The carbon isotope signature of fossil phytoliths: The dynamics of C3 and C4 grasses in the Neogene.

摘要

The relative dominance of C₃ and C₄ grasses in past, present and future grassland ecosystems reflects the complex interplay between climate, atmospheric CO₂/O₂ ratios, and inter-specific competition. Reconstructed shifts in the proportion of C₃ and C ₄ 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 13C), none of these proxies provide a record of C₃ and C₄ grasses specifically. The δ13C 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 C₃ and C₄ grasses (versus C ₃/C₄ of total vegetation) and detect pure C₃ grasslands. The δ13C system in phytoliths from modern plants and soils are characterized and used to interpret the fossil record. The δ 13C signatures of Neogene fossil phytoliths from the Great Plains, USA, indicate a strong trend from pure C₃ grasses at around 8.5 Ma to over 80% C₄ grasses by 3 Ma. These results reveal the presence of mid-latitude pure C₃ grasslands 8.5 Ma, an ecosystem unlike any found today in the Great Plains. Thus, the previously identified shift from a C₃ to C₄ diet recorded in the δ 13C of herbivore tooth enamel at around 7 Ma in North America was a shift from C₃ grasses to C₄ grasses rather than from C₃ shrubs/trees to C₄ grasses. The proportion of C ₄ grass 3–4 Ma (84%) is much greater than what is seen today in northern Nebraska (50%), suggesting that the early Pliocene warming favored C₄ grasses, and that there was not a simple monotonic increase from pure C₃ grasslands 8.5 Ma to today's mixed grasslands of the central Great Plains.