Carbon dioxide and coccolithophore physiology in ancient oceans

Coccolithophores form an important and dynamically evolving component of the carbon cycle. These ubiquitous single-celled marine calcifying phytoplankton are re- sponsible for half of the calcium carbonate production in the modern surface ocean, and their adorning calcite plates (coccoliths), produced intracellularly, have con- tributed to sedimentary carbonate for over 200 million years. They constitute a significant control on the partitioning of carbon between the atmosphere, ocean and sedimentary reservoirs on timescales from the instantaneous to the geological. Coc- colithophores are also uniquely placed to record aspects of the carbonate chemistry of the surface ocean, because the carbon isotopic composition of the organic matter (δ13Corg) and calcite (δ13Ccal) that they produce is a function of many parameters, including ambient aqueous carbon dioxide concentration [CO2]. This thesis addresses the bidirectional interaction between coccolithophores and the carbon cycle in the geological past, by asking how cellular carbon fluxes relate to physical evidence that is preserved throughout geological time. First, I present and calibrate a novel rationale for size-normalising coccolith mass, and show that over two glacial-interglacial cycles, coccolithophores appear to calcify more under high [CO2] conditions; a result that is manifest on evolutionary timescales, and is necessarily elusive to experiments. Second, I investigate the parameters controlling δ13Ccal and δ13Corg in coccolithophores through in vivo experimentation, and devel- opment of a model of cellular isotopic fluxes. I show that so called "vital effects" in coccolithophores arise as a result of differences in calcification to photosynthesis ratios. Third, using a combination of novel and established protocols for extraction and isotopic analysis of specific organic molecules from fossils taxonomically separated by size, I show the very first size-specific geologic time series of coccolith-associated δ13Corg, and the first time-series of size-separated coccolith δ13Ccal over a glacial cycle. A novel means of inferring past carbon dioxide concentrations, based on an iterative inverse modelling approach, is presented and tested.