QUATERNARY STABLE CARBON ISOTOPE SIGNATURES ACROSS THE GREAT PLAINS OF NORTH AMERICA

Stable carbon isotopes (δ¹³C) are a well-established tool for reconstructing past plant communities (C₃ vs C₄) and inferring paleoclimatic conditions for grasslands. Here we present a comprehensive dataset using δ¹³C values from existing literature and from new localities within the Great Plains of North America. We improve on previous studies by compiling a dataset of over 500 δ¹³C values from buried soils in alluvium, loess and dunes, extending back to 11,800 ¹⁴C yr B.P. Spatial interpolation was performed for select time periods to generate a series of interpolation maps that depict spatial changes in plant communities through the Holocene. Time periods were chosen to facilitate comparisons with previous studies.

From the latest Pleistocene through the Younger Dryas (11,800-9700 ¹⁴C yr B.P.), C₄ plant communities notably expand northward and eastward across the Great Plains. The crossover latitude, representing greater than 50% C₄ biomass, expands from approximately 41° to 46° N. The proportion of C₄ biomass remains similar during the early Holocene (9700-6700 ¹⁴C yr B.P.) in the Central Plains. However, C₄ plant communities retract southward in the Northern Plains and westward in the Southern Plains. During the mid-Holocene (6700-4200 ¹⁴C yr B.P.), the spatial extent of C₄ vegetation is similar to the early Holocene, however, the proportion of C₄ biomass in the Central and Southern Plains increases. This pattern persists through the late Holocene until 2100 ¹⁴C yr B.P. when the proportion of C₄ vegetation increases and expands eastward across the Central Plains. From 800 ¹⁴C yr B.P to present, the opposite pattern is observed with a reduction in the proportion of C₄ biomass in the eastern Central Plains.

Some outliers were observed in the interpolation maps that could be indicative of either microbiomes or inadequate sampling density. Also, it is interesting to note that δ¹³C data from 3000-2100 ¹⁴C yr B.P. is scarce. Based on regional studies of geomorphic response, this was a period of aggradation in river systems and, therefore, the scarcity of data likely results from a lack of appropriate buried soils to sample. Overall, the interpolation maps provide an improved snapshot of changes in C₃ and C₄ vegetation over time in the Great Plains that can be used to infer climatic change and assess associated geomorphic response.