PP23B-2298
New Iinsights Iinto Great Plains C4 Grassland Evolution and Paleoenvironmental Change From Paleosol Sedimentary Organic Matter d13C Records Over the Past 5 Myr
Tuesday, 15 December 2015
Poster Hall (Moscone South)
Kyle L Chambers, University of Puget Sound, Department of Geology, Tacoma, WA, United States, Kena Fox-Dobbs, University of Puget Sound, Tacoma, WA, United States, David L. Fox, University of Minnesota Twin Cities, Minneapolis, MN, United States, Andrew William Haveles, University of Minnesota, Minneapolis, MN, United States, Kathryn E Snell, University of Colorado at Boulder, Boulder, CO, United States, Kevin T Uno, Lamont -Doherty Earth Observatory, Palisades, NY, United States, Pratigya J Polissar, Lamont-Doherty Earth Observato, Nyack, NY, United States and Robert Martin, Murray State University, Murray, KY, United States
Abstract:
The Meade Basin (MB) of Southwestern Kansas, USA, contains abundant paleosols and mammalian fossil deposits that span the past 5 Myr. Geochemical records derived from paleosols provide insights into paleoenvironmental conditions in MB during the evolution of the Great Plains C4 grassland ecosystem. We measured carbon isotopes in pedogenic carbonates, plant waxes, and bulk sedimentary organic matter (OM) from the same stratigraphic level to directly compare the paleovegetation signal recorded in each proxy; to further understand carbon isotope systematics; and to estimate the relative proportions of C3 plant versus C4 grass biomass. Carbon isotope (δ13C) records were derived from OM preserved in the paleosol matrix and occluded in large carbonate nodules, and used to estimate %C3 plant versus %C4 grass biomass on the landscape. Carbonate δ13C records show a steady increase in C4 grass dominance in MB from <10% C4 biomass in the Miocene to near modern (~80%) levels by the mid Pleistocene. Leaf wax %C4 estimates were more variable, and also generally higher than the carbonate estimates. Our δ13C records of OM occluded in carbonate nodules are highly variable; much more so than the carbonate record generated from the same nodules, and the OM record does not show a clear increase in C4 grass dominance over time. We are able to rule out incomplete removal of carbonate as the source of high variability in OM δ13C values. A potential explanation is that OM occluded in nodules provides a spatial and temporal “snapshot” of aboveground biomass, while nodule carbonate reflects an integrated signal of paleovegetation. When combined, these proxies yield a more comprehensive landscape reconstruction. Specifically, the OM dataset gives insight into changes in paleovegetation heterogeneity over time. Our new understanding of the paleovegetation history in MB is being paired with paleoclimate records such as MAP (from elemental and magnetic proxies) and temperature (from clumped isotope paleothermometry) estimates for an integrated view of ecosystem evolution.
