These data were produced within the objectives of the NERC grant (alongside collaborator Gibbs at NOC, Southampton) and predominantly comprise biometric data collected under light microscope at x1500 magnification from the coccolithophore taxon Coccolithus pelagicus, a heavily calcified taxon with a long fossil record. The data was collected as part of a collaborative research effort bringing together the modern and fossil consortia within the UK Ocean Acidification research programme. The data are from batch culture experiments on both modern sub-species of C. pelagicus and provide cell size, coccosphere size, coccolith size and number of coccoliths per cell. The same parameters were measured from C. pelagicus from North Atlantic field and sediment trap samples from inside and outside bloom conditions. Again, the same parameters were also measured from C. pelagicus from exceptionally well-preserved fossil material from several shelf and off-shelf marine locations including New Jersey, Tanzania, California and the Bay of Biscay.

Global warming during the Palaeocene-Eocene Thermal Maximum (PETM, ~56 Ma) is commonly interpreted as being driven by massive destabilization of carbon from surficial sedimentary reservoirs. If correct, this has important implications for the amplification of future fossil fuel emissions via carbon-climate feedbacks. In our study we provided new paired records of boron and carbon isotope changes in the ocean that questions this long-held interpretation. Our data are implemented in an Earth system model to reconstruct the unfolding carbon cycle dynamics across the event. Strong evidence for a larger (>10,000 PgC) and on average isotopically heavier (> -17‰) carbon source leads us to identify volcanism associated with the North Atlantic Igneous Province as the main driver of the PETM. We also find that although organic carbon feedbacks with climate played a more minor role in driving the event than previously thought, organic matter burial was important in ultimately sequestering this carbon and driving the recovery of the system. Data presented in this data set comprise geochemical elemental, as well as boron, carbon and oxygen isotopic data from surface dwelling foraminifera Morozovella Subbotina. Alongside the boron isotopic data we also provide reconstructed surface water pH with corresponding uncertainties for our preferred pH reconstruction.