PHYTOLITH CARBON ISOTOPE RECORDS OF NEOGENE GRASSES

Although a variety of proxies have been used to characterize past grasslands (i.e. paleobotanical records, vertebrate functional morphology, paleosol characteristics, herbivore tooth enamel d¹³C, and soil carbonate d¹³C), none captures a record of the photosynthetic pathway (C₃ or C₄) of grasses specifically, as distinct from overall vegetation. Carbon isotope ratios of fossil grass phytoliths (silica bodies produced primarily by grasses), on the other hand, provide a direct record of grass photosynthetic pathway and therefore can detect proportions of C₃ and C₄ grasses.

Phytoliths from modern C₃ and C₄ grasses exhibit a compressed and depleted carbon isotope scale relative to that of the whole plant tissue. Analysis of modern grass phytoliths using TMAH thermochemolysis reveals that they contain lipids (and potentially cellulose). The isotopic compression and depletion of modern grass phytoliths can be explained, therefore, by the differential fractionation in lipids from C₃ and C₄ plants (Collister et al., 1994). Phytoliths from modern soils do not exhibit this same compression and depletion. Instead, pedogenesis appears to remove the lipid fraction, leaving more refractory (potentially cellulosic) compounds with carbon isotope ratios similar to whole plant values. Fossil phytoliths from paleosols, therefore, can be interpreted relative to the carbon isotope ratios of grass tissue.

In contrast to the tooth enamel d¹³C records, which indicate no C₄ vegetation prior to about 7 Ma in North America, Phytoliths from Neogene paleosols of the Great Plains show that C₄ grasses represented a significant proportion of the grass community (50%) as early as 12 Ma. Following this early occurrence of C₄, the phytolith carbon isotope record is highly variable between 12 and 3 Ma, suggesting large shifts in grassland ecosystems during this interval. These shifts in C₃/C₄ composition of Neogene grasslands provide a context for examining future changes in croplands and grasslands in response to anthropogenic climatic and atmospheric changes.