Refining the alkenone-pCO2 method I: Lessons from the Quaternary glacial cycles

Abstract The alkenone- p CO 2 method is one of the most widely used approaches to reconstruct atmospheric CO 2 in the Cenozoic. The method depends upon fractionation of stable carbon isotopes during algal photosynthesis, expressed as e p37:2 , and a physiological scaling parameter, b , that accounts for biological factors such as growth rate, cell size, and membrane permeability. Alkenone-derived CO 2 records for the late Pleistocene, however, are poorly correlated with ice core CO 2 , challenging the classic model that considers most of the CO 2 to be acquired through simple diffusion. In this study, we investigate the nature of the b term and the underlying patterns of the sensitivity of e p37:2 to p CO 2 changes. We generated two new e p37:2 records from the South China Sea (MD01-2392) and tropical Atlantic Ocean (ODP 668B) and compiled other published e p37:2 records over glacial-interglacial cycles. Using the e p37:2 data, ocean temperature estimates, and ice core CO 2 , we were able to back-calculate the corresponding values of b . At all locations, b varies over glacial cycles. The highest values of b correspond to peak interglacial stages, indicating that the phytoplankton growth rate is faster or cell size is smaller during interglacials than during glacial periods. We further show that the range of e p37:2 between glacial and interglacial conditions, Δe p37:2 , scales with growth conditions, consistent with the predictions of the carbon isotope fractionation model based on CO 2 diffusion. In other words, the sensitivity of e p37:2 to p CO 2 changes increases where the modern b values are large, contradicting the recommendations that oligotrophic sites are the best for alkenone-CO 2 applications because of the presumed stability of b . Using the average back-calculated b value for each site, the composite p CO 2 estimates from MD01-2392 and ODP 668B – the two sites with adequate Δe p37:2 sensitivity – show broad agreement with the ice core CO 2 record.