Rapid late-glacial atmospheric CO2 changes reconstructed from the stomatal density record of fossil leaves

The Younger Dryas stadial (11 000-10 000 yr BP) was an abrupt return to a glacial climate during the termination of the last glaciation. We have reconstructed atmospheric CO2 concentrations from a high-resolution sequence of fossil Salix herbacea leaves through this climatic oscillation from Krakenes, western Norway, using the relationship between leaf stomatal density and atmospheric CO2 concentration. High Allerod CO2 values (median 273 ppmv) decreased rapidly during 130-200 C-14-years of the late Allerod to ca. 210 ppmv at the start of the Younger Dryas. They then increased steadily through the Younger Dryas, reaching typical interglacial values once more (ca. 275 ppmv) in the Holocene. The rapid late Allerod decrease in CO2 concentration preceded the Younger Dryas temperature drop, possibly by several decades. This striking pattern of changes has not so far been recorded unambiguously in temporally coarse measurements of atmospheric CO2 from ice cores. Our observed late-glacial CO2 changes have implications for global modelling of the ocean-atmosphere-biosphere over the last glacial-interglacial transition.