LATE MIOCENE EXPANSION OF GRASSLANDSIN NW ARGENTINA LINKED TO SHIFTING HYDROCLIMATE

IFFAT AZMI; ETHAN HYLAND; COTTON JENNIFER; ADIT GHOSH; RAIGEMBORN MARIA SOL; DAVID TINEO; INSEL NADJA

PORTLAND

Congreso; gsa cOONECTS 2021; 2021

GSA

The Late Miocene marks the global expansion of Cgrasslands, with several factors beingsuggested as potential drivers in different regions. In South America, Cexpansion has beenlinked to the rise of the Andes and the development of the Summer Monsoon (SASM);however, spatio-temporal variability of Cabundance in different regions indicates thecomplexity of the event. This study aims to identify the effects of SASM on the distributionand abundance of Cgrasses in NW Argentina through proxy-based reconstruction of LateMiocene paleoclimate and paleovegetation. It presents a multi-proxy paleoclimate andpaleovegetation reconstruction through bulk paleosol geochemistry, organic carbon isotopesof soil organic matter, and phytoliths preserved in paleosols at two different sites in southernSalta Province. The first site contains the Palo Pintado Formation (7.24?5.96 Ma) depositedin an intermontane setting, and the second site (La Viña) contains the Jesus Maria,Guanaco, and Piquete Formations (17.01?4.08 Ma). Bulk geochemical analyses of paleosolB-horizons resulted in average mean annual precipitation (MAP) and mean annualtemperature (MAT) values from Palo Pintado of ~900 mm/yr and 10C, with a slightdecrease in precipitation after 6.3 Ma to ~800 mm/yr. From La Viña, average MAP and MATwere estimated to be ~800 mm/yr and 10C with a gradual decrease in precipitation to aminimum of ~300 mm/yr by 8.5 Ma. This study also uses both a δCmixing model andphytolith assemblages for paleovegetation reconstructions. We find minimal Cvegetation inthe Palo Pintado Formation while a substantial increase of Cgrasses is detected in La Viñabeginning around 8 Ma. We identified 26 different phytolith morphotypes and a >5? carbonisotope excursion within the studied intervals indicating a progressive increase in Cvegetation. Paleoclimatic conditions (including preliminary estimates of growing seasonprecipitation) coupled with this paleovegetation reconstruction suggests a significantrelationship between the increase in Cgrasslands and changing/variable hydroclimate inthe region. The onset of this Late Miocene aridification and shifts coincide with modeleduplift in the region and illustrates the complicated interplay between tectonics, climate, andecosystems in South America.