Exploring the carbon isotopic ratio and chemistry of coccoliths and coccolith associated polysaccharides as a proxy for CO2

Our planet is habitable, in part, due to the carbon dioxide (CO2) that resides in our at- mosphere. Whilst the presence of CO2 ensures that ambient surface temperatures remain above freezing, additional inputs of CO2 drive global temperatures higher. The long-term increase in temperature associated with an increase in the mixing ratio of CO22 in the at- mosphere (pCO2) is, however, not certain. In order to understand how much our planet’s temperature rises given an increase pCO2, we look to the sedimentary record to reconstruct coupled changes in past temperature and pCO2. Yet, reconstructing pCO2 is marred by inaccurate proxies, which often require numerous assumptions and only provide snapshots of past change. Recent work has linked chemical signals recorded in coccolithophores, unicellular phytoplankton, to changes in the CO2 concentration of seawater ([CO2(aq)), which reflects pCO2 in certain ocean settings. Such signals manifest in calcium carbonate plates, know as coccoliths, that coccolithophores produce and their associated polysac- charides, which assist with coccolith calcification. This thesis set out to investigate the use of coccoliths and coccolith associated polysaccharides (CAPs) as a proxy for pCO2. I first invert an existing coccolithophore cellular carbon isotopic flux model to reconstruct pCO2 given novel carbon isotopic data from size separated coccoliths dating to the Eocene period (56-34 million years ago). This work takes advantage of the understanding that the external concentration of CO2 in seawater, in part, controls the carbon isotopic composition of coccolith calcite. I find that pCO2 declined over the Eocene period with a corresponding shift to less calcified coccolithophore species. Second, I recalibrate an existing cellular carbon isotopic flux model with novel isotopic data measured from culture experiments to derive insight into modern day species carbon acquisition strategies. I find that two modern coccolithophore species, Gephyrocapsa Oceanica and Coccolithus pelagicus, have differing responses to CO2(aq) limitation. Whilst G. Oceanica likely upregulates HCO−3 transporters under carbon limitation, Coccolithus pelagicus utilises HCO−3 over all carbonate parameter space. Finally, I investigate the chemical and isotopic response of CAPs trapped within coccoliths from both culture and the sedimentary record to changes in CO2(aq). I find that both the monosaccharide composition and carbon isotopic ratio of CAPs vary over CO2(aq) space. Whilst species specific behaviour is apparent, collective changes observed across species point towards the potential development of a CAP based pCO2 proxy. Taken together the work presented in this thesis lays the foundation for a suite of novel pCO2 proxies that may, with supplementary work, reconstruct pCO2 back to the Triassic period when coccoliths first appeared in the fossil record.