The Period 1995 - 1999 of the Paleopedology Commission

(Some problems and suggestions for discussion)

Arnt Bronger

One of the most important uses of buried Quaternary paleosols has been the paleoclimatic interpretation of the multiple soils in long loess sequences of Central and Eastern Europe, Central Asia, China and other regions. These sequences have provided detailed records of climatic change for the last 2.6 million years, which are potentially at least as detailed as the oxygen isotope record obtained from deep oceanic cores. Like the oceanic cores, they also show a good relationship to the variations of solar radiance resulting from perturbations of the earth's orbit, the so-called Milankovitch curves. Paleosols have undoubted importance for understanding the processes driving natural climatic change, not just to the Quaternary (glacials, interglacials, interstadials) but throughout geological time, because the Milankovitch curves have now been identified in cyclic sediment sequences of many pre-Quaternary periods. The same processes will undoubtedly control future climatic change, though the more immediate effects of atmospheric pollutants (greenhouse gases) may be superimposed on those related to the Milankovitch curves.

The main advantage of the loess-paleosol sequences in reconstructing past global climate change is that they cover all or much of the Quaternary period in many different geographical regions. When the paleosols are adequately dated and sufficiently interpreted, they will allow geographical variation of past climates to be assessed more reliably and in more detail than is possible by any other method known at present. This is one of the tasks of the Commission on Paleopedology of INQUA.

Paleopedology is also important for a proper understanding of the agrochemical and agroenvironmental behaviour of surface (non-buried) paleosols, and it plays a vital role in the multidisciplinary pedological study of soils. We therefore see paleopedology as occupying an important position in soil science (Fig. 1) with considerable potential for a major co-ordinating role through a genetic system of soil classification (Bronger & Catt 1996). This is a main task of paleopedology in the ISSS especially in Commission V, where we have only a status of a Working Group (see Protocol of the business meeting in Berlin and below).

For these areas of responsibility we need first to solve the problems of defining paleosols. Considerable progress was made at the Inter-Congress Symposium in Champaign, IL, USA 1993 especially by the Working Group "Definitions, used in Paleopedology (Paleopedology Glossary)" ran by J.A.Catt. First results are summarized in Newsletter (NL) 11/2, 35-37 Oct.1995). Some problems may be emphasized briefly:

1. There is a considerable measure of agreement over what is meant by a buried paleosol. It is a soil buried by younger sediment sufficiently thick that any subsequent pedogenesis has not affected its entire thickness and therefore has not influenced the buried soil. Problems start, however, with postpedogenic (or diagenetic) changes after burial. A frequent example is the recalcification of (almost) all paleosols buried by loess deposition. This causes severe problems for paleosol classification if property based systems are recommended (see below) although soil micromorphology can distinguish between primary and secondary calcites. Another difficulty is caused by the fact that most buried paleosols in loess sequences are more or less truncated. therefore it is difficult e.g. to separate Mollisols (Phaeozems, Chernozems, Kastanozems) from Luvisols. Because of the rapid decrease of humus content even in buried Holocene Chernozems e.g. in the Central Loess Plateau of China (Bronger and Heinkele 1989) it may be even difficult to distinguish them from Kastanozems because of colour as in the FAO-UNESCO system.

2. Non-buried paleosols (relict soils) do not suffer from these difficulties but there are even more problems of defining and recognizing them. A non-buried paleosols show two or more sets of properties which can be related to different combinations of soil-forming factors (esp. different climates and with it vegetation) through sets of often incompatible soil forming processes. Incompatibility of processes implies two or more environmentally different periods of soil development.

Thus also non-buried paleosols play an important role for a full and correct understanding of landscape history, which is essential for a proper appreciation of many modern environmental problems. For example soil cover of central and south India consists mainly of Vertisols ("Black soils") and Lixisols (Rhodustalfs or "Red soils") both of which are non-buried paleosols because they formed in an earlier period of much more moist climate than the present. In the present distinct seasonal semi-arid conditions (<1000 mm annual rainfall and very high evapotranspiration) earlier soil- forming processes such as deep weathering and strong kaolinite formation have almost ceased; instead secondary carbonate is accumulating in the saprolite (Cr) and lower parts of the Lixisol Bt horizons (Bronger and Bruhn, 1989), and under arable agriculture soil erosion is becoming a severe problem. It is widely accepted that soil erosion can be tolerated from a crop production viewpoint provided soil formation keeps pace with it to compensate for the losses. However, in today semiarid India the soil development processes have changed and the rate of compensatory regeneration of soil is in effect (almost) zero. The erosion there is consequently a permanent loss of the country's most important natural resources, but it has not been recognized as such because the soils were not identified as paleosols. The erosion now occurring in many other countries (Oldeman et al., 1990) is probably a similar irreplaceable loss of resources inherited from earlier geological periods. - This example may emphasize the importance of paleopedology even for applied aspects in the Commission V of ISSS, because in the tropics and subtropics most soils are paleosols. We will submit another proposal for upgrading Paleopedology Working Group to a Subcommission in the ISSS at the next World Soil Congress at Montpellier 20 to 26 August 1998.

The delimitation of non-buried paleosols from so-called "modern" soils may be still unsatisfactory. To stimulate further discussion the vetusol concept of M. Cremaschi (1987) may be introduced again. A vetusol (vetus, Latin: old) is a non-buried soil "which underwent the same or very similar processes of soil formation over a generally long period of time, including at least some part of the Pleistocene" (op. cit., 234).

With this suggestion we are back to the question of more or less climatic changes of the Quaternary in different regions.

For our paleopedological approach the concept of a pedostratigraphic unit introduced by R.B. Morrison, chairperson of the WG "Pedostratigraphy" seems very useful. His valuable ideas are recently summarized in NL 11/1, 5-8 (July 1995) and are an important contribution towards a (trans)continental pedostratigraphy which our Paleopedology Commission should develop.

The "fundamental and only unit in pedostratigraphic classification is a geosol" comprising one or more differentiated pedologic horizons. A geosol "has a consistent stratigraphic position and is defined at a type locality where the horizons are buried by younger deposits ..."(J.A.Catt in NL11/2, op.cit.). - In the North American Stratigraphic Code (NASC), article 57a a composite geosol can still be regarded as a pedostratigraphic unit. It is a polygenetic paleosol, recording more than one soil-forming episode (R.B.Morrison); also termed "welded paleosol" after Ruhe (1967). R.B.Morrison names as an example the "Yarmouth-Sangamon paleosol which represents about 700 000 in Iowa" (op.cit. p.6). However up to a dozen paleosols, partly in pedocomplexes (see below) and more or less separated by loess can be found in this time interval in long loess sequences (up to 65 m) in China or Tadjikistan (Bronger and Heinkele 1989, Bronger et al. 1995). Therefore the idea of R.B.Morrison that stratigrafic codes as the NASC "should recognise pedostratigraphic ranks subordinate to geosol" which he termed "pedomembers" (op.cit. p.6) should be supported.

From composite geosols R.B. Morrison distinguishes compounded (multistory) paleosols defined as "two or more paleosols separated by small thickness of not soil-sediment (C horizon)..." (p.6). These compounded paleosols (compounded geosols?) are described as pedocomplexes - which is the name for a fundamental pedostratigraphic unit in Central Europe, used since decades in very many publications. R.B.Morrison continues with compounded paleosols if "traced laterally, these pedocomplexes commonly merge into a single composite paleosol". The necessity to distinguish between composite and compounded paleosol (geosol) might be questionable; the composite geosol is also "multistory" or polygenetic.

Finally R.B. Morrison proposes litho-pedostratigraphic complexes "that exist throughout the World. These complexes comprise stacks of numerous paleosols ... closely spaced ..." (p.6) His conclusion that "the individual paleosols cannot be reliably differentiated and traced very far; the whole complex is best treated and mapped as lithostratigraphic unit" (p.6-7) may be valid only in certain areas. Their existence as world-wide litho-pedostratigraphic complexes would show that in other areas, e.g. in Central Asia and China (see above) the equivalent units can tell the most detailed paleoenvironmental history.

Even more discussion is needed for the classification of paleosols. Two proposals have been submitted (see NL11/2, 12-13 and 33-34); they will be published together with several other papers presented at the Inter-Congress Symposium in Champaign 1993 in a next volume of Quaternary International hopefully this year. Both proposals are based on properties.

The proposal from P. Buurmann is based mainly on the FAO-UNESCO soil classification system, the proposal of the WG on "Classification of Paleosols" chaired by W.D. Nettleton recommended the U.S. Soil Taxonomy as a basis. Both international classification systems do not distinguish, however, between a soil in situ and a soil sediment (redeposited soil material) which is essential for stratigraphic, geomorphological and paleoenvironmental interpretation. Especially they do not distinguish profiles which have been truncated by erosion or contain discontinuities resulting from erosion followed by redeposition and renewed pedogenesis. Both systems also do not allow for postpedogenic (diagenetic) changes (see above). - The Soil Taxonomy uses present climatic factors at high taxonomic level which leads to confusion over the recognition and paleoclimatic interpretation of relict features, formed in past periods of different climate.

Only some major problems and suggestions for discussion are mentioned, encouraged by the well-organized Inter-Congress Symposium 1993 in Champaign, IL, USA with very stimulating contributions (see also Protocol of the business meeting in Berlin). For this I want to thank especially L.F. Follmer and D.L. Johnson. - However, we need much more exchange of ideas and concepts, and discussion; the gap between the last but one Symposium on Paleopedology in Amsterdam 1970 (!) and that in Champaign 1993 can be still recognized very much. Therefore it is my main aim to have more meetings in the near future, also to build up an active core group (as large as possible) of members of the Paleopedology Commission (WG in ISSS).

References:

Bronger, A. and Heinkele, Th. 1989 in: Geoderma, 45: 123-143
Bronger, A. and Bruhn, N. 1989 in: Catena Supl.16: 107-128
Bronger, A. et al. 1995 in. Quaternary Proceedings, 4: 69-81
Bronger, A. and Catt, J.A. 1996 in: Quaternary International (in press)
Cremaschi, M. 1987. Paleosols and vetusols in the Central Po Plain (Northern Italy) etc. Profschrift Univ. Amsterdam, E. Unicopli, Milano
Oldeman, L.R. et al. 1990 World map of the status of human-induced soil degradation: GLASOD. - ISRIC/UNEP etc.
Ruhe, R.V. 1956 in: Soil Science 82: 441-455
Ruhe, R.V. 1967 in: McGraw-Hill Encyclopedia of Science and Technology, New York, p.522
Yaalon, D.H. 1971 in: Paleopedology: origin, nature and dating of paleosols, Jerusalem, p.29-39

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