Polygenesis of a Pleistocene paleosol in southern Iowa

Abstract

The Yarmouth-Sangamon Paleosol (YSP) is a major soil-stratigraphic unit occurring across a large portion of the U.S. Midwest. Study of a YSP exposed in a toposequence near Earlham, Iowa, demonstrates the paleosol9s usefulness for deciphering paleoenvironments in the U.S. midcontinent before the last glacial maximum, environments for which very little other evidence exists. Field description and micromorphological, textural, and mineralogical analyses were used to identify the major processes and conditions of Pleistocene soil evolution at the site. The results suggest that the YSP is a complex polygenetic soil that developed in multiple parent materials: (i) a Pre-Illinoian glacial till composing the base of the paleosol; (ii) a middle unit of colluvium and loess; and (iii) an upper unit of loess and reworked loess, possibly Illinoian in age. A stone zone at the top of the till indicates a period of erosion before burial by the fine-textured sediments. Filled animal burrows and granular and spongy microstructure at the top of the middle unit suggest a former surface horizon and imply that the YSP is a composite, welded profile consisting of at least two sola. Micromorphological and mineralogical features suggest soil formation under paleoenvironmental conditions that ranged from periglacial to intensely seasonal climates. Profile differentiation during periods of landscape stability was offset by the homogenizing effects of shrink/swell processes and postburial diagenesis, resulting in an isotropic profile that belies a genetically complex history. As a result of diagenesis, toposequence members at the site exhibit little chemical and mineralogical variability.