The importance of accurate and precise temperature reconstruction for alkenone paleobarometry

By measuring the carbon isotope ratio of long chain ketones produced by haptophyte algae (alkenones) the isotope fractionation during photosynthesis (εp) can be determined. Fractionation is dependent on the concentration of carbon dioxide in surface waters, thus, past atmospheric CO2 can be reconstructed. Recent studies have successfully applied this technique at across several intervals of rapid climatic change in the Cenozoic, and efforts have been made to use alkenone paleobarometry to constrain climate (or Earth system) sensitivity. Several factors are important to the accurate and precise determination of atmospheric CO2; for example, recent work has focused on the effect of changes in algal cell size. However, reconstructions are also highly dependent on the accurate and precise determination of sea surface temperature (SST), as this effects both the determination of εp from alkenone δ13C values, and the conversion of [CO2](aq) to atmospheric pCO2. This has become of particular importance given the multitude of proxies now applied to the reconstruction of SST, and uncertainty about exactly what each proxy represents. Here we assess the sensitivity of alkenone pCO2 estimates to inaccurate and/or imprecise SST reconstructions using new high resolution data from the Pliocene, and investigate the possible implications for previously published records. We highlight the importance of these results, especially the revised uncertainties of paleo-pCO2 estimates, to understanding climates of the past and estimating climate sensitivity (or Earth system sensitivity) for models of the future