High-resolution isotopic record of C4 photosynthesis in a Miocene grassland

The origin 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 mixing 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, Montana (USA) that was deposited during the Miocene (10.2–8.9 Ma ago). Phytoliths were extracted from paleosol samples to compare vegetation assemblages to inferences based on isotopic compositions, with both proxies giving similar results. Isotopic results from organic matter indicate both a small component of C4 photosynthetic plants locally prior to their regional expansion to dominance in the late Miocene through the early Pleistocene, and large variation in the abundance of C4 plants (0–25%) in this ecosystem both laterally and on a 100 Kyr timescale. In contrast, pedogenic carbonate δ13C values from this site indicate a high proportion of C4 photosynthesis that is at odds with both phytolith and δ13Corg results, suggesting that the carbonate values are biased by diagenesis or diffusion of atmospheric CO2, and that a similar issue may impact previous paleovegetation reconstructions based on pedogenic carbonates. Quantitative reconstructions of mean annual temperature and mean annual precipitation indicate little local variability through time and that the fluctuations in C4 proportion were not climatically driven. Instead, the variable proportion of C4 photosynthesis is best explained by ecosystem-scale variables such as succession, fluvial avulsion, and fire regime.