The Environmental Factor Approach to the Interpretation of Paleosols

Paleosols ranging in geological age to 3 billion years are widely used as evidence for ancient surface environments—an enterprise dependent on the enormous literature on factors in soil formation, popularized by Hans Jenny. This kind of inference inverts the logic of Jenny in a manner common to geological sciences—deducing paleoenvironments from observed paleosol features, rather than deducing variation in soil features with observed environmental differences. A paleosol is a single product of many past influences, including alteration after burial, and because of this, some environmental relationships with soil color, clayeyness and organic matter are not useful for interpreting paleosols. One relationship that has proven useful for paleosols is that between depth to calcic horizon and mean annual rainfall. A new compilation of data presented here demonstrates that this relationship holds for aridland soils worldwide. The use of this relationship for interpreting paleoclimate from paleosols is illustrated with an example of the Eocene and Oligocene paleosols of Badlands National Park, South Dakota. Other approaches for the study of paleosols include identifying paleosols within a soil taxonomy, and simulating ancient soil development with mathematical process models. Identification of paleosols leads to broad areas on soil maps unless done with several paleosols. Process models often founder on assumptions, and those of the form δx/δt (where x is a measured soil property) are difficult to apply because time of formation (t) of a paleosol is estimable only to an order of magnitude. Thus the environmental factor approach to the interpretation of paleosols is likely to remain popular for some time to come.