COUPLING BETWEEN ATMOSPHERIC CO2 AND TEMPERATURE DURING THE LAST MILLENNIUM

The Little Ice Age of the Last Millennium is the most recent cool pulse in a series of Holocene climate oscillations. Following the Medieval Climatic Optimum, a marked shift to a more colder time began in the 1200s. The climate was coldest between 1600 AD and 1850 AD. Fluctuations in the pre-industrial CO2 record from Antarctic ice cores suggest a causal link between the Little Ice Age climate deterioration and atmospheric CO2 concentration. However, comparison of ice-derived CO2 data and temperature records is hampered by generally low time-resolution of the CO2 measurements in ice cores. By applying the inverse relation between numbers of leaf stomata and atmospheric CO2 concentrations, high-resolution stomatal frequency analysis of buried tree leaves provides a more accurate method for detecting CO2 changes. To corroborate the concept of a coupling between CO2 and natural climate variability of the Last Millennium, we studied oak (Quercus robur) leaves from sediment cores of an organic-rich oxbow-lake infill of the river Roer (SE Netherlands). This infill consists of 4 meters laminated organic rich clays. 14C dating of the profile suggests the presence of an undisturbed oak leaf record from 1000 AD to 1560 AD. Stomatal frequency analysis of these leaves provide us with a CO2 reconstruction for this period on a decadal resolution. This reconstruction suggests shifts in atmospheric CO2 considerably larger than previously recorded in ice-core records. There is every indication that the reconstructed CO2 trends show good temporal correlation with many proxy records of temperature changes. Ongoing research is focusing upon fine-tuning the time framework for the leaf-based CO2 reconstructions from the Roer site and extending the CO2 curve into the second half of the Last Millennium. Therefore, new sites in Denmark are being incorporated in our research.