Oak leaves as biosensors of late Neogene and early Pleistocene paleoatmospheric CO2 concentrations

Complementary to the interpretation of delta(13)C values of biogenic carbonate and sedimentary organic carbon in marine sediments, paleoatmospheric CO2 levels can be estimated by considering the inverse relationship between atmospheric CO2, concentration and stomatal parameters (frequency, size) on leaves of land plants. In woody plants, the significance of this (species-specific) physiological response to changing CO2 regimes is now repeatedly confirmed, both experimentally and from historical sequences of leaves collected since the onset of industrialization. A corollary of this relationship is that analysis of stomatal parameters on fossil leaves has the potential of determining changes in paleoatmospheric CO2 levels at different time scales, Well-preserved cuticle remains of oak leaves from late Miocene, Pliocene and early Pleistocene sediments of the Lower Rhine Embayment (Germany, The Netherlands) give promise of extending the record of stomatal frequency response to the last 10 Ma. During intervals with warm-temperate,to subtropical climatic conditions, oak leaves are characterized by a high stomatal resistance (or low conductance) to CO2 diffusion and low stomatal frequencies; during cooler intervals we observe an opposite picture. Comparison with historical relations between CO2 concentration and stomatal properties suggests that paleoatmospheric CO2 concentrations were not significantly higher than during the last 200 years and fluctuated several times between 280 and 370 ppmv in covariation with contrasting regional climatic conditions. On a global scale, intervals with reduced CO2 levels match glacial pulses characterized by the occurrence of ice-rafted detritus in high-latitude oceanic sediments.