Quaternary International, Vols 51/52, pp. 109-114, 1998. © 1998 Published by INQUA/Eisevier Science Ltd Pergamon All rights reserved. Printed in Great Britain. PII: S1040- 6182(97)00037- 2 1040-6182/98/$19.00 SOIL TAXONOMY AND PALEOENVIRONMENTAL RECONSTRUCTION: A CRITICAL COMMENTARY Dennis E. Dahms* and Vance T. Hollidayt * Department of Geography, University of Northern Iowa, Cedar Falls, 1.4 50614-0406, USA t Department of Geography, 550 North Park Street, University of Wisconsin, Mcidison, WI 53706, USA INTRODUCTION 1990; Lehman, 1989; Mack et al., 1993). These applica- tions of the soil taxonomy improve communication An important focus of paleopedology is the recon- only when the assumptions used as the basis for the struction of past environments. In particular, consider- taxonomy also can be made for the ancient sedimen- able interest in ancient (mostly pre-Quaternary) soils tary record (Soil Survey Staff, 1975). Classification of has appeared since the last paleopedology symposium paleosols has precisely the same utility as classification (Yaalon, 1971), with an emphasis on paleoenvironmen- of sediments and sedimentary rocks. Clearly, there is tal reconstruction (e.g. Retallack, 1990). A significant demand for classification systems that can be applied amount of current research uses soil-like features in the to paleosols, including quaternary and pre-quaternary sedimentary record as stratigraphic marker zones, as buried, relict, and lithified soils (e.g. Mack et al., 1993; evidence to infer past environmental conditions, and Nettleton et al., 1998). By classifying these soils, how- as a basis for paleo-geomorphic interpretations by ever, we must ask the larger questions: (1) how useful is applying the principles of soil geomorphology and the U.S. soil taxonomy for classifying lithified soils, pedology to paleosols. Some investigators view the (2) can the U.S. soil taxonomy be used without access classification of paleosols as an important step in their to past (diagnostic) soil conditions without violating interpretation (Mack et al., 1993; Retallack, 1993, 1994; the underlying principles on which it is based, and/or Nettleton et al., 1998). Classification of these ancient (3) more broadly, which system, if any, is best for soils can serve several useful purposes: to order the application to such profiles? profiles that have been described, to provide initial descriptive information, and as a convenient short- hand reference. INTERPRETIVE APPROACHES IN A more contentious issue is use of the U.S. soil PALEOPEDOLOGY taxonomy as a direct indicator of past environments (e.g. Fastovsky and McSweeney, 1987). Soil taxonomy A review of the literature shows that geoscientists was developed primarily for agricultural, mapping, and working with pre-quaternary soils use paleopedology land use purposes and is based on a number of present to construct ancient environments chiefly in two ways. soil-forming conditions and processes. The problem In the first way, soil-like sequences found in the sedi- with using soil taxonomy for ancient lithified soils is mentary record serve to identify the stratigraphic posi- that classification to even the great group level (even to tion of ancient land surfaces and to correlate facies order level in some cases, e.g. Aridisols) requires that changes within rock-stratigraphic units. Once identi- the nature of the soil-forming paleoenvironments must fied, the paleogeography of stable land surfaces can be be known first. The system is based entirely upon interpreted together with the facies changes of sedi- diagnostic terms that require knowledge of current, ments that accumulated on and adjacent to them. observable conditions of the soil and its local environ- Examples of such a use of paleosols is that of Bown and ment. However, ancient soils often do not preserve the Kraus (1981, 1987), Kraus (1987), Sigleo and Reinhardt information required to assign diagnostic terms used in (1988), and Kraus and Asian (1993). These studies use soil taxonomy due to diagenetic overprints in the sedi- paleosols (including lateral variations) and sedimen- mentary rocks. If diagnostic terms cannot be assigned tologic characteristics to interpret the processes and because these soil conditions cannot be determined, paleogeography of ancient floodplain construction. then further classification using the soil taxonomy is Such work represents an extension of quaternary groundless. In this paper we argue that the system paleopedology to the pre-quaternary sedimentary rock simply does not permit the classification to be used to record in those places where it is based on accepted infer past environments. soil-geomorphic principles that seek to explain the Some paleopedologists use terms and concepts taken variability of modern surface soils over contrasting directly or modified from soil taxonomy (Retallack, landscapes (Ruhe, 1956; Ruhe and Walker, 1968; 109 110 D.E. Dahms and V.T. Holliday Daniels et al., 1971; Conacher and Dalrymple, 1977; more) different phases of pedogenesis under different Valentine and Dalrymple, 1976; Jenny, 1980; Birke- conditions. land, 1984). A second way pre-quaternary paleosols are used by geoscientists is described by Retallack: '(This) approach DISCUSSION to interpreting paleosols is to identify them within Problems of Post-burial Alteration a soil classification and compare them to modern soils ...' (Retallack et al., 1990, p. 1325; see also Retal- The problem with applying diagnostic h0'rizon terms lack, 1993). Retallack refers to this approach as 'taxo- to both modern and ancient soils is that post-burial nomic uniformitarianism' (1994, pp. 51-53; and this diagenetic processes confuse an interpretation based on volume). This approach involves describing the profiles modern pedogenesis. In order to identify diagnostio and applying diagnostic terms to classify ancient soils, surface horizons in an ancient soil one has to be certain using the nomenclature and classification system of the that the properties were formed at the land surface and U.S. soil taxonomy. Ancient environments and pro- are not the result of post-burial diagenesis. We offer cesses that influenced the ancient soils then are inferred that this is the case only in a few instances to date. from the classification. Recent work by Patterson and others (1988, 1990) in Retallack likens soil taxonomy to biological taxo- Eocene floodplain settings of Wyoming suggests that nomy in the sense that 'identification of a paleosol differences in lithology can create diagenetic pathways within a modern soil taxonomy may be taken to imply that mimic soil horizonation. Walker and others (1978) past conditions similar to those enjoyed by such soils show that pellicular clay coatings that mimic argillans today' (1994, p. 51). The problem with taxonomic uni- can form through infiltration below the zone of soil formitarianism is well stated in a recent critique by formation. Even a few thousand years of burial is Fastovsky (1991, p. 182): enough to deplete much of the original organic carbon 'The idea that analogies can be drawn between soil from the A-horizons of many late Pleistocene-Holo- classifications and biological classifications is mis- cene soils (Dahms, 1994; Holliday, 1988). Thus, a num- leading. Biological classifications are developed for ber of processes can either mimic, block, or otherwise their abilities to reflect genetic relationships among interfere with our ability to see through the 'veil of organisms. The assumption underlying this is that diagenesis' (Patterson et al., 1990, p. 845) to past envir- there exists one true "phylogeny, the reflection of onmental conditions (also see Blodgett, 1988; Nesbitt which is sought in the classification. Biological clas- and Young, 1989; Patterson, 1991). sifications are hierarchical because the character dis- To classify ancient soils according to the orders, tributions in nature that they reflect are likewise suborders, or great groups of soil taxonomy is to risk hierarchical. Established soil taxa, however, have no compounding the interpretive error. The problem may single, inferred historical and genetic connection, be the greatest when interpreting soils influenced by such as is presumed to exist among organisms. In- aquic moisture regimes. For example, the designation deed, soil classifications have been developed for aquept requires detailed information about how long multiple purposes and impose an arbitrary typologi- a soil remains saturated during the year, information cal system upon natural continua of overlapping that is simply not available for ancient soils. Further- processes. Because of this, soil taxonomies do not more, calling an ancient soil an aquept (Retallack, 1990, have the predictive power inherent in biological Fig. 4.11, p. 85) implies that present oxidation-reduc- classifications'. tion states resemble those of the past. But, analysis of Relatively few specific soil features or types of surface modern mineral oxidation states and redox calcu- or buried soils are related to unique or easily identified lations cannot proxy for ancient conditions even if we environments of formation. Most soil taxonomic terms could reconstruct diagenetic pathways more directly. are influenced by all of the soil-forming factors, not just In the case of buried soils in floodplain environments, it climate and organisms (which are of most interest in should be very hard to tell pre-burial aquic conditions paleopedology). Aroillic horizons, for example, occur from aquic conditions