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

Where it is thick enough to extend below the surface soil profile, the late Wisconsinan Peoria Loess in the midcontinent of North America has characte ristics 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 ret ention of detrital carbonate grains. Late Wisconsinan climatic conditions p rovide one potential explanation for these observations. This paper conside rs two other, non-climatic, explanations: (1) loess accumulation was more r apid than carbonate dissolution or OC accumulation, and (2) post-deposition al diagenesis has removed evidence of syndepositional pedogenesis. The appr oach used is comparison of potential rates for pedogenic processes and loes s 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 di ssolution to keep pace. On the other hand, data from modern tundra and bore al forest environments suggest that soil OC accumulation could have kept pa ce with loess deposition in many cases. Low OC contents observed in the loe ss today probably reflect post-depositional oxidation. Climatic factors may provide partial or local explanations for the apparent minor extent of syn depositional pedogenesis, but loess accumulation rates and diagenesis are k ey factors as well. (C) 1998 Published by INQUA/Elsevier Science Ltd. All r ights reserved.