Using Biomarkers to Investigate Climate Controls on Late Miocene C4 Grass Expansion in Central Argentina

RYAN KELLIS; COTTON, JENNIFER M.; ETHAN HYLAND; SCOTT HAUSWIRTH; SHELBY LITTLETON; IFFAT AZMI; MARIA SOL RAIGEMBORN; D.E. TINEO; INSEL, NADJA

San Francisco

Congreso; AGU23 meeting; 2023

American Geophysical Union (AGU)

The Late Miocene/Early Pliocene 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 changing seasonal precipitation and wind patterns during this time as possible 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 (leaf waxes) are composed of organic hydrocarbons in long carbon-hydrogen chains known as n-alkanes, and are prevalent in soils wherever vegetation has grown. Using Late Miocene paleosols sampled from the Cacheuta Basin in Mendoza province and numerous sites across the La Pampa province in central Argentina, we measure stable isotopes of n-alkanes to link hydroclimate change to the expansion of C4 grasses. δ13C values will be used to determine C4 vegetation abundance, while δD values reflect changes in the amount, timing, transport, or sources of precipitation. Additionally, 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, and specific PAH concentrations can be used to determine the abundance of gymnosperm vs angiosperm vegetation. Preliminary PAH data for Retene, DMP-y and DMP-x proxies from the La Pampa region samples (~10-5.4 Ma) indicate vegetation dominated by herbaceous angiosperms, suggesting that a grassland ecosystem similar to the modern was in place by the Late Miocene. 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.