Refining the alkenone-pCO2 method: The role of algal growth conditions

Abstract:

The alkenone-pCO₂ method based on carbon isotope fractionation during growth of haptophyte algae is one of the most widely used approaches to reconstruct atmospheric CO₂ level in the Cenozoic. Based on the fractionation of stable carbon isotopes between dissolved CO₂ and phytoplankton biomass, as represented by alkenone lipid biomarkers, this relationship (known as ε_p37:2) scales inversely with growth rate and cell volume to surface area ratio, and positively with CO₂. Recently-published estimates for late Pleistocene CO₂ levels, however, are poorly correlated with ice core CO₂ records, suggesting that alkenone paleobarometry needs to be refined. Here we compiled published records over recent glacial-interglcial (G-IG) cycles and revised the relationship between algal growth rate, as expressed by the physiological parameter ‘b’, and dissolved phosphate concentration. We further show that the magnitude of change in ε_p37:2 over glacial-interglacial cycles at different sites is dependent on local nutrient conditions, highlighting the importance of constraining b for accurate CO₂ estimates. The correlation between GDGT-2/3 ratio and back-calculated b at Ceara Rise (ODP Site 925) suggests that archaeal lipids could be used as proxies to calibrate b. Application of our variable-b method to reported data yields pCO₂ estimates that are similar in both trends and magnitude to ice core-derived records.