STABLE C ISOTOPES IN BIGNELL LOESS AND SOILS AS A PROXY INDICATOR OF ENVIRONMENTAL AND CLIMATIC CHANGE ON THE CENTRAL GREAT PLAINS

Bignell Loess of the central Great Plains accumulated episodically throughout the Holocene in response to regional drought that destabilized dune fields, which were the immediate source of dust. Stratigraphic sections proximal to dune fields are often several meters thick and typically contain four buried soils within the loess unit and overlying the Brady Soil and Peoria Loess. To further test the environmental and climatic significance of Bignell Loess we analyzed stable C isotopes of organic matter in loess and buried soils at two high-resolution proximal sections and one swale fill in an adjacent dune field.

Wauneta Road Cut in southwest Nebraska contains 6 m of Bignell Loess with 4 buried soils. Twelve OSL ages in Bignell Loess range from 10,231 ± 622 yrs to 108 ± 33 yrs, indicating sedimentation throughout the Holocene. Samples were collected throughout the entire section, including the Peoria Loess, Brady Soil, Bignell Loess, and buried soils. Delta ¹³C values in Peoria Loess and the Brady Soil are <-17 ‰, with a sample from the Brady Bk at -21 ‰. The upper Brady Soil A horizon and 8 of 9 samples throughout the Bignell Loess have d¹³C values >-16 ‰. We interpret the environmental record to indicate that vegetation with a C4 photosynthetic pathway became dominant at Wauneta during the earliest Holocene.

The Lewis Site is a 1.3 m thick organic-rich swale fill buried by dune sand ~2 km from Wauneta. The basal 20 cm and middle portions have d¹³C values of –18 to –19, while the upper 20 cm is –14.7, indicating a shift from a mixed C3-C4 vegetation community to a warm season C4 community prior to burial by active dune sand before 10,105 ± 450 yrs (OSL).

Logan Road Cut in central Nebraska contains 3.3 m of Bignell Loess with 4 buried soils similar to Wauneta. In contrast to Wauneta, however, the d¹³C values at Logan are dominantly in the range between –17 to –20 ‰, which are interpreted as indicating a mixed C3 and C4 vegetation community. Exceptions include the Peoria Loess and modern surface, which are <-21 ‰, indicating a strong C3 input. We interpret the C isotopes to indicate a mixed C3-C4 vegetation community for most of the Holocene. Humus enrichment of ¹³C was regionally synchronous beginning during the late Pleistocene-Holocene transition, indicating a shift to warmer temperatures and increased dominance of warm season vegetation.