HOW VARIABLE WERE ATMOSPHERIC CO₂ LEVELS DURING THE LAST DEGLACIATION? EVIDENCE FROM STOMATAL INDEX MEASUREMENTS OF A SWEDISH HIGH-RESOLUTION LEAF RECORD

Data from ice cores suggest that Lateglacial and early Holocene atmospheric CO₂ variations were rather conservative, the most important change being a gradual Younger Dryas increase. By contrast, palaeo-CO₂ records based on the inverse relationship between CO₂ partial pressure and stomatal frequency of terrestrial plant leaves reflect a more dynamic CO₂ evolution. If correct, stomatal-based CO₂ estimates suggest that the global carbon cycle experienced significant reorganization during the last deglaciation.

Here we present a Lateglacial and early Holocene CO₂ record based on stomatal index data from leaves preserved in the sediments of Lake Madtjärn, a small lake in southwestern Sweden. Based on an extensive radiocarbon dating programme in combination with lithostratigraphic and palaeobotanical investigations, it was possible to correlate our CO₂ record to the INTIMATE event stratigraphy.

Three independent CO₂ records constructed from stomatal index data of Salix polaris, Salix herbacea and Betula nana leaves were combined to form a high-resolution reconstruction for the period 12,800-10,800 cal yr BP. Atmospheric CO₂ concentrations were found to have decreased rapidly from c. 260 ppmv to 210-215 ppmv within 200 years during the Allerød (GI-1)-Younger Dryas (GS-1) transition. After 100-200 years, CO₂ concentration started to gradually increase to 270-290 ppmv at the end of the Younger Dryas stadial (GS-1). CO₂ concentrations were relatively stable during the early Holocene, except for a short-lived period of lower (240-250 ppmv) values c. 11,350-11,200 cal yr BP.

The CO₂ record from Lake Madtjärn resembles in many respects the GRIP oxygen isotope record, suggesting that atmospheric CO₂ may have played an important role in climate dynamics during the last deglaciation. Our CO₂ record also partly resembles previous stomatal-based reconstructions, and the overall deglacial trend is almost identical to that seen in ice-core records. The amplitude of change is, however, markedly higher in the Swedish stomatal-based record compared to the ice cores. This difference may partly be accounted for by the inherent smoothing of ice-core CO₂ records caused by diffusion, but a major part of the difference in amplitude between ice-core and stomatal-based CO₂ records still remains to be explained.