Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book Interpretation of Micromorphological Features of Soils and Regoliths. The copy attached is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research, and educational use. This includes without limitation use in instruction at your institution, distribution to specific colleagues, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From Nicolas Fedoroff, Palaeosoils and Relict Soils. In: Georges Stoops, Vera Marcelino and Florias Mees, editors, Interpretation of Micromorphological Features of Soils and Regoliths. Elsevier, 2010, p. 623. ISBN: 978-0-444-53156-8. © Copyright 2010, Elsevier B.V. Elsevier. Author's personal copy 27 Palaeosoils and Relict Soils 1,*,z 2 3 Nicolas Fedoroff , Marie-Agne` s Courty , Zhengtang Guo 1 RETIRED FROM AGROTECH, PARIS, FRANCE 2 CNRS UMR 7194 & IPES, FRANCE AND UNIVERSIDAD ROVIRA Y VIRGILI, TARRAGONA, SPAIN 3 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING, CHINA 1. Introduction Soils have played an important role in the history of the Earth history since 3000 million years ago (Retallack 1990; Retallack & Mindszenty, 1994). Primitive soils, which formed by weathering, appeared before vegetation developed (e.g. Driese et al., 2007). Later soils formed in conditions with different atmospheric compositions than those existing at present and with a different impact of cosmic events (e.g. Jones & Bo Lim, 2000). Definitions of palaeosoils vary widely. For Morrison (1977), palaeosoils are soils of obvious antiquity, whereas Butzer (1971) defines them as ancient soils. For other authors, they are soils formed in a landscape of the past (Ruhe, 1965; Yaalon, 1971), or in a past environment (Yaalon, 1983). In this chapter, the term ‘palaeosoil’ is restricted to buried soils of any age, whose functioning was totally or partially inhibited by burial. We also apply it to truncated soils, even if just a thin basal stump is preserved. Relict soils are defined as soils belonging to the present-day soil cover but which have characteristics inherited from the past, resulting from different environmental conditions than those occurring today. The aims and history of palaeopedology differ from those of pedology (Fedoroff & Courty, 2002). Palaeosoils have been approached with different objectives. Many authors (e.g. Kukla & An Zhisheng, 1989) considered palaeosoils as stratigraphic markers in con- tinental sequences, without attempting to analyse them, whereas others considered some layers in such sequences as palaeosoils just because of their colour. Analysis and interpreta- tion of palaesoils with methods used in pedology, including soil micromorphology, began at a relatively late stage (e.g. Fedoroff & Goldberg, 1982). The palaeoenvironmental significance of palaeosoils has also only recently been taken into account (e.g. Fedoroff et al., 1990; Kemp, 1999; Felix-Henningsen & Mauz, 2004; Ferraro et al., 2004). Proxy indicators for * Corresponding Author: Tel.: +33 468 591171 E-mail: [email protected] z Retired Interpretation of Micromorphological Features of Soils and Regoliths DOI: 10.1016/B978-0-444-53156-8.00027-1 623 ISBN 978-0-444-53156-8, Ó 2010 Elsevier B.V. All rights reserved. Author's personal copy 624 MICROMORPHOLOGICAL FEATURES OF SOILS AND REGOLITHS defining climatic characteristics under which palaeosoils have developed have been used by some authors (e.g. Wang & Follmer, 1998; Retalllack, 2005a). Soil micromorphology is at present commonly applied in investigations of palaeosoils and relict soils. Mu¨cher and Morozova (1983) have presented a foundation for the micromorphological description of palaeosoils and related sediments, whereas Fedoroff et al. (1990) drafted keys for the recognition of environmental changes in the past. Most papers deal with Quaternary palaeosoils, especially those formed during the last inter- glacial, the last glacial cycle and the Holocene (e.g. Kemp et al., 1994; Cremaschi & Trombino, 1998; Kemp, 1998; Srivastava & Parkash, 2002), but pre-Quaternary palaeosoils have also been studied (e.g. Tate & Retallack, 1995; McCarthy et al., 1998; Driese et al., 2007). Micromorphological analyses of palaeosoils often show differences with present- day soil functioning. Elementary attributes of pedogenic facies of the past are similar to present ones, but they can be different by showing a higher degree of development or by the presence of assemblages of pedofeatures unknown in modern soils. Pedofeatures specific of pedogenesis of the past are rare. Micromorphological studies also reveal that almost all palaeosoils were affected by processes of in situ reworking, erosion, transport, deposition and allochthonous aggradation (e.g. Retallack, 2005b; Sephton et al., 2005), which are often underestimated or not even recognised by other observations. Examination in thin sections of well-developed soils, especially those dating back to the Pleistocene, nearly always reveals the presence of relict characteristics (e.g. Fedoroff, 1997). In some relict soils, such as Ferralsols, the history of the soil is almost totally wiped out, but in others, such as calcretes and ferricretes (e.g. Achyuthan & Fedoroff, 2008), the history is preserved to a large extent. In this chapter, an interdisciplinary approach is used, dealing with analysis of sets of interacting entities and the interactions within those systems (systems analysis). Such an approach is commonly utilised by sedimentary petrologists (e.g. Humbert, 1976a, b). 2. Methodology 2.1 Recognition of Palaeosoils and Relict Soils The first task when investigating continental sequences is to decide whether the layers defined in the field as palaeosoils are soils that formed in situ, transported soil materials (pedosediments), or something else (e.g. McCarthy, 2002). In situ soils are characterised in thin sections by a continuous pedogenic facies recognised by at least one of the following features: (i) undisturbed features resulting from soil biological activity, such as passage features and channels with root residues or excrements; (ii) a pedogenic micro- structure; (iii) a pedogenic b-fabric; or (iv) one or more types of undisturbed pedofeatures. In contrast, pedosediments are recognised by: (i) an absence of in situ biogenic features, but the common presence of tissue and charcoal fragments; (ii) a massive microstructure, and occasionally a structure dominated by packing of rounded aggregates; or (iii) sedi- mentary features (see also Mu¨cher et al., 2010, this book). Some layers which seem to be free of pedogenic characteristics in the field can show a pedogenic facies in thin sections. Author's personal copy Chapter 27 Palaeosoils and Relict Soils 625 For example, the typic Late Pleistocene loess layers of the Loess Plateau of China consist entirely of passage features and various types of excrements (Guo, 1990). Consequently this examination must be extended to under- and overlying horizons or layers in order to characterise variations in pedogenic facies, identify other possible types of facies, com- pare groundmass characteristics and identify features related to erosion and aggradation. Palaeosoils are very rarely preserved as complete and undisturbed profiles. Some discontinuities are easy to identify in the field, such as truncations, stone lines and the superimposition of allochthonous materials on pedogenic horizons. The field diagnosis can be considerably improved by examination in thin section, which can reveal disconti- nuities in pedofeatures, as well as features due to erosion, in situ reworking (see Fig. 15), mass-transportation or minor aggradation (e.g. accumulation of aeolian dust). It also contributes to the determination of which in situ horizon or horizons are part of the investigated palaeosoil, on the basis of its pedogenic facies. Palaeosoils commonly alternate with sediments, as in loess sequences (e.g. Kukla & An Zhisheng, 1989) or in deltas and alluvial plains (Srivastava & Parkash, 2002), creating palaeosoil–sediment sequences termed ‘pedocomplexes’ (e.g. Morrison, 1977; Feng & Wang, 2005). The palaeosoils can be: (i) juxtaposed, i.e lying in a close vertical succession, without penetration of pedofeatures of the overlying into the underlying palaeosoil; (ii) superimposed, i.e with penetration of pedofeatures of the overlying into the underlying palaeosoil; or (iii) cumulic, i.e palaeosoils in which a type of pedofeature (in general a textural pedofeature) occurs without significant change in a thick homogeneous horizon (Fig. 1)(e.g.Yin & Guo, 2006). An example of a well-studied pedocomplex is the marine FIG. 1 Possible relationships between palaeosoils and related sediments in palaeosoil–sediment sequences. Author's personal copy 626 MICROMORPHOLOGICAL FEATURES OF SOILS AND REGOLITHS isotope stage 5 (MIS 5) pedocomplex, also known as the Eemian pedocomplex, i.e present throughout the loess belt of the northern hemisphere, whose characterisation includes