Alkenone-derived estimates of Cretaceous pCO (sub 2)

Alkenones are long-chain ketones produced by phytoplankton of the order Isochrysidales. They are widely used in reconstructing past sea surface temperatures, benefiting from their ubiquitous occurrence in the Cenozoic ocean. Carbon isotope fractionation (epsilon (sub p) ) between alkenones and dissolved inorganic carbon may also be used as a proxy for past atmospheric pCO (sub 2) and has provided continuous pCO (sub 2) estimates back to ca. 45 Ma. Here, an extended occurrence of alkenones from ca. 130 Ma is reported. We characterize the molecular structure and distribution of these Mesozoic alkenones and evaluate their potential phylogenetic relationship with Cenozoic alkenones. Using delta (super 13) C values of the C (sub 37) methyl alkenone (C (sub 37:2) Me), the first alkenone-based pCO (sub 2) estimates for the Mesozoic are derived. These estimates suggest elevated pCO (sub 2) with a range of 548-4090 ppm (908 ppm median) during the super-greenhouse climate of the Early Cretaceous, in agreement with phytane-based pCO (sub 2) reconstructions. Finally, insights into the identity of the Cretaceous coccolithophores that possibly synthesized alkenones are also offered.