| Abstract: |
The relative dominance of C<sub>3</sub> and C<sub>4</sub> grasses in past, present and future grassland ecosystems reflects the complex interplay between climate, atmospheric CO<sub>2</sub>/O<sub>2</sub> ratios, and inter-specific competition. Reconstructed shifts in the proportion of C<sub>3</sub> and C<sub> 4</sub> grasses in the Neogene provide insight into the complex environmental dynamics that have influenced the biogeography of grasses in the past, and thus the likely responses of grasslands and croplands to present and future changes in climate and atmospheric conditions. The biogeography of Neogene grasses also provides a context within which to examine paleoecological and evolutionary changes in grassland faunas. Although several methods have been used to reconstruct different aspects of past grasslands (i.e. vertebrate functional morphology, paleopedology, and tooth enamel and soil carbonate δ<super> 13</super>C), none of these proxies provide a record of C<sub>3</sub> and C<sub>4</sub> grasses specifically. The δ<super>13</super>C of grass phytoliths (microscopic silica bodies produced in abundance by grasses), on the other hand, provide a direct record of grasses, and therefore can both estimate proportions of C<sub>3</sub> and C<sub>4</sub> grasses (versus C<sub> 3</sub>/C<sub>4</sub> of total vegetation) and detect pure C<sub>3</sub> grasslands. The δ<super>13</super>C system in phytoliths from modern plants and soils are characterized and used to interpret the fossil record. The δ<super> 13</super>C signatures of Neogene fossil phytoliths from the Great Plains, USA, indicate a strong trend from pure C<sub>3</sub> grasses at around 8.5 Ma to over 80% C<sub>4</sub> grasses by 3 Ma. These results reveal the presence of mid-latitude pure C<sub>3</sub> grasslands 8.5 Ma, an ecosystem unlike any found today in the Great Plains. Thus, the previously identified shift from a C<sub>3</sub> to C<sub>4</sub> diet recorded in the δ<super> 13</super>C of herbivore tooth enamel at around 7 Ma in North America was a shift from C<sub>3</sub> grasses to C<sub>4</sub> grasses rather than from C<sub>3</sub> shrubs/trees to C<sub>4</sub> grasses. The proportion of C<sub> 4</sub> grass 34 Ma (84%) is much greater than what is seen today in northern Nebraska (50%), suggesting that the early Pliocene warming favored C<sub>4</sub> grasses, and that there was not a simple monotonic increase from pure C<sub>3</sub> grasslands 8.5 Ma to today's mixed grasslands of the central Great Plains.
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