epsilon(p) measured on natural fossil samples across the glacial-interglacial (G-I) CO2 variations of marine isotope stages 12 to 9 interval (454-334 ka) at the western tropical Atlantic Ocean Drilling Program Site 925 together with a set of organic and inorganic geochemical, micropaleontological and morphometrical data from Gephyrocapsa coccoliths in the same samples. The similar to 2 parts per thousand variation in epsilon(p) is significantly correlated with the CO2[aq] concentrations calculated from assumption of air-sea equilibrium with measured ice core pCO(2) concentrations. The sensitivity of epsilon(p) to CO2[aq] is similar to that derived from a multiple regression model of culture observations and is not well simulated with the classical purely diffusive model of algal CO2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non-CO2 effects on epsilon(p) at this location, via either direct size effect or growth rate correlated to cell size. Primary productivity, potentially triggered by shifting growth rates and light levels, may also affect epsilon(p.) Proposed productivity proxies % Florisphaera profunda and the ratio between the C-37 to C-38.et alkenone (C37/C38.et ratio) both correlates modestly with the non-CO2 effects on epsilon(p). When the observed G-I epsilon(p) to CO2 sensitivity at this site is used to estimate pCO(2) from epsilon(p) since the Miocene, the inferred pCO(2) declines are larger in amplitude compared to that calculated from a theoretical epsilon(p) diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric-separated Gephyrocapsa coccoliths (respectively Delta delta O-18(Gephyrocapsa-Trilobatus sacculifer) and epsilon(coccolith)) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.

Automated summary

Understanding the sensitivity of coccolithophores to changing CO2 and its impact on cellular photosynthetic carbon isotopic fractionation is crucial for predicting future adaptations and past CO2 estimates. Measurement of epsilon(p) across glacial-interglacial CO2 variations showed significant correlation with CO2[aq] concentrations. The effects of non-CO2 factors on epsilon(p) cannot be fully explained by variations in cell size of Gephyrocapsa.