| dc.description.abstract | The stable carbon isotope composition (δ¹³C) of single-species dinocysts has been suggested as an alternative to the alkenone-δ¹³C proxy for surface ocean pCO2. However, uncertainties remain regarding the conditions that affect the uptake of inorganic carbon and how to properly account for these in the assumptions underlying a potential dinocyst pCO2 proxy. To reduce these uncertainties, the δ¹³C of two dinocyst species, Nematosphaeropsis labyrinthus and Impagidinium aculeatum, were measured alongside alkenones at several glacial and interglacial maxima throughout the Pleistocene. This period was selected for its availability of atmospheric pCO2 from ice core records and for its variability in atmospheric pCO2, ranging between approximately 170 and 300 ppm. The samples used in this study were collected north of the modern position of the subtropical front (STF), where surface ocean pCO2 in summer is at near-equilibrium with the atmosphere. The δ¹³C measurements were used to calculate the fractionation (εp) of each species, which was compared to surface pCO2 reconstructions based on alkenone δ¹³C from the same samples, and ice core records. In accordance with previous studies, the results indicate higher fractionation with steeper slopes with respect to CO2 for dinocysts than for motile dinoflagellates. However, the present study found an unusually high fractionation for N. labyrinthus of 23.1‰ during one of the interglacials. This anomaly is hypothesised to be a consequence of carbon concentrating mechanisms influenced by environmental conditions and oversimplification of the equations used to calculate fractionation, but cannot be explained fully. Furthermore, fractionation of N. labyrinthus was found to be higher during interglacials than glacials, while I. aculeatum and alkenones portrayed an opposite trend. Together, these findings underline the importance of considering species separately and of further constraining species-specific responses to environmental conditions in order to establish a εp-pCO2 relationship that can be applied as a dinocyst pCO2 proxy for the geological record. | |
