LATE QUATERNARY EVOLUTION OF NORTH AMERICAN GRASSLANDS INFERRED FROM STABLE CARBON ISOTOPES OF BURIED SOIL ORGANIC MATTER

Evolution of late Quaternary grasslands in the North American Great Plains is inferred from geographically restricted fossil pollen spectra and low resolution biome models leading to deficiencies in our understanding of vegetation dynamics in the much of the region. In this study we utilize 95 δ¹³C values from buried soil A horizons from across the Great Plains to develop a time series analysis of C4/C3 plant migrations for the last 12 ka (1000 radiocarbon yrs). The δ¹³C database is a literature compilation of fluvial and eolian localities within an area of 47-32 °N latitude and 104-94°W longitude. All samples were analyzed by mass spectrometry after removal of carbonate and roots, reported relative to the PDB standard, and age bracketed by either radiocarbon or optically stimulated luminescence techniques. An assessment of vegetation change was performed after plotting each buried soil δ¹³C value as a deviation from its nearest modern counterpart, thus normalizing a single temporal curve for the entire Great Plains. The modern data were derived from surface δ¹³C values of 61 native prairies within the same geographical area as the buried soils. Five isotopic stages are identified: Stage V (12-10.5 ka) documents a 15 to 40% deficiency in C4 production; Stage IV (10.5-9.6 ka) records a dramatic increase in C4 grasses to near modern conditions; C4 production in Stage III (9.6-6.8 ka) is similar to Stage I, but fluctuates by 15 to 20% with a 500 year periodicity; during Stage II (6.8-4.2 ka) C4 contributions were highest for the Holocene when values were as much as 15% greater than present; Stage I spans the last 4.2 ka and exhibits a ±10% fluctuation in C4 production below (4.2-3.2 ka, 2.2-1.4 ka, after 0.8 ka) and above (intervening times) modern. At the orbital scale, C4 grasses between 12 and 7 ka were responding to increasing solar insolation, but periodically suppressed by glacial meltwater pulses to the Gulf of Mexico and North Atlantic. At the millennial scale, most pronounced after 7 ka, C4 production oscillated in response to variations in solar irradiance with a periodicity of 1 to 2 ka. Our vegetation record captures expected climate characteristics of the Younger Dryas, 8.2 ka cool event, Altithermal, Medieval Warm Period, and Little Ice Age.