Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.fadr.msu.ru/inqua/nl-14/discussion.html
Дата изменения: Mon Jan 12 17:54:23 1998 Дата индексирования: Tue Oct 2 06:02:32 2012 Кодировка: Поисковые слова: horizon |
Ancient soils
Soils of the past may be studied with widely different background and for various reasons. Modern soil scientists when trying to understand the response of various substrata to actual climatic and topographic conditions, are often inhibited in doing so be anomalies caused by relict or exhumed soils blanketing the actual landsurface but not related to the actual climate. For them, the study of ancient (but not necessarily very old) soils is necessary to comprehend the anomalies observed. Geologists , when studying sedimentary rocks, often encounter buried and fossilized soil horizons or even complete soil profiles which by their shear presence are useful indicators of subaerial exposure. Also, such fossil soils may provide information about past climatic (and/or topographic) conditions. One thing should be kept in mind, however, namely that the environmental information provided by ancient soils (be it relict, buried, exhumed or fossil) will never be as direct and unequivocal as it may be in the case of modern soils fully in equilibrium with the actual environment. Ancient soil profiles are very rarely complete phisically and very rarely monogenetic geochemically mineralogically and texturally. What is more, ancient soils encountered in the stratigraphic column will invariably have been subject to the rpocesses of diagenesis - familiar to the geologist but unusual to scientists of modern soils (just as many biologically mediated elementary processes are less well known for the geolodists).
Paleopedology - soil science or geology?
Very clearly paleopedology is a discipline halfway between pedology and sedimentary geology and scholars coming from either side can not avoid getting acquainted at least with the essentials of the other side. Under "essentials" not only methods and subdisciplines (like microbiology, botanics, mineralogy, geomorphology or basic structural geology) shold be consedered but rather the approach - a quality extremely difficult to define. The approach of a scientists is a cumulative property acquired normally during hai basic training, when he is comtinuously surrounded by a particular scientific atmosphere created by his professors (for him the representatives of his chosen discipline). The geological approach, for example, enables the scholar to look at all natural phenomena (be it mineralogical, biological or chemical) with a time/space framework in mind. He combines the conventional three-dimensional view of natural sciences (like geomorphology) with the historical perspective< with time as a fourth dimension, much like archeologists do.
Althought time - manifested as the duration of pedogenesis - is a parameter realized also by modern soil scientists, their concept of a "long time" is a radically different from that of the geologist. For most soil scientists one million year would be a vary long time, whereas for the geologists dealing with hundreds of million years as a routine, one million year would be an "instant", and most geologists working with pre-Quaternary profiles would be extremely happy when having data sets with a time-resolution of one million year. Quite naturally, it is an over-simplification to say that the difference between the approach of the two groups would be resulted only by the different time concepts. There are additional differences rooted in basic training, in the organization of curricula, etc. The time concept, however, is crucial in determining the approach.
The Meeting
The Meeting in Rauischholzhausen provided excellent opportunity for people of different approach to interact, exchange ideas and make more than one important step toward the establishment of a common language - the prerequisite of mutual understanding. That the progress was not quite as big as it might have been was by no means the foult of those present, but rather of those whom we greatly missed. Over 90% of the participants were soil scientists having modern pedology as their basic training (and specizlized in Holocene to Pleistocene soils). Geologists interested in older soils were in minority. One reason for that was obviously the unfortunate almost-overlap of the Rauischholzhausen workshop with the European Meeting of IAS (the International Associztion of Sedimentologists) in Heidelberg (just two weeks before Rauischholzhausen). Two meetings, as closed as temporally, often discourages participation at one or the other. They should be either consecutive, or much farther apart. Probably this is why many sedimentologists trained in paleopedology have not come, though we all could have had greatly benefited from their presence.
Still, those present, have their best to bridge the gap between the two disciplines, and the effort was undoubtedly worthwhile. We, geologists have learned a great deal from our fellow soil scientists and the struggle to understand the other side and to make ourselves understood has proved to be an invaluable help in clarifuing our ideas about paleopedology in general and about our older-than-Pleistocene soils in particular. For the opportunity to achieve this, sincere thanks should go to all the organizers and among them particularly to P.Felix-Henningsen and Thomas Scholten Special thanks are due to Drs a.Semmel, M.Weidenfeller and L.Zoller, that guides of the field-trips in the Rheingraben and surrounding.
About the Paleopedology Glossary
The best Glossaries are always open to additions. The 2nd draft distributed in Rauischholzhausen is certainly a thoughtful and an almost perfectly concise one, so that it is hard to come up with any additions. The more so that it was apparently preceeded by long brain-storming sessions of a community of soil scientists far more authentic in the matter of terminology than an average geologist would be.
After these intriductory remarks let me take the liberty and comment on some of the items included in it.
Geosols
In order to clarify the difference between a geosol and a paleosol perhaps it could be emphasized, that the term geosol refers to an assemblage of ancient soils rather then to any particular paleosol. The geosol is "...a whole soilscape that can be recognized as a laterally extensive stratigraphic horizon". (Retallack 1990, Follmer 1978, Morrison 1978 etc.) In addition ti the terms cumulic and compround perhaps also composite should be mentioned. This latter term was used by Wright and Marriot (1995) to describe the situation referred to as "Lateral merging af several geosols" in the Glossary.
Pedofacies
A reference to terms like catena, toposequence and chronosequence would be useful here.
Polygenetic soil
The last sentence of this entry ("Minor episodes of deposition may have occured between or within the periods of soil development, but were not sufficient to leave layers of unaltered material") seems to be simewhat out of place here. It could be added rather to the entry on geosols, where the term cumulic soil is explained.
Soil
Implict in the present definition it is included that sediments modified by pedogenic precesses to an extent which permits the recognition of the aleration also in the lithified sedimentary rock (e.g. fossilized root molds and/or associated primitive gley spots, etc.) but not displaying distinct horizontation (like weakly developed entisols for example) would not be considered as soil. The stratigraphic record developed counterparts would be the sources of valuable information for the geologist looking for the record of episodes of subaerial exposure within a shallow-marine or a fluvial sedimentary rock suite. In a study focusing to such paleosol sequences it would be very inconvenient to exclude these weakly developed soils from under the term soil, so it is highly recommended to adjust the definition (in the paleopedological context) to meet also the requirement of geology. The proposed new sentence would read as follows: "a three-dimentional body on the surface of the earth composed of mineral and/or arganic material, air and water, and formed by the impact of environmental factors acting on parent materials over a period of time to produce recognizable alteration of the parent material in some times resulting in the establishment of a sequence of horizons".
Soil sediment
As a synonym the term pedorelict (Retallack 1983, 1990) should be indicated. Also, reference to Ida Valeton's Laterite Derived Facies (LDF) could be made here (Valeton 1983 and Valeton & Wilke 1993).
Vetusol
This is a very loose category. Also some of the statements included in the definition may not be of overall validity.
Soils occuring a major regional unconformities (the geological term for long-lasting subaerial episodes recognized in the stratigraphic record) are very rarely "monogenetic" (if not rof other reasons, at least because of the addition of aeolian dust from sources other than their parent rock!). In fact the overwhelming majority of geosols recognized by geologists are known to be poligenetic in origin.
In the case of soils formed over very long periods of time, the absence of "detectably different assemblages of soils features" is very probably not the result of "landsurface stability" or of the "small variability of environmental factors" but rather of the inertia of the soil formed during earlier stages of the long history of the subaerial exposure phase. Certain well-developed soils (like oxisols or ferrasols) may not have the peogenetic potential to reacr to high-frequency climate changes having a duration of 20.000 years. As an example the case of Mesozoic shallow water carbonate sediments, and their intercalated bouxites (presumably representing several million years of exposure) could be cited (see also D'Argenio & Mindszenty 1995). Carbonate depositional environments being very sensitive to environmental changes were shown to record high frequency climate changes wihthin the intertropical belt (Fischer 1964, 1991; Goldhammer et al.1990, D'Argenio et al 1992 etc.). Such high frequency cyclicity detected in carbonate rocks of a given locality indicate that high frequency changes did take place in that area in the past. Bauxites (equivalents of oxisols) intercalated in the sequence generally do not record such high frequency changes, which does not necessarily mean that for several million years high frequency changes would have been stopped, Rather it means that the soil which during a number of those changes finally has riched the stage of an oxisol, was unable to reflect any more changes and gives the false impression of unchanged conditions throughout the period studied!
By now means the definition should suggest restrictions in time, like the remark in the last sentence referring to Quaternary climate changes. Climate changes as significant as the Quaternary ones have taken place several tmes during the Earths' history, so there is no reason for the discrimination.
With the above considerations in mind the definition of vetusols could be changed as follows:
"soils formed over a very long period of time (order of magnitude >105 years) during which (a) either the influence of pedogenetic and eventual sedimentary processes (like repeated reworking) has resulted in an assemblage of often polygenetic soil features which may not permit to inequivocally identify them as belonging to one or the other of the categories of Soil Taxonomy. In some cases, however, they may be identified as oxisols or ultisols or their partly resedimented rpoducts (like the LDF of I.Valeton).; or (b) vetusols may be produced also by long lasting conditions of stable land surface and minor or no variations of environmental factors. Most vetusols occur on old geomorphic surfaces in tropical and subtropical regions where the influence of high frequency climate changes was minor."
Classification of paleosols
Efforts to elaborate a reasonable classification of paleosols have been numerous. Following the first arttemps of Polynov (1927) whose four categories (secondary soils, two-stage soils, fossil soils and ancient weathering products) even if rather straightforward, were essentialy genetic categories, there was a general tendency towards a more phenomenological ckassification utilizing more and more directly the terminology of modern soil taxonomy of which the best examples are the classifications of Retallack (1990) and Mack et al (1993). The former simply adopted all the categories of the US Soil taxonomy for paleosols while the latter have modified it an ended up with only 9 paleosol clsses. This approach has two inherent shortcomings: (a) every attempt to categorize a given paleosol would necessarily be biased by the fact that most criteria used in modern soil science (like ion exchange capacity, soil moisture regime, etc.) instead of being directly measured, will have to be "guessed" on the basis of their supposed effects on the soil. (b) after having identified the paleosol as the fossil equivalent of its modern counterpart by using the above "guessed" criteria, the result will be highly temptating to be used for an analogy-based environmental reconstruction. Mental exrcises like this would obviously violate the rules of formal logic (conclusion used as a premiss in the same context).
May be a simple phenomenilogical classification, based in geological rather than pedological criteria and combined with the interpretation of the effects of those elementary pedogenic processes which do have a high enough preservation potential to survive throughout diagenesis and perhaps even incipient metamorphism would be the right solution.
To elaborate a geological classification, paleosols should be treated as sedimentary rock suites. The relationship of any sedimentary rock body to the sediments it is enclosed by is one of the crucial criteria of such a classification. Being purely phenomenological, this criterion has also the benefit of being very easy to convert to genetical terms.
With such an approach we may say that there are principally two kinds of paleosols
(a) is obviously something more or less equivalent to vetusols in the Glossary. It may be added, that most major subaerial unconformities are associated with mature paleosols like e.g. laterites, bauxites, terra rossa, silcretes and xertain calcretes. They may be partly or entirely resedimented, and may contain abudant pedorelics. These paleosols do not continually or uniformly cover quite all the former subaerial landscape, because when, after a break, sedimentation is resumed again, erosion may destroy part of the soils formed during the break. Also they may form paleo-catenae reflecting the underlying paleorelief and therefore useful in the paleotopographic reconstruction.
(b) Paleosols may occur in every sedimentary environment where deposition is more or less regularly interrupted by shorter or longer episodes of non-deposition and suaerial exposure. Environments like this are e.g. most aeolian and alluvial settings, delta-platforms, volcano sedimentary terrains and shallow-water carbonate platforms. Depending on the rate at which sedimentation resumes after an episode of interruption, these soils may be singles (=compounds), composites (=nerging) and cumulic (=cumulates, soil-sediment complexes) (c.f. Wright & Marriott 1996).
Intraformational paleosols are substantially different from those associated with major regional unconformities, not only because of the generally much longer duration of pedogenesis of the latter. At major regional unconformities the parent material, i.e. the substratum of soil formation, is generally a mature rock surface consisting of sediments already passed through all stages on near-surface and burial diagenesis, sometimes even of metamorphism. Their pedogenetic alteration can thus be considered as an epigenetic or telogenetic process. On the contrary, intraformational (or synsedimentary) paleosols often form on the surface of unconsolidated, sometimes even loose sediments, i.e. pedogenetic alteration in this case is essentially a diagenetic process taking place under meteoric conditions. In the first case, the beginnings of pedogenesis are necessarily restricted to joints and fissures, where plants may penetrate the hard rock substrate and the saprolite consists of fragmented hard rock displaying a distinct porosity increase as compared to the parent material. Intraformational pedogenesis, on the other hand, attacking the fresh, uncompacted sediment, penetrates the substratum along primary pores and forms a diffuse weathering front along which sometimes, due to early cementation, primary porosity is in fact plugged instead of being increased (cf.calcretes!). With time such cementative phenomena may result in lithification of the originally unconsolidated sediment and thus in a gradual change of the intraformational regime into that known from situations at major regional unconformities, i.e. early and late (intraformational and telogenetic) pedogenesis may overlap.
This simple twofold division is easy to combine with any one of the previously suggested detailed classifications (depending on the amount of information avaolable). Classifications with too many categories should be discouraged just as the forced analogy to modern soils is. The study of as many soil features as possible and their simple interpetation in terms of elementary soil forming processes would probably be the most fruitful approach - at least when paleopedological research is intended to contribute to the paleoenvironmental reconstruction.
What else?
Further collaboration of soil scientists and geologists to facilitate a betterunderstanding of past soil forming environments is highly desirable:
Analogy-based interpretation of fossil soil features observed in older-than-Pleistocene paleosols would be easier when geologists dealing with such old soils would have appropriate chronosequences and/or climosequences available from Pleistocene/Holocene settings studied by soil scientists.
The correct identification of paleosols as approximate equivalents of one or the other of modern soil taxa could be facilitated when the preservation potential of diagnostic soil features were known. Research focussed to improve our understanding of the fossilization of diagnostic soil features and the organization of further interdisciplinary workshop to discuss these problems should be encouraged.
References cited
D'Argenio, B.-Ferreri, V.-Iorio, M.-Longo, G.-Raspini, A. (1992): Early Crateceous eustatic oscillation under astronomical forcing and relkated problems in time calibration. The case of Southern Italy. In: CNRS-IFP Sequence Stratigraphy of European Basins, 406-407, Dijon, Abs.Vol.