Relative rates of Peoria Loess accumulation and pedogenic processes: Implications for paleoclimatic inference

Abstract

Abstract Where it is thick enough to extend below the surface soil profile, the late Wisconsinan Peoria Loess in the midcontinent of North America has characteristics suggesting relatively little syndepositional pedogenesis. In these thicker sections, the Peoria Loess below the surface soil profile commonly displays massive macroscopic structure, low organic carbon content, and retention of detrital carbonate grains. Late Wisconsinan climatic conditions provide one potential explanation for these observations. This paper considers two other, non-climatic, explanations: (1) loess accumulation was more rapid than carbonate dissolution or OC accumulation, and (2) post-depositional diagenesis has removed evidence of syndepositional pedogenesis. The approach used is comparison of potential rates for pedogenic processes and loess accumulation. Even when wide ranges of controlling factors (precipitation, evapotranspiration, and soil carbon dioxide levels) are considered, loess accumulation was probably too rapid at many sites for carbonate mineral dissolution to keep pace. On the other hand, data from modern tundra and boreal forest environments suggest that soil OC accumulation could have kept pace with loess deposition in many cases. Low OC contents observed in the loess today probably reflect post-depositional oxidation. Climatic factors may provide partial or local explanations for the apparent minor extent of syndepositional pedogenesis, but loess accumulation rates and diagenesis are key factors as well.