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

Abstract 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.