HIGH RESOLUTION ISOTOPIC RECORD OF C4 PHOTOSYNTHESIS OF A MID-MIOCENE GRASSLAND

The origination and spread of grasslands is one of the key evolutionary events in the Cenozoic, which is characterized by long-term cooling and drying climatic conditions. One way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols. Paleosols that preserve organic matter in the form of carbonaceous root traces provide direct evidence of the proportion of C₃ to C₄ biomass that grew in the soil, in contrast with pedogenic carbonate δ¹³C values, which may also reflect complicating factors including contamination from atmospheric δ¹³C in low productivity ecosystems. A high temporal and spatial resolution reconstruction of past ecosystems was derived from thirty-five paleosols in a thirty-four meter section of the Sixmile Creek Formation at Timber Hills (MT) that was deposited during the Miocene (10.2-8.9 Ma). Samples were collected vertically and laterally throughout the stratigraphic section and as vertical depth profiles within single paleosols, and were analyzed for δ¹³C of the preserved organic matter. Results show large variation in the abundance of C₄ plants (0-25%) in this ecosystem on a 100ky timescale. Quantitative reconstruction of mean annual temperature (9.2°C) indicates that the fluctuations in C₄ proportion were not climatically driven. Conventional wisdom suggests the global spread of C₄ grasses began 7Ma ago, but these results indicate the presence of a significant proportion of C₄ plants prior to that time and show significant short-term ecological variation prior to the global expansion of grasslands.