Root Traces and the Identification of Paleosols

Margaret E. Berry and James R. Staub,
Department of Geology, Southern Illinois University, Carbondale, IL 62901-4324

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).

Problems with the Premise

1. Although root traces indicate that a surface was colonized by plants, they do not indicate that genetic soil horizons were developed, or that the surface was subaerially exposed for prolonged periods of time. Geomorphic surfaces can be vegetated during and shortly after their formation; the roots may penetrate to considerable depth, but if the surface is young, the associated genetic soil will be very shallow or nonexistent (e.g. Birkeland, 1984; Fanning and Fanning, 1989).
2. Some roots are morphologically similar to burrows. Because of this, the discrimination of the two types of trace fossils can be difficult, and often is based on preconceived ideas about the depositional environment of the rock in which they are found; via circular reasoning, the trace fossils then become evidence to support the environmental interpretation (Boyd, 1975).
3. Vegetation grows in a variety of environments, including subaqueous ones where alteration of the substrate is due to the chemistry of the groundwater rather than to subaerial weathering and soil development. For example, vegetation grows in marshes and ponds; calcium carbonate accumulating in perennially-saturated sediments below the water table in these types of environments is a function of the groundwater, and thus lacks the environmental and time connotations associated with pedogenic carbonates (J.L. Slate, personal communication, 1993). Peat swamps and mires provide other examples. Although deep rooting is sometimes used as an indicator of well-drained soils (Retallack, 1988), current research on peat swamp environments in the Rajang River delta in Sarawak, East Malaysia illustrates that root systems can penetrate deeply (4 m or more) in perennially saturated substrates (Staub, unpublished data). The processes involved in the development of a peat mantle is more accurately described as geogenetic than pedogenetic because peat swamps and mires depend on the continual deposition of organic material for existence (Scott, 1985). Below the peat mantle the alteration of the perennially saturated substrate, which is commonly rooted depends on the chemistry of the groundwater. Leached, clay-rich zones found under many peat mantles are formed by the humic-acid solutions generated in the peat mantle, and their kaolinite content can be positively correlated to peat thickness (Staub and Cohen, 1978). The development of these kaolinite-enriched zones is substrate-facies dependent, and does not occur where the peat is underlain by relatively sandy sediments. Although some may view these systems as organic soils, and thus paleosols when preserved in the rock record as coals and underclays (e.g. Retallack, 1988; Mack and James, 1992), they are strongly affected by the subaqueous environments in which they are formed, thus their environmental and time connotations are quite different from those associated with genetic soils and paleosols formed by pedogenic processes in subaerial environments.

Position Advocated.

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.

References Cited

Birkeland, P. W., 1984, Soils and Geomorphology: Oxford University Press, New York, 372 p.

Boyd, D. W., 1975, False or misleading traces, in R. W. Frey, The Study of Trace Fossils--a Synthesis of Principles, Problems, and Procedures in Ichonology: Springer-Verlag, New York, p. 65-83.

Buol, S. W., Hole, F. D., and McCracken, R.J., 1973, Soil Genesis and Classification, first edition: Iowa State University Press, Ames, 360 p.

Buol, S. W., Hole, F. D., and McCracken, R. J., 1989, Soil Genesis and Classification, third edition: Iowa State University Press, Ames, 446 p.

Fanning, D. S., and Fanning, M.C.B., 1989, Soil--Morphology, Genesis, and Classification: John Wiley and Sons, New York, 395.

Fenwick, I., 1985, Paleosols--problems of recognition and interpretation, in J. Boardman, Soils and Quaternary Landscape Evolution, John Wiley and Sons Ltd., p. 3-21.

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Retallack, G. J., 1988, Field recognition of paleosols, in J. Reinhardt and W. R. Sigleo, eds., Paleosols and Weathering Through Geologic Time--Principles and Applications: Geological Society of America Special Paper 216, p. 1-20.

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Staub, J. R., and Cohen, A. D., 1978, Kaolinite-enrichment beneath coals; a modern analog, Snuggedy Swamp, South Carolina: Journal of Sedimentary Petrology, v. 48, p. 203-210.

Yaalon, D. H. (chairman), 1971, Criteria for the recognition and classification of Paleosols, in D. H. Yaalon, ed., Paleopedology: Israel University Press, Jerusalem, p. 153-158.