Actualistic versus non-actualistic conditions in the Precambrian sedimentary record: reappraisal of an enduring discussion




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Actualistic versus non-actualistic conditions in the Precambrian sedimentary record: reappraisal of an enduring discussion


J.A. DONALDSON*, P.G. ERIKSSON- and W. ALTERMANN#


* Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, Ottawa, ON, K1S 5B6, Canada. E-mail, corresponding author (JAD): jadonald@ccs.carleton.ca


- Department of Earth Sciences, University of Pretoria, Pretoria 0002, South Africa


# Institut fŸr Allgemeine und Angewandte Geologie, Ludwig-Maximilians-UniversitŠt MŸnchen, Luisenstrasse 37, D-80333 MŸnchen, Germany


Running title: Actualistic conditions in the Precambrian


Abstract


Actualistic models can be successfully applied to virtually all sedimentary successions preserved in the Precambrian rock record, allowing for differences in the relative rates and intensities of those processes that control weathering, erosion, transportation, deposition and lithification. As the basis for this statement, we understand actualism to be the principle that the same processes and natural laws applied in the past as those active today. This definition readily accommodates events of even catastrophic character, such as bolide impacts and tsunamis. Non-actualistic models introduce semantic problems and are of little help in understanding geological processes. A review of the English and German literature on actualism reveals that many misconceptions can arise because of the two principal meanings of the term "actual": (1) real, factual; (2) present-day. However, if the full range of possible outcomes subject to natural laws are considered, rather than just the expression of these laws as observed in present environments, all Precambrian settings can be investigated and understood by applying the actualistic approach, which is fundamental but not unique to the geological sciences.


Precambrian sedimentary environments evolved in the absence of advanced life which significantly influenced the progress of sedimentation through Phanerozoic time. Yet even today, some unusual settings, such as hypersaline environments in which stromatolites thrive, closely resemble those that existed during the Precambrian. Terrestrial sediments accumulated in the absence of plant cover are strikingly different from those formed in environments with abundant flora. Present-day periglacial and desert environments provide excellent analogues for Precambrian fluvial and aeolian deposits. Several Precambrian glacial episodes have been clearly documented by comparisons to present-day and Pleistocene analogues, and even the unique conditions of the recently modelled snowball Earth can be understood from the actualistic point of view. Throughout the Precambrian the interplay of volcanic and sedimentary processes has produced strata that closely mimic present-day deposits. Normal effects of tides, ocean currents and gravity are demonstrable for many Precambrian successions, and a case for tidal ranges significantly different from the present-day range can be evaluated by comparisons to modern examples.


Concerns about actualistic/non-actualistic assessments are best illustrated by the problem of atmospheric and hydrospheric evolution. The Precambrian atmosphere and hydrosphere certainly obeyed the same laws of chemical equilibria that apply today, and thus the evolution of the atmosphere and hydrosphere can be evaluated by an actualistic approach. Although the rates and intensities of oxidation, reduction, hydration and carbonation have changed through time, their records in paleosols can be evaluated in the same way as modern soil profiles. Even the possibility of atmospheric density changes can be accommodated by wind-tunnel experiments, or by numerical modelling. Thus, all studies involving Precambrian sedimentary rocks can be approached by means of comparisons to present-day environments. Non-actualistic models should be reserved for speculations about early Hadean environments, cosmology (e.g. black holes, wormholes, origin of the universe), religion, and philosophical conjectures about ''parallel'' worlds (such as those inhabited by unicorns).


Keywords: actualism, Precambrian sedimentary record, relative intensities of natural processes


Received ; revision received ; accepted


Introduction


Throughout the history of geology, new terms have been introduced, old terms redefined or abandoned, and familiar terms assigned new meaning in the scientific literature. Sedimentological publications contain recent examples of evolving terminology, including: paradigm (example, pattern or model), architecture (arrangement, structure), model or system (complex entity). This paper deals specifically with two terms that have appeared in a number of recent papers on Precambrian sedimentology: actualistic and non-actualistic. Although the former is clearly understood, the latter is commonly used with meanings that seem to be at variance with the meaning initially assigned to it. This may stem from the various meanings assigned to the root word, actual (e.g. Sykes, 1982): existing in fact and not merely potential or possible; real (as distinct from ideal); current, acting at the present moment.

Some geologists, primarily sedimentologists, paleontologists and geochemists, in their attempt to emphasize differences between Precambrian and present-day conditions, landscapes, biotic communities and environments, have begun to refer to non-actualistic Precambrian settings, biota and sediments (e.g. PflŸger 1995; PflŸger and Gresse 1996). Although there is a need to draw attention to the numerous differences between the Precambrian and the present day, many are simply differences of degree, and we argue here that application of the adjective non-actualistic is confusing, at times misleading, and therefore inappropriate for the purpose of stressing different and/or unique events that occurred, and conditions that prevailed, during segments of Precambrian time. Where past paleoenvironmental conditions have resulted in the formation of rocks or structures for which modern analogues are difficult or impossible to find, terms such as unique or distinctive can serve to draw attention to inferences about conditions unlike those observed today. In light of the duration of the Precambrian Era for almost 90% of Earth's history, it is surprising how rarely such unique rock records have been preserved. Of course, the present surface and interior of the Earth do not necessarily reflect all past environments, because with time, new environments continuously evolved which were not present in early Earth history. Conditions prevailing today thus are the result of the evolution of our planet, an evolution constrained by invariant physical, chemical and biological laws. Furthermore, many important evolutionary events, such as the appearance of land plants, the emergence of grasslands and forests, the development of coral reefs, and the ascendancy of silica-secreting organisms, all took place during the Phanerozoic, thus significantly diminishing the value of using the term "non-actualistic" to stress differences between Precambrian and modern environments.


Definitions and related terminology


Actualism is not listed in all standard language dictionaries, but where it is, the meaning provided generally relates to its usage in philosophy (see Conclusions). Because of entrenchment in the geological literature, however, it appears in most geoscience dictionaries. For example, the Oxford Dictionary of Earth Sciences (Allaby and Allaby 1999) defines actualism as: "the theory that present-day processes provide a sufficient explanation for past geomorphological phenomena, although the rate of activity of these processes may have varied". Another example is provided by Challinor's Dictionary of Geology (Wyatt 1986): "Actualism was the geologic parallel to the 18th Century postulate of universal physical determinacy: invariance of the laws of nature through time and space; as stated by Holmes (1965) it is the principle that the same processes and natural laws prevailed in the past as those we can now observe or infer from observations". Thus, these two well-known sources provide definitions that correspond, in terms of natural laws and processes, to accepted definitions of modern uniformitarianism. Unfortunately, the widely used Glossary of Geology (Jackson 1997) defines actualism as: "the same forces presently in evidence, acting over time with energies and frequencies now observable, are sufficient causes of all geological results, relationships and configurations". The qualification: ''with energies and frequencies now observable'', which is at variance with the views of many authorities, including those presented in the papers reviewed below, may have unduly influenced the use of this term. In this regard it is interesting to note that Preston Cloud (1988), in his book, Oasis in Space, nowhere uses the term actualism, but instead described as fundamental to geology the "principle of natural causes". He argued that, averaged over sufficient time, all rates are essentially uniform, thus resolving the apparent conflict between uniformitarianism and catastrophism. In other words, given enough time, anything can happen that is consistent with natural law.

Paradoxically, although the definitions discussed above would appear to characterize actualism as the basis of all science, the term seems not to be used in other scientific disciplines. For instance, it is not listed in the McGraw-Hill Encyclopedia of the Geological Sciences (Parker 1991), nor in Van Nostrand's Scientific Encyclopedia (Considine 1995). Perhaps the principal doctrine of geology that the present is the key to the past is so self- evident in other natural sciences, that it does not require any definition. Physicists, biologists and chemists can continuously observe the phenomena of acting forces and reactions. The demonstrable reproducibility of scientific experiments is an indispensable prerequisite to acceptance of the validity of hypotheses arrived at through deduction. In geology, natural processes that tend to be extremely slow are commonly difficult to evaluate, and can rarely be directly observed. Their products, however, can be explained in accordance with natural laws by inductive reasoning, related as closely as possible to presently observed processes.

In more than 30 recent (post-1970) English texts on physical geology consulted for this review, all discuss uniformitarianism and catastrophism, but actualism is mentioned in only two (Dott and Batten1980; Judson and Richardson 1995). Dott and Batten (1980, reprinted from first edition, 1971), state that: "... (the) necessity for clear distinction of modern from Lyellian uniformitarianism is evidenced by the impact of neo-catastrophists, such as Immanuel Velikovsky, author of Worlds in Collision (1950), and Earth in Upheaval (1955). Such writers repeatedly misinterpret modern uniformitarianism, or more correctly, actualism. They take as authority Charles Lyell's (1830) principles, assuming that the strict uniformity (gradualism) expressed therein is still the guiding doctrine of geology. The problem is compounded by confusion of what is meant by catastrophic processes and by a lack of appreciation of geologic time. Geologists today routinely accept sudden, violent, and even certain unique events as perfectly consistent with contemporary Earth history. Only by substituting the term actualism can misconceptions be minimized.''

A similar definition of actualism appears in the earlier, widely used text by Arthur Holmes (1965), who stated that "actualism... conveys much more appropriately (than does uniformitarianism) the real meaning of Hutton's (1788) inspired appeal to actual causes: the principle that the same processes and natural laws prevailed in the past as those we can now observe or infer from observations".

One author of a physical geology textbook out of the 30 examined suggested (Hamblin 1991) that the word naturalism should take the place of uniformitarianism. However, the term naturalism does carry other meanings, as listed below from the Canadian Oxford Dictionary (Barber 1998):


- the theory or practice in art & and literature of representing nature,

character, etc., realistically and in great detail

- a theory of the world that excludes the supernatural or spiritual

- any moral or religious system based on this theory

- action based on natural instincts

- indifference to conventions


The first meaning is in accord with actualism, but the other four hardly provide a successor term to uniformitarianism that clarifies the essence of geological methodology.

In contrast to the scarcity of references to actualism in English physical geology texts, 18 of 30 recent historical geology texts consulted contain discussions of the term, and provide definitions that closely follow the Holmes (1965) and Dott & Batten (1980) definitions. For instance, in The Earth Through Time, Levin (1999) defines actualism as the principle that natural laws governing both past and present processes on Earth have been the same, and in Origin and Evolution of the Earth: Principles of Historical Geology, Condie and Sloan (1999) define actualism as "the principle by which modern geologic processes are used to understand ancient ones". The relationship of actualism to uniformitarianism is emphasized by Stanley(1989) in his Earth and Life Through Time: "The principle of uniformitarianism, sometimes called actualism, governs geologists' interpretations of the most ancient rocks on Earth". In addition, the Encyclopedia of Evolution (Milner 1990) defines actualism as the concept that "geological processes are governed by natural physical and chemical laws", attributing this definition to the scientist L.C. Prevost (1825). Thus, Prevost appears to have discussed actualism eight years before Lyell's expansion and promulgation of the Hutton-Playford enunciations on uniformitarianism, and 80 years before Geike's (1903) application of the phrase "the present is the key to the past" to uniformitarianism. As pointed out by Gordeev (1961) and by Muller et al. (1991), the methodology of actualism was introduced in this sense much earlier by M.V. Lomonosov (1763), and even earlier by Buffon (1749), who thus both preceded Hutton's application of actualistic concepts which were subsequently incorporated in the principle of uniformitarianism. Similarly, von Hoff (1822) accumulated substantial data to document actualism; in a review of publications and correspondence, Hamm (1993) argued that von Hoff's writings influenced Lyell's (1830) pronouncements on uniformitarianism.


In consulted German physical geology books (1970 to 2000), only one out of six does not include an explanation of actualism. Even in the German translation of Press and SieverÕs (1994) Understanding the Earth, the term actualism (Aktualismus) was added to the glossary, although it is missing in the English original. This contrast underlines the long-lasting debate on the principle of actualism in Germany. In German literature the rise of the idea of actualism is credited mainly to von Hoff (1822), who concluded that all presently active forces can adequately explain all changes on Earth's surface, especially the migration of land and sea, if unlimited time for Earth history is provided. Von Hoff supported this conjecture by direct observation of processes and with an extensive collection of geological examples, without developing a theoretical framework around it. Therefore, some German authors distinguish between von HoffÕs Òactualistic methodÓ and LyellÕs Òprinciple of actualismÓ (e.g. Schwartz 1978). According to this dichotomy, the principle of actualism proclaims that throughout Earth history, all processes and forces have remained unchanged, and therefore no other causes and effects have been active in the past than those observed in the present. The principle, as thus interpreted, attempts to explain all geological phenomena in terms of present-day processes.

From the twenties on into the forties of the 20th Century, a wide and prolonged discussion of actualism transpired in geological publications in the German language. Strict followers of the principle, as defined above (e.g. Salomon 1926, who demanded that only processes observable today can be accepted for the past), were soon succeeded by subscribers to the actualistic method who allowed for new ideas and new observations not necessarily applicable to every period of Earth history. The most powerful and influential among them were Erich Kaiser (1931) and von Bubnoff (1937). Kaiser discussed pre-vegetational Earth environments, concluding that primitive Earth deserts must have included humid climatic realms, with different sedimentation and erosion patterns than are known from recent examples. Von Bubnoff questioned whether the sediment accumulation rates observed in modern environments are applicable to old sedimentary rocks, which he inferred to have accumulated at slower rates in early Earth history. However, von Bubnoff did not question the universal validity of the physical processes responsible for sedimentation (e.g., Fig. 1).

Discussion of actualism as a principle was also triggered by observations that glacial and interglacial periods are not the ÒusualÓ state in Earth history, but are simply exceptions from long-lasting warmer time intervals. Accordingly, Kummerow (1932) concluded that the present is not the key to the past, and that actualism is not a valid principle applicable to periodic events. Strangely enough, the same phenomena (e.g. the climate of the Tertiary) served Lyell as argument for his ÒPrinciples of GeologyÓ and his discussion of Ò...how far the former changes of the earth surface are referable to causes now in operationÓ. The discussion went so far as to construct a contradiction between actualism and the historical character of the geological sciences, whereby the followers of actualism were accused of dogmatism. For example, Beurlen (1935, and several subsequent papers), proclaimed that actualism was non-historic and dogmatic. In his criticisms of actualism, Beurlen (1935) applied the political ideas of national socialistic race philosophy, extending his arguments and those of his followers ad absurdum.

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