C discrimination by fossil leaves during the late-glacial climate oscillation 12-10 ka BP: measurements and physiological controls.

The late-glacial climatic oscillation, 12-10 ka BP, is characterised in ice core oxygen isotope profiles by a rapid and abrupt return to glacial climate. Recent work has shown that associated with this cooling was a drop in atmospheric CO concentration of ca. 50 ppm. In this paper, the impact of these environmental changes on C discrimination is reported, based on measurements made on a continuous sequence of fossil Salix herbacea leaves from a single site. The plant responses were interpreted using an integrated model of stomatal conductance, CO assimilation and intercellular CO concentration, influenced by external environmental factors. According to the model, temperature exerts a marked influence on C discrimination by leaves and the pattern of C changes recorded by the fossil leaves is consistent with other palaeotemperature curves for 12-10 ka BP, particularly the deuterium isotope record from Alaskan Salix woods, which generally reflects ocean temperatures. The gas exchange model correctly accounts for these changes and so permits the reconstruction of ancient rates of leaf CO uptake and loss of water vapour in response to the abrupt late-glacial changes in global climate and CO. The approach provides the required physiological underpinning for extracting quantitative estimates of past temperatures and for contributing an ecophysiological explanation for changes in C discrimination in the fossil record.