USING FACTORS OF SOIL FORMATION TO INVESTIGATE STABLE CARBON ISOTOPE DISEQUILIBRIUM IN LATE PLEISTOCENE (MIS 3) BURIED SOILS OF THE CENTRAL GREAT PLAINS, USA

Buried soils serve as important archives of environmental change and landscape history. In particular, the stable carbon isotope (δ¹³C) compositions of preserved organic matter and pedogenic carbonates in buried soils have proved to be important proxies for reconstructing late Quaternary ecological and climatic change. In this study, we investigate a series of buried soils from the High Plains of western Kansas. Three cores were obtained that contain three superposed loess units: the Gilman Canyon Formation, Peoria Loess, and Bignell Loess. All cores contained buried soils that date to ca. 28-48 ka (Marine Isotope Stage [MIS] 3) and stratigraphically correlate to the Gilman Canyon Formation. These buried soils have Ak-Bk horizonation and pedogenic carbonates consists of small (<10 mm) nodules and filaments (Stage I-II). δ¹³C analyses were performed on bulk pedogenic carbonates (δ¹³C_carb) and co-occurring soil organic matter (δ¹³C_SOM) to reconstruct late-Quaternary bioclimatic change in the region.

Results indicate a unique decoupling between δ¹³C_carb and δ¹³C_SOM in the buried soils. Theoretically, δ¹³C_carb values are enriched by 14-17‰ relative to δ¹³C_SOM. However, Δ¹³C values (δ¹³C_{carb – SOM}) are 2 to 7 per mil higher than expected equilibrium values in the Bk horizons. In contrast, Δ¹³C values are 1 to 5 per mil lower than expected equilibrium values in the Ak horizons. Considering the different soil-forming factors allows for the establishment of multiple working hypotheses regarding the influence of climate, vegetation, parent material, and time on the observed isotopic disequilibrium. In particular, we consider the effect of temperature, aridity, seasonality, and pCO₂ levels during MIS 3 as well as potential changes in parent material sources, phases of carbonate growth, and diagenetic overprinting over time. A variety of techniques are utilized, including strontium isotope (⁸⁷Sr/⁸⁶Sr) and particle-size analyses together with petrographic and cathodoluminescence imaging of carbonate components. Given that pedogenic carbonates are widely used as paleoenvironmental proxies, our results highlight the necessity of screening pedogenic carbonates for isotopic equilibrium as well as understanding landscape histories when interpreting isotopic trends.