Reconstructing Vegetation and Hydrological Changes from Biomarkers in Modern Soils and Late Neogene Paleosols, Meade Basin, KS, USA

Abstract:

Paleoclimate and paleovegetation proxies can help us understand how C₄ grasslands have expanded worldwide and responded to climate change during the late Neogene. The Miocene-Holocene deposits of the Meade Basin, southwestern Kansas preserve a record of C₄ ecosystem expansion as well as a rich fossil record of small mammals. These records combined provide an excellent opportunity to study the cause(s) of the C₄ ecosystem expansion and the consequences for faunal evolution. As part of an interdisciplinary project, we are investigating the organic geochemical record of vegetation and hydrological change in the Meade Basin. Preliminary findings from three modern soil pits suggest n-alkane δ¹³C values have similar trends to n-alkanoic acid δ¹³C values in each soil profile, but the absolute values of these two different types of plant-wax compounds are offset by up to 6.8‰. This result was unexpected because the absolute δ¹³C values of n-alkanes and n-alkanoic acids are assumed to be very similar in the plants themselves. Moreover, the δ¹³C values of the n-alkanoic acids more accurately reflect the modern vegetation at our sites. Few studies compare carbon isotope data in n-alkanes and n-alkanoic acids and it is important to constrain how these two types of plant-wax compounds reflect vegetation differently. We will use hydrogen isotope measurements on these same plant-wax samples to evaluate how the modern precipitation isotopic ratios are recorded in biomarkers, which in turn will be used to interpret hydrogen isotope data from paleosol biomarkers. Ongoing work will use plant-wax isotopic records from the modern soil profiles as a framework for interpreting the plant-wax isotopic records from Meade Basin paleosol and fluvial sediments to reconstruct the late Neogene C₄ grassland expansion and hydroclimate change in this region.