Controls on Alkenone Carbon Isotope Fractionation in the Modern Ocean

Carbon isotope records from alkenone biomarkers (ε p37:2 ) produced by haptophyte algae are frequently used for atmospheric CO 2 paleobarometry, but this method has yielded inconsistent results during periods where CO 2 variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that ε p37:2 indeed records CO 2 information: ε p37:2 increases as aqueous CO 2 concentrations increase relative to carbon demand. However, interpretations of ε p37:2 are complicated by irradiance, where higher irradiance yields higher ε p37:2 . Here we examine the roles of physiology and environment in setting ε p37:2 in the ocean. We compile water‐column and sediment core‐top ε p37:2 data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on ε p37:2 in the modern ocean. We test a culture‐derived model of ε p37:2 and find that the quantitative relationships calibrated in culture experiments can be used to predict ε p37:2 in sediment samples. In water‐column samples, the model substantially overestimates ε p37:2 , largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the “ b ” parameter, is not supported by field data and should not be used to reconstruct past CO 2 changes. Future estimates of CO 2 from ε p37:2 should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.