Using carbon isotopes in paleosols to constrain the Late Miocene expansion of C4 grasses in Tucumán and La Rioja provinces, Northwestern Argentina

Abstract

Grasses cover a significant amount of the non-glaciated land on Earth’s surface. C4 grasses evolved and rapidly spread during the Neogene Period, but when and why they spread is widely debated, particularly in South America. In this study, we investigate δ13C of paleosol organic carbon to better understand the expansion of C4 grasses and expand upon previous studies on C4 grasses in South America. We hypothesize that the spread of C4 grasses can be attributed to increased precipitation and seasonality from the strengthening of seasonal monsoons across South America (SASM). Paleosols were collected from two sites in northwestern Argentina, within the present day region influenced by the SASM. The Late Miocene India Muerta Formation, located in Tucumán province, preserves paleosols within fluvially derived coarse to fine sandstones and siltstones, and is dated to 9.8 to 3.7 Ma. The Salicas Formation, located in La Rioja province, is an important fossiliferous mammal locality that preserves paleosols within fluvial fine sands and is dated to approximately 7 Ma. Preliminary data from the India Muerta samples between 9.8 and 8.3 million years ago shows an expansion in C4 grasses with δ13C values of up to -18.5‰, indicating vegetation composed of approximately 35% C4 grasses based on an isotope mixing model. Estimates appear to return primarily to C3 vegetation after 8 Ma, with δ13C values mostly ranging from -25 to -23‰. These preliminary δ13C results are in agreement with phytolith data from this site and others that show Late Miocene increases in C4 vegetation, though C4 grasses never expanded to comprise a majority of the vegetation at any site. We will also present additional δ13C data from the Salicas Formation, which will help us to better understand the timing and regional extent of C4 grass expansion in South America. Furthermore, this data will be paired with local paleoclimate reconstructions showing the evolution of mean annual (MAP) and growing season (GSP) precipitation through the Late Miocene, in order to elucidate potential drivers of this observed ecosystem transition.