Over the last 10 million years, the Great Plains transitioned to the modern C4 grass dominated ecosystem. Well-preserved late Miocene to Holocene fossils and paleosols make the Meade Basin in southwest Kansas, USA a unique place to determine how paleoenvironmental conditions changed during C4 grassland evolution. δ18O values of paleosol carbonates (δ18Ocarb) in the Meade Basin decreased from the Miocene to Holocene while δ13C values increased; these trends were interpreted as an increase in temperature and/or in aridity coincident with an increase of C4 grass biomass on the landscape. Estimating temperature from δ18Ocarb is complicated, however, by the role of source water δ18O (δ18Owater) values in δ18Ocarb values. Thus, we used carbonate clumped isotope (Δ47) thermometry of paleosol carbonate nodules to develop independent paleotemperature estimates and estimated δ18Owater by combining temperature and δ18Ocarb values.
Preliminary temperature estimates (5-1.8 Ma) in the Meade Basin range from 17°C to 24°C with no systematic change through time, when compared to the modern mean annual (14°C) and warm season (24°C) temperatures. In contrast, δ18Owater values increased through time. We preliminarily suggest that local/regional temperature change was not the primary factor that drove grassland ecosystem evolution in the Meade Basin, while increasing δ18Owater values suggest increased aridity may have been a bigger influence on C4 biomass and faunal changes, although we cannot rule out atmospheric CO2 (pCO2)changes. Additionally, Δ47 temperatures and δ18Owater values may reflect numerous factors besides air temperature and aridity changes, respectively, including depositional environment differences, soil type/depth, and source water changes. Further analyses and detailed organic biomarker records currently underway will help constrain the roles of paleoenvironmental factors in C4 grassland expansion.
Kena Fox-Dobbs, University of Puget Sound
Kathryn E. Snell, University of Colorado, Boulder
Bachelor of Science in Geology
Date of Award
Fetrow, Anne, "Insights into the evolution of the Great Plains grassland ecosystem over the last 5 million years from paleotemperature and paleovegetation records" (2015). Honors Program Theses. 13.