| Summary: | This study investigates the biological and morphological factors controlling carbon and oxygen stable isotope fractionation in foraminifera (δ13C and δ18O) and uses these findings to address two major questions of paleoclimate—the effect of warm climates on organic carbon cycling and the strength of polar amplification through geologic time. Using a chemical model of the foraminifera’s diffusive boundary layer, it is shown that metabolism and the physical arrangement of algal symbionts on the spines and rhizopodia of planktonic foraminifera can significantly affect the δ13C signal recorded in their shells. This finding implies that vertical gradients of δ13CDIC in the Eocene were likely less different from modern profiles than previously proposed, meaning that the temperature-dependence of organic carbon cycling may be a weaker climate feedback than previously proposed. This theoretical investigation of spines and rhizopodia is supplemented with in situ observations using the autonomous imaging platform Zooglider, which demonstrate that spines increase the effective prey encounter volume of spinose foraminifera by two to three orders of magnitude, opening new predatory ecological niches. Novel observations are presented of exceptionally large hastigerinid foraminifera with a total prey encounter volume close to 40 cm3 (about the size of a golf ball), as well as newly observed features of the hastigerinid bubble capsule. Expanding the boundary-layer model described above with a new model of carbon isotope behavior in the cell interior provides for the first time an explanation of the “carbonate ion effect” on δ13C, in which δ13C covaries strongly with seawater pH or [CO32-]. It is shown how changes in pH alter the dynamics of CO2 diffusing into or out of the calcifying fluid, fractionating 13C and creating the carbonate ion effect. This process implies a linkage between photosymbiosis and the foraminifera’s internal pH and can also explain why δ13C is abnormally low in small foraminifera, an effect previously ...
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