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 CO 2 and its role modulating cellular photosynthetic carbon isotopic fractionation (ε p ) is crucial to understand the future adaptation of these organisms to higher CO 2 world and to assess the reliability of ε p for past CO 2 estimation. Here, we present ε p measured on natural fossil samples across the glacial‐interglacial (G‐I) CO 2 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 ∼2‰ variation in ε p is significantly correlated with the CO 2 [aq] concentrations calculated from assumption of air‐sea equilibrium with measured ice core p CO 2 concentrations. The sensitivity of ε p to CO 2 [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 CO 2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non‐CO 2 effects on ε 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 ε 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‐CO 2 effects on ε p . When the observed G‐I ε p to CO 2 sensitivity at this site is used to estimate p CO 2 from ε p since the Miocene, the inferred p CO 2 declines are larger in amplitude compared to that calculated from a theoretical ε p diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric‐separated Gephyrocapsa coccoliths (respectively Δδ 18 O Gephyrocapsa–Trilobatus sacculifer and ε coccolith ) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.