HOW DECAY AND DIAGENESIS INFLUENCE ISOTOPE SIGNALS FROM PLANT FOSSILS?

Both molecular and bulk stable isotope data are being increasingly applied to study plant compression fossils that yield rich evolutionary, taxonomical, physiological, ecological, and environmental information for paleobotanists and organic geochemists. However, how decay and diagenesis influence the fidelity of molecular and stable isotope signals in plant fossils remain elusive. Our laboratory investigates a variety of fossil and archeological plant remains as well as their modern counterparts with various degrees of degradation using integrated morphological (light microscope and scanning electron microscope (SEM), bio-molecular (Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and nuclear magnetic resonance (NMR), and isotopic (Isotope ratio-mass spectrometry (IRMS) approaches. Bulk and in situ molecular carbon isotope analyses of modern and ancient plants indicate that bulk carbon isotope values (δ¹³C) of these samples are closely linked to the degree of tissue degradation, while the δ¹³C and δD values of lipid molecules show little change during the decay process. The initial 1‰ negative shift of bulk δ¹³C in early decay samples is due to the rapid removal of ¹³C enriched liable components such as mono- and di-saccharides, amino acids, hemicelluloses, and pectin. In long-term fossilization, the δ¹³C values of most Cenozoic fossil material exhibit 4‰ to 5‰ positive shift in comparison with their modern counterparts. Morphological (SEM) observations and molecular (Py-GC-MS) analyses show that the positive shift is primarily due to the removal of ¹²C enriched polymer components such as carbohydrates and lignin, as well as the background change in atmospheric CO₂ δ¹³C. Decay experiments, in both laboratory and natural settings, will shed new lights on the process, the mechanism as well as the taphonomic impacts on the applications of morphological, molecular, and isotopic data from plant fossils.