Last interglacial atmospheric CO2 changes from stomatal index data and their relation to climate variations

A high-resolution atmospheric CO2 reconstruction based on stomatal index data obtained from Betula and Quercus leaf fragments extracted from the Danish Hollerup lake sediment section provides a unique insight into last interglacial CO2 dynamics. According to pollen stratigraphic correlations the CO2 record covers the first c. 7400 years of the Eemian, as palaeobotanically defined in northwestern Europe. The first c. 3000 years of the reconstruction are characterised by centennial to millennial CO2 variability in the interval 250-290 ppmv, while the remaining part of the record is generally more stable with slightly higher values (290-300 ppmv). According to pollen stratigraphic correlations this shift in CO2 dynamics is coincident with the end of the early Eemian climatic optimum in northwestern Europe. Pollen data from this region indicate that early Eemian CO2 instability may be linked to vegetation succession following deglaciation in Europe, but vegetation dynamics on other northern continents were probably also important. In addition, palaeoceanographic records from the Nordic seas indicate an influence of oceanic processes on the reconstructed Eemian CO2 evolution. A 300-year period of rapid CO2 oscillations immediately before the establishment of stable conditions is synchronous with a dry and cool event previously inferred from proxy data from the same sediment sequence, suggesting that this was a climatic event of regional or global significance. The presented CO2 reconstruction is in general agreement with previous ice core and stomatal-based CO2 data, although a larger variability compared with Vostok ice core data is evident. This may be explained partly by the different resolution of the two records and the inherent smoothing of ice core gas records