Opening the carbon isotope "vital effect" black box, 2, Quantitative model for interpreting foraminiferal carbon isotope data

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Abstract
Interpretation of carbon isotope records from late Quaternary planktonic foraminifers are confounded due to the presence of a significant physiological component in the carbon isotopic signal. A quantitative carbon isotope (QC) model is presented which relates the carbon isotopic composition of a foraminiferal shell to the physiological processes of respiration and symbiont photosynthesis and to the δ13C value of seawater ΣCO2. The QC model is calibrated with physiological and stable isotopic data from laboratory experiments with living planktonic foraminifers. Model simulations of chamber and shell δ13C values with the symbiont‐bearing foraminifers, Orbulina universa and Globigerinoides sacculifer, suggest (1) variations in symbiont density and photosynthetic rate (light level or habitat depth) are the primary physiological parameters controlling intraspecific carbon isotopic variability in these species, (2) respiration has little effect on the δ13C of O. universa, and (3) each chamber in a multichambered foraminiferal test will have a distinct δ13C value depending on its position in the test whorl. Size:δ13C value relationships reported for G. sacculifer from fossil assemblages can be explained as a function of increasing symbiont density during ontogenetic development.

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