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  1. Free, publicly-accessible full text available February 1, 2024
  2. Abstract

    Carbon isotope records from alkenone biomarkers (εp37:2) produced by haptophyte algae are frequently used for atmospheric CO2paleobarometry, but this method has yielded inconsistent results during periods where CO2variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that εp37:2indeed records CO2information: εp37:2increases as aqueous CO2concentrations increase relative to carbon demand. However, interpretations of εp37:2are complicated by irradiance, where higher irradiance yields higher εp37:2. Here we examine the roles of physiology and environment in setting εp37:2in the ocean. We compile water‐column and sediment core‐top εp37:2data 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 εp37:2in the modern ocean. We test a culture‐derived model of εp37:2and find that the quantitative relationships calibrated in culture experiments can be used to predict εp37:2in sediment samples. In water‐column samples, the model substantially overestimates εp37:2, 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 CO2changes. Future estimates of CO2from εp37:2should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations.

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