USING BIOMARKERS TO DETERMINE THE IMPACTS OF FIRE FREQUENCY ON LATE MIOCENE C4 GRASS EXPANSION IN CENTRAL ARGENTINA

The Late Miocene transition from C₃ plants to C₄ grasses was one of Earth’s most dramatic ecological changes in the past ~60 million years. The mechanisms driving the rise in C₄ grasses are poorly understood and presently debated, though recent studies have been increasingly focused on growing season precipitation and wind patterns during this time as contributing factors. Here we examine the potential connection between C₄ expansion and the enhanced South American Summer Monsoon (SASM), using the geochemistry of biomarkers preserved in paleosols in regions influenced by the SASM. Plant lipid remains are composed of organic hydrocarbons known as n-alkanes, and are prevalent in soils wherever vegetation has grown. Another important factor to consider is fire incidence, which could have played a key role in clearing plant biomass in order to make room for a new vegetation regime. Using Late Miocene paleosols sampled from the Cacheuta (12.78-7.2 Ma) and Choromoro (9.81-8.43 Ma) Basins in Mendoza and Tucuman provinces respectively, and numerous sites across the La Pampa (10-5.7 Ma) province in central Argentina, we will use pyrogenic polycyclic aromatic hydrocarbons (PAHs) to reconstruct fire frequency and paleovegetation. PAHs are produced through the incomplete combustion and pyrolysis of organic matter and are preserved in paleosols. Specific PAH concentrations can be used to determine the abundance of gymnosperm vs angiosperm vegetation, as well as indicating whether they are plant-, oil-, or fire-derived. Preliminary PAH paleovegetation proxy data, including Retene, DMP-y and DMP-x, taken from samples covering the above three regions, indicate vegetation dominated by herbaceous angiosperms, suggesting that a grassland ecosystem similar to the modern was in place by the Late Miocene. PAH fire proxy data show a mixture of pyrogenic, petrogenic, and biogenic inputs dominated by petrogenic sources during this time, as well as a pronounced combustion event at ~8 Ma in Cacheuta. We will present further results from these locations for PAH and plant lipid concentrations as well as geochemical and isotope data that will constrain C₃ and C₄ vegetation growth and hydrological change over the Late Miocene. Unearthing the primary drivers for the C₃-C₄ turnover is crucial for responding to impacts on modern ecosystems, such as those due to anthropogenic climate change.