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43 3 Paleoenvironmental and Paleoclimatic Interpretations from Paleosols Clues to ancient paleoenvironmental and paleoclimatic conditions in the Badlands can be found in many sources, including fossils, sediments that make up the buttes and spires, and the enigmatic stripes of red, brown, and beige that cut across the Badlands. These razor-sharp lines of color are actually ancient soils (paleosols) representing former landscapes that have been buried and lithified (turned to rock). In order to utilize paleosols to reconstruct past climates and environments, we need a working knowledge of modern soils and the environmental conditions under which they form. What Is a Soil? Soils are zones of physical, chemical, and biological activity at the surface of the Earth that modify geological materials into a more stable form. Physical weathering is the process by which geological materials are mechanically broken down into progressively smaller pieces. Chemical weathering is the interaction of geological materials with water and organic acids and alters the minerals that comprise the rock. These various forms of weathering and modification represent a quest for equilibrium between the forces of weathering and the stability of the geological material that is being weathered. As a soil develops, the downward movement of soil water and dissolved/suspended materials will create distinct accumulations (soil horizons) that reflect the particular physical, chemical, and biological processes regulated by five primary factors of soil formation: climate, organisms, relief, parent material, and time (CLORPT) (Jenny, 1941). Variations in these five factors have generated over 18,000 different soils in the United States alone (Soil Survey Staff, 1999). In terms of soil formation, climate can be thought of as the predominant temperature and precipitation conditions for any given area. Is it hot or cold? Is it rainy most of the time, or is it dry like a desert? Warm, humid environments will promote faster rates of chemical weathering, whereas extremely cold climates promote physical weathering by freeze–thaw processes. Humid conditions favor the downward movement (eluviation) and accumulation (illuviation) of clays, whereas more arid conditions favor the accumulation of calcium carbonate by evaporation of soil water. The influence of organisms can be both physical and chemical in nature, and includes the smallest microbes to the largest animals as well as all varieties of plants. Relief is simply the shape of the landscape, otherwise known as topography. Changes in relief , if significant enough, will influence rates of soil erosion and stability, with areas of higher relief experiencing greater erosion. Parent material refers to the physical and chemical makeup of the geologic material that is being altered by soil formation. The last factor of soil formation is time. In the simplest sense, the more time that is available for weathering , the greater the degree of physical and chemical change that can be induced. Persistent deposition of new material or erosion of existing soil resets the time clock. Soils are classified into 12 distinct orders (Soil Survey Staff, 1999). These soil orders are three-dimensional bodies of material that are composed of distinct combinations of horizons (Table 3.1). The types of horizons present in the soil are the direct result of CLORPT conditions. The first level of horizon classification is the master horizon (Table 3.1). O horizons are accumulations of purely organic matter at the land surface and can vary from a thin layer of leaf litter or a thick mat of marsh and swamp vegetation. A horizons are zones at the top of soil profiles (topsoil) that are defined by a mixture of geological materials and organic matter. B horizons are the primary zones of accumulation that form from the downward percolation of soil water and deposition of finer materials in pores and fractures, or the precipitation of minerals that were dissolved in the soil water. The C horizon represents unaltered parent material. These master horizons can be further classified on the basis of the presence of particular types of mineral and organic matter that accumulate within them (Table 3.1). Common diagnostic subsurface soil horizons include B horizons that are enriched in clay (argillic horizons, Bt) or calcium carbonate (calcic horizons, Bk). These horizons are stacked vertically and are referred to as a soil profile (Fig. 3.1). 44 The White River Badlands Paleopedology and the Recognition of Paleosols Paleopedology is the study of fossilized soils (paleosols). This is a relatively new field within geology and has been steadily gaining in popularity over the past couple of decades. Today, the study of paleosols is used in a...


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