The Neogene Record of Grass Photosynthetic Pathway from the Carbon Isotope Signature of Fossil Phytoliths

Francesca Smith

Department of Geophysical Sciences, University of Chicago, and INSTAAR, University of Colorado, Boulder

Phytoliths (microscopic silica bodies produced in abundance by grasses) have the potential to tell us a great deal about the relationship between past grassland ecosystems and climate and atmospheric CO₂. The carbon isotope ratio of organic matter encased in phytoliths retains a record of the photosynthetic pathway of the grass that produced them. To decipher this record in the ancient, however, we need to look first at the modern system. The research presented here finds that although the carbon isotope scale in phytoliths extracted from fresh grasses is both compressed and depleted relative to the whole plant, the carbon isotope scale for phytoliths extracted from soils is neither. Pedogenesis appears to remove the components (lipids) that cause this isotopic depletion and compression. Turning then to the fossil record of phytoliths from Neogene (Miocene and Pliocene) paleosols from the Great Plains, we see the transition from C₃ to C₄ grasses at about the same time as the C₃ to C₄ shift detected in other proxy records (7 Ma) (Cerling et al, 1997). The significance of finding this transition from C₃ to C₄ within phytoliths is that this is the first direct evidence that this transition was not from C₃ shrubs and trees to C₄ grasses, but rather from C₃ grasses to C₄ grasses in the Great Plains.