The rise of C4 grasses in South America: Linking grassland transition to the South American summer monsoon

Abstract

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 this study, we aim to determine the drivers of C4 grass expansion in South America. We hypothesize that this expansion was driven by the strengthening of the South American Summer Monsoon (SASM), which occurred as a result of regional climatic change due to local tectonic and global climatic changes. Using carbon isotopes of bulk organic matter preserved in paleosols, we reconstruct the abundance of C4 grasses across 7 sites in Argentina to assess the timing of this ecological transition in South America. We pair this C4 grass reconstruction with phytolith and biomarker analysis to gain a more detailed perspective of vegetation and fire history, and we use soil geochemistry proxies to assess changes in hydroclimate across the region from the late Miocene-Pliocene. We find that in the absence of temperature and atmospheric CO2 changes, hydrologic change is likely the driver of C4 grass abundances in the Miocene. In far northwest Argentina, our data suggests that increased seasonality brought about by the intensification of the South American Monsoon drove the expansion of C4 grasses, at least in the more humid regions. These results are supported by Community Earth System Model simulations that suggest an increase in summer precipitation along the eastern foothills of the Andes in Northern Argentina between 8 and 3Ma. Our data does not show a correlation between fire frequency and C4 grass abundance in Argentina, unlike in the Siwaliks of South Asia. We also note that we only find moderate to high abundances of C4 grasses in areas where proxy-based precipitation estimates exceed ~500mm/yr. We also do not observe C4 grasses at sites with modern elevations higher than ~3,000 feet or behind rain shadows, showing that tectonically-driven climate changes are important controls on the distribution and spread of C4 grasses. This work will allow us to better predict changes to modern grasslands in the future.