A CONSTRAINED TEST ON THE EFFECT OF ATMOSPHERIC CO2 PARTIAL PRESSURE (pCO2) ON CARBON ISOTOPE FRACTIONATIONS IN C3 PLANTS FROM CENOZOIC FOSSIL LAGERSTÄTTEN

The stable carbon isotope (¹²C and ¹³C) signals of higher plant tissues are determined primarily by plant carbon supply (from carbon source to biochemical reaction sites) and the fractionations of carbon isotopes during biochemical reactions, two processes both influenced by plant types and various climatic/environmental parameters. Stable isotope (IRMS) analyses of exceptionally-preserved coniferous fossils (Glyptostrobus, Larix, Metasequoia, and Pseudolarix) from well-studied Cenozoic fossil lagerstätten from the Arctic suggest that the offset of δ¹³C values between lipid molecules and bulk tissues was maintained in a similar fashion as their modern counterparts despite a marked positive shift (~4‰) of δ¹³C in fossil material. Both morphological (light microscope and SEM observations) and bio-molecular (Py-GC-MS and NMR analyses) evidence suggested that this constant ~4‰ isotopic shift between fossils and their living counterparts was unlikely caused by tissue decay, and constructed models demonstrated that neither the change of δ¹³C in atmospheric CO₂ nor the change of c_i/c_a ratio (the ratio of leaf intercellular and atmospheric partial pressures of CO₂) can satisfactorily explain this positive shift of δ¹³C values in these Cenozoic fossils. Recent studies on the effect of atmospheric CO₂ concentration/partial pressure (pCO₂) on carbon isotope fractionations in C₃ plants shed new lights toward a possible means to reconcile this long standing discrepancy. We test this hypothesis using these evolutionary conservative conifers from well-documented fossil lagerstätten deposited under humid climatic conditions against known environmental, climatic, and atmospheric conditions (including pCO₂) reconstructed using various proxies. We believe that the combination of all these advantages from Cenozoic fossil lagerstätten provides an ideal material for testing the new method to infer the atmospheric CO₂ concentration by using bulk and molecular carbon isotope data from C₃ plants.