Using the factors of soil formation to assess stable carbon isotope disequilibrium in late Pleistocene (MIS 3) buried soils of the Great Plains, North America

The stable carbon isotope composition of both soil organic matter (SOM) and pedogenic carbonate are widely used as paleoenvironmental proxies. This study utilizes 813C analyses to reconstruct bioclimatic change from a series of buried soils in the central Great Plains of North America that developed between ca. 44-24 ka. Results revealed a paradoxical isotopic disequilibrium between the isotopic composition of bulk SOM (813CSOM) and pedogenic carbonate (813Ccarb). Specifically, & UDelta;13C values are 0.1 to 6.3 per mil greater than the highest expected equilibrium value of 17 per mil in the Bk horizons. In contrast, & UDelta;13C values are 0.1 to 4.8 per mil lower than the lowest expected equilibrium value of 14 per mil in the Ak horizons. A soil-forming factor approach was utilized to establish multiple working hypotheses regarding the influence of climate, vegetation, parent material, and time on the observed isotopic disequilibrium.Of the various hypotheses presented, we suggest that the following most likely explain the observed isotopic disequilibrium. The greater-than-expected & UDelta;13C values in the Bk horizons most likely reflects seasonal bias in pedogenic carbonate formation, resulting in an apparent C4-biased signal. The lower-than-expected & UDelta;13C values in the Ak horizons remains perplexing. The most likely explanation is that detrital carbonate contributions affected the 813Ccarb record or that the 813Ccarb and 813CSOM records are asynchronous. Overall, it appears that different factors have affected the 813CSOM and 813Ccarb records independently and therefore results of this study highlight the importance of assessing pedogenic carbonates for isotopic equilibrium as well as the need to understand past environmental conditions (i.e., soil-forming factors) when interpreting isotopic trends.