TEMPORAL AND SPATIAL CARBON ISOTOPE PATTERNS IN LATE QUATERNARY PALEOSOLS OF THE CENTRAL GREAT PLAINS AND PALEOCLIMATIC LINKAGES

Stable carbon isotope signals have been extracted from soils formed in upland loess deposits during the Pleistocene-Holocene (P-H) transition and during the Middle Wisconsin (MW). Buried paleosols, representing these two distinct intervals of landscape stability, have been sampled from western Missouri through Kansas and southern Nebraska into eastern Colorado. The P-H soil, known regionally as the Brady soil (developed in the uppermost Late Wisconsin Peoria loess), is coeval with the Younger Dryas (HO) and exhibits limiting ages of about 10.8 and 9.5ka (c. 12.8 and 11 cal ka). Temporally, the Brady soil displays a marked increase in d¹³C during the course of its 1000-2000 years of development, reflecting the P-H transition from C₃- to C₄-dominated plant communities. Spatially, the eastern limit of C₄-dominated communities migrated northeastward from southwestern Kansas into western Missouri during the period of Brady pedogenesis. Similarly, the MW soil, expressed regionally as the dominant feature of the Gilman Canyon Formation loess unit, is coeval with late MIS 3 (H2, 3 & 4). At most localities, the isotopic signature depicts a notable bimodal expansion (“pulsing”) of C₄-dominated grassland during its 36-20ka (c. 42-24 cal ka) interval of development. Both soil-forming intervals appear to correlate with distinct paleoclimatic events and with proxy data from elsewhere in the central continent. To provide modern analog data, existing surface soils are being sampled from undisturbed upland (loessal) locations in pioneer cemeteries and preserves along the ecocline from western Missouri into eastern Colorado. Modern soils typically exhibit the well-documented isotopic inversion within the upper 20-30 cm, reflecting modern biotic, atmospheric and other influences, whereas below that soil depth, values reflect ¹³C enrichment.