OAK LEAVES AS BIOSENSORS OF LATE NEOGENE AND EARLY PLEISTOCENE PALEOATMOSPHERIC CO2 CONCENTRATIONS

Complementary to the interpretation of delta(13)C values of biogenic c arbonate and sedimentary organic carbon in marine sediments, paleoatmo spheric CO2 levels can be estimated by considering the inverse relatio nship between atmospheric CO2, concentration and stomatal parameters ( frequency, size) on leaves of land plants. In woody plants, the signif icance of this (species-specific) physiological response to changing C O2 regimes is now repeatedly confirmed, both experimentally and from h istorical sequences of leaves collected since the onset of industriali zation. A corollary of this relationship is that analysis of stomatal parameters on fossil leaves has the potential of determining changes i n paleoatmospheric CO2 levels at different time scales, Well-preserved cuticle remains of oak leaves from late Miocene, Pliocene and early P leistocene sediments of the Lower Rhine Embayment (Germany, The Nether lands) give promise of extending the record of stomatal frequency resp onse to the last 10 Ma. During intervals with warm-temperate,to subtro pical climatic conditions, oak leaves are characterized by a high stom atal 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 sto matal properties suggests that paleoatmospheric CO2 concentrations wer e not significantly higher than during the last 200 years and fluctuat ed several times between 280 and 370 ppmv in covariation with contrast ing regional climatic conditions. On a global scale, intervals with re duced CO2 levels match glacial pulses characterized by the occurrence of ice-rafted detritus in high-latitude oceanic sediments.