RUI/Collaborative Research: The rise of C4 grasses in South America: Linking grassland transitions to the South American summer monsoon

The expansion of C4 grasses is one of the most dramatic ecological changes in the past 65 million years. Beginning in the late Miocene (~7 million years ago), these tropical and subtropical grasses began to spread and now cover roughly 25% of the Earth's surface. C4 grasses include economically important crops such as corn, sugarcane and sorghum, but the environmental conditions that drove this global expansion are poorly understood. In order to predict how these grasses will respond to future climate change, we must first understand the environmental factors controlling their past distributions. We hypothesize that their expansion was driven by increases in summer precipitation in many regions globally, at approximately the same time. In this work, we plan to investigate whether the major ecological change in South America was driven by the strengthening of the South American Summer Monsoon (SASM), and if this regional climatic change occurred as a result of tectonic or global climatic changes. As the precise timing of the rise of C4 grasses at low elevation sites across South America is poorly understood, we will first precisely constrain the timing of C4 grass expansion at seven different well-dated, low elevation fossil soil deposits in Argentina and Bolivia by producing a multi-proxy paleovegetation reconstruction using stable carbon isotopes and phytoliths. We will then develop a high spatial resolution regional climate model to simulate seasonal paleoprecipitation at different time slices through the late Miocene to Pliocene. This modeling work will allow us to quantify the spatial and temporal variation in regional climate in response to changes in global climate and topography of the Andes. Additionally, we will carry out a multi-proxy paleoprecipitation reconstruction using both soil geochemistry and pedogenic carbonates precipitation proxies to validate modeled precipitation results and independently determine whether precipitation changes occurred concurrently with the rise in abundance of C4 grasses. Finally, we will correlate the paleovegetation reconstruction with both our model and proxy based paleoprecipitation reconstructions to identify climatic and tectonic drivers of the expansion of C4 grasses, and define relationships between the abundance of C4 grasses and climatic variables to produce a predictive map for the spatiotemporal changes in the abundance of C4 grasses across South America.