Two conclusions drawn by the working group at the 1970 Paleopedology Symposium in Amsterdam were that paleosols should be studied by the same methods as those used for present-day soils, and that field recognition should be based on the identification of more than one distinct pedogenic feature (Yaalon, 1971). These are guidelines that should be followed when evaluating potential paleosols of any age. However, a premise promoted for the recognition of paleosols in the rock record--namely, that paleosols can be recognized solely on the presence of root traces (e.g. Retallack, p. 1-2, 1988; Mack and James, 1992, p. 3)--is conceptually at odds with these guidelines. The premise not only bases the identification of a paleosol on a single attribute, but also on one that is biogenic rather than pedogenic in origin. Thus, it uses an agriculturally-based definition (Fanning and Fanning, 1989) that primarily considers a soil as a medium for the growth of land plants (e.g. Hilgard, 1914; Soil Survey Staff, 1990). Although this definition may be useful for some purposes, it is unsatisfactory for pedologic and sedimentologic research because it does not take soil genesis into account (see discussion in Buol, Hole, and McCracken, 1973, 1989). A preferred definition considers a soil as a natural body that differs from the parent material in its morphology, its physical, chemical, and mineralogical properties, and its biological characteristics, with at least some of these differences being pedogenic in origin (e.g. Joffe, 1949; Buol, Hole, and McCracken, 1973, 1989; Birkeland, 1984). The soil's formation is a function of the topographic setting, the parent material, the organisms in the environment, the length of time involved, and perhaps most importantly, the regional climate (Jenny, 1941).
The use of root traces alone to prove the presence of a paleosol is problematic for several reasons, which are enumerated below, and can easily lead to biased interpretations of the genesis of other features in the rock record that could be due to subaerial weathering and soil development, ground water alteration, or diagenesis. These various modes of alteration have very different implications. Their resolution can be difficult, yet is critical to the evaluation of possible paleosols (Pavich and Obermeier, 1985; Patterson and others, 1990).
Because paleosols are typically used to interpret past climates and lengths of time of subaerial exposure, a definition based on soil genesis, rather than plant growth, is advocated. This type of definition would better conform to that used by many pedologists working on present-day soils, and would have greater environmental significance. In addition, the use of such a definition might reduce the risk of circular reasoning in identifying the origin of alteration features in possible paleosols. Finally, field recognition of paleosols should be based on the identification of more than a single distinct pedogenic feature, as was stressed by the working group at the 1970 Paleopedology Symposium, and has been supported by others conducting research on paleosols (e.g. Fenwick, 1985). Root traces should be used as evidence of plant growth, not as conclusive evidence of a paleosol.
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