The soil-evolution model as a framework for evaluating pedoturbation in archaeological site formation

Abstract

Soil and near-surface material affected by pedogenesis is the principal medium in which field archaeologists dig. Soil encases the cultural resources from which prehistoric human behavior is inferred. In fact, the various soil properties, factors, and conditions often determine the extent to which perishable materials are preserved, modified, moved, or destroyed by post-burial processes. The soil is not, however, the same material that was originally present when the artifacts were deposited. The dynamic nature of the soil system is such that changes in its morphology, thickness, and biophysicochemistry occur over time, and the sum of the changes is different from one site to another. Changes may include evolution of new properties, features, and horizons, such as illuvial cutans of clay, humus, iron and manganese, argillic horizons, and concretions, among others. Such endogenous or intrinsically evolved properties and features are termed "pedogenic accessions." Once formed, such accessions may persist, undergo further change, or be obliterated, as when clay cutans are broken up in Bt horizons via shrink-swell action, or when carbonate nodules dissolve following an increase in soil acidity. Further, some accessions may cause feedback processes that redirect pedogenic pathways in progressive or regressive directions. For example, certain subsoil horizons may gradually increase in density until threshold development is reached (fragipans, clay pans) after which they become barriers to downward water percolation and function as aquitards or aquicludes. Feedbacks such as perched water tables and attendant profile gleying, mottling, and cessation of profile deepening may be triggered. Then there are the

The good earthworm: How natural processes preserve upland Archaic archaeological sites of western Illinois, U.S.A.

“…Soil adhering to the uplifted roots of uprooted trees eventually forms a mound adjacent to the original excavation (cradle-and-knoll topography). Soil horizons are disrupted and mixed together, the repeated action ultimately creating what has been termed a floralmantle (see reviews by Johnson, 1990;Johnson and Watson-Stegner, 1990;Wood and Johnson, 1978). Clearly, tree plowing is a process that can move existing particles (including artifacts) in any direction, bringing up materials from below as well as introducing surface materials to lower levels in the soil.…”

“…Activities of pocket gophers, which excavate burrows about 6 cm in diameter, have received perhaps the most attention recently (Erlandson, 1984;Bocek, 1986;Johnson, 1989Johnson, , 1990Johnson and Watson-Stegner, 1990). Western North American gophers concentrate large artifacts (Ͼ6 cm) but mix up smaller materials throughout the upper biomantle.…”

“…As the process continues, the absolute elevation of the soil surface changes little, except perhaps to rise slightly from increases of soil pore space due to burrowing. Theoretically, then, a three-tiered soil profile results, consisting of a fine-grained upper layer, a coarse-grained subsurface concentration of stones and artifacts (the stone zone), and the relatively unaffected materials beneath the stone zone (Johnson, 1990(Johnson, , 1993a(Johnson, , 1993bJohnson and Watson-Stegner, 1990;Johnson and Balek, 1991;Balek, 1995).…”

Buried artifacts in stable upland sites and the role of bioturbation: A review

“…Depth of artifact burial ultimately corresponds to the base of major biologic activity (Johnson and Watson-Stegner, 1990), demonstrated by a noticeable decrease in fecal pellet and biovoid (burrow) abundance (Balek, 1995). In Mollisols and Alfisols of the Midwestern United States, and in other soils outside the United States (e.g., Australian "duplex" soils of Bishop et al [1980]), this depth commonly approximates the uppermost part of the B horizon.…”

“…All soils are by nature dynamic (Johnson and Watson-Stegner, 1990;Johnson, this issue), even where developed on relatively stable geomorphic surfaces such as nearly level uplands, because they form through the vertical and lateral translocation of particles by less than obvious biomechanical processes. Where not environmentally restricted, biologic processes dominate in every soil exposed at the surface.…”