Full paper:
http://www.geosci.monash.edu.au/precsite/docs/workshop/moscow07/transaction.pdf
82.2mb
|
M.A. Fedonkin1, B.S. Sokolov1 M.A. Semikhatov1,
N.M.Chumakov2
|
Philosophy and methodology of stratigraphy is based on a long-term experience of the Phanerozoic research and on the essentially European tradition. Immense length of the Achaean and Proterozoic geological record, the duration of which was particularly realized after introduction of the radiometric dating of the rocks, became the major challenge for the stratigraphers and the earth scientists in general. Long lasting and seemingly poor fossil record, domination of the problematic organic remains of uncertain nature, loose bases for a stable systematics and nomenclature of the fossils made a strong contrast to rich fossiliferous strata of the Phanerozoic deposits. This contrast compelled the geologists to rely upon the non-biological stratigraphic approaches to the Precambrian sequences.
Methods of historical geology, for example, lithostratigraphy, tectonics and climatostratigraphy, added later by the radiometric dating and supported later on by the Hi-Tech chemostratigraphy and magnetostratigraphy, formed the base for the Precambrian chronostratigraphic chart. However, being effective at the the scale of regional geology, the whole set of the methods mentioned above can not help to overcome the complex of inferiority, every experienced stratigrapher (we mean the Phanerozoic experience) should feel: none of these methods gives something even distantly similar to the biostratigraphy at the global scale. The fundamental, quality difference between the biostratigraphy and all other abiotic approaches to the periodization of the geological record is the uniqueness of a signal the biostratigraphy deals with. This unique signal is a biological species with its recognizable image, specific environment, limited time range of existence, and its place in the evolutionary or phylogenetic succession.
Classical biostratigraphy has proved its effectiveness and reliability during two centuries of practical work in the Phanerozoic interval of the geological record. Irreversible process of the biological evolution marked by a succession of the fossil species makes a solid frame for the stratigraphy and opens enormous possibilities for more detailed elaboration of the scales and high precision of the correlation with the help of all other methods available at present. So, the key rule of the classical stratigraphy is to establish a succession of unique signals (biological species) and then to trace it laterally using all possible characters, methods and techniques according to the Principle of the Chronological Interchangeability of the Characters formulated by S.V. Meyen).
The Precambrian stratigraphy in many cases could not follow this principle in the absence of the fossils. But the non-biological stratigraphic methods alone, including the modern ones with their topmost precision, could not replace the biostratigraphy: neither the geochemical signals (such as a carbon isotope ratio) nor the geophysical ones (such as paleomagnetic orientation) are time-specific or time-relevant. The sets of these signals have much in common with the borehole logging in regards of bitumen, gas, magnetic susceptibility, electrical conductivity, redox potential etc. But the borehole logs can not be, and are not used as the geochronometric instrument. Nevertheless, recent practice of the Precambrian stratigraphy demonstrates growing role of the non-biostratigraphic methods that built some sort of the logging traces without much care of the pitfalls of theses methods, for example, their inability to detect the hidden time-gaps in the sedimentary sequence or the actual magnitude of the signal distortion by the postgenetic processes.
But is the biostratigraphy hopeless indeed in the realm of the Proterozoic? Modern paleobiology shows that duration of life on Earth exceeds the time span of the preserved geological record and that most of the geological record is actually fossiliferous (Schopf, 1992; Sergeev et al., 2007). Even the prokaryotic fossil microorganisms demonstrate some trends such as an increasing cell size, diversity, and morphological complexity through the Achaean and Proterozoic (Schopf, 1999; Knoll, 2003). As to the eukaryotic fossil record that seemed to start in the late Achaean, it is becoming a recognizable stratigraphic instrument only Mezoproterozoic and later on (Yakshin, Nagovitsin, Faizullin, 2004; Butterfleld, 2000, 2004). Particularly fast evolution is demonstrated by the planktonic unicellular microorganisms that inhabited the open marine environments (Sergeev, 2005 and references therein). Even the megascopic eukaryotic fossils are becoming the noticeable time markers from 1.9 Ga (Hofmann, 1994; Fedonkin and Yochelson, 2002) and later, particularly, after 850 Ma ago (Fedonkin, 2003; Porter, 2004).
The late Neoproterozoic, especially the time interval corresponding to the Vendian/Ediacaran, was a period of dramatic change in biota related to the fast growing importance of the eukaryotic organisms, global expansion of the megascopic life forms, increasing biodiversity in terms of morphology, physiology and life style, and growing complexity of the ecosystems. All these events have been accompanied by the contrast climatic changes, increasing oxygenation of the atmosphere and ocean, fast paleogeographic changes etc.
The Vendian biota with its very special characteristics represents its transitional nature. It was transition from the archaic ecosystems dominated by the prokaryotic organisms to the modern style ecosystems in which a key role belong to the eukaryotes. The further strategy of the Proterozoic stratigraphy depends very much now on our choice: 1) shall we continue the study of diverse and abundant fossils in order to understand their nature, ecology, systematics so that this rich and invaluable information could serve as an instrument for the division and correlation of the sedimentary sequences; or 2) we neglect these data accumulated by the generations of paleontologists and will continue our exercises with non-biological techniques and methods. One should not forget though that the latter techniques and methods are becoming meaningful and effective indeed in the well-defined biostratigraphic framework.
Representatives of the Russian geological school develop for decades the Phanerozoic (chronostratigraphic) approach to the division of the Proterozoic. They were the first who applied the biostratigraphic methods to the division and correlation of the Riphean and Vendian deposits on the basis of the organic-walled microfossils (papers by Timofeev, Volkova, Yankauskas and other) and stromatolites (see the papers by Krylov, Komar, Serebryakov, Raaben, Semikhatov and other). This approach was accepted and developed in a number of other countries. At present the paleontological study combined with that from sedimentology, lithochemistry, chemostratigraphy and radiometric dating are forming a basis for the system approach to the geological record. This multidisciplinary approach is documenting the changes both in biota and in the environment during the extended time intervals thus creating the most powerful instrument of the Proterozoic stratigraphy.
The late Neoproterozoic, in particular, the lime interval that is equivalent to the Sinian, Vendian or Ediacaran, makes a strong and happy contrast to the most of the Proterozoic fossil record. On the background of the dramatic decline of the stromatolites one can see the growing domination of the eukaryotic organisms among the primary (photosynthesizing) producers, acceleration of the evolutionary processes and of the species renewal, global expansion of the megascopic multicellular animals and algae in marine environments, increasing heterotrophy, biofiltering, bioturbation, and biomineralization. All these processes exerted tremendous influence upon the global biogeochemical cycles, sedimentogenesis, ocean and atmosphere chemistry and climate. The Vendian was a culmination period in the process of transition from the archaic biosphere to the modern one. Long term paleontological studies of the Vendian of the Eastern-European Platform (Sokolov, 1997), for instance, the successions in Ukraine, White Sea, and more recently, in the Urals revealed a uniquely rich fossil record. This record includes abundant microfossils, megascopic algae, metazoan fossils, and ichnofossils. Every fossil group demonstrates its space-time distribution pattern which, in addition to the evolutionary factor, is controlled by a number of factors related to the paleogeography, paleoecology, taphonomy and paleoclimate. These factors being under the systematic multidisciplinary study reveal the fossil record as interplay of biotic and abiotic processes and create a causal determinacy of the Vendian stratigraphy at the regional and global scale. The Vendian in its type area consists of large subdivisions such as Laplandian, Redkino, Kotlin and Rovno Regional stages (Sokolov and Fedonkin, 1984; The Vendian System, 1990). Three latter stages have distinct paleontological characteristics which were essentially enriched during a couple of last decades. The data accumulated up to now lets us establish the lower rank biostratigraphic subdivisions that can be traced globally. The task of the stratotype selection for the Vendian subdivisions and their boundaries can be put now on the agenda on the Ediacaran Subcomission of the ICS. Following the principle that the subdivision boundaries make the boundaries of the geological system, the problems of the upper and lower boundaries of the Vendian can be considered as well.
The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian (Sokolov, 1952, 1956, 1997). Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern-European Platform, Sokolov, 1965) and for the carbonate basin (base of the Tommotian Stage of the Siberian Platform, Rozanov et al., 1969). The body fossils only were involved into the boundary definition. The GSSP of the lower boundary of the Cambrian on the SE Newfoundland approved by the International Commission on Stratigraphy as a preferred alternative to the base of the Tommotian Stage in Siberia was selected on the basis of the ichnofossils. In the history of stratigraphy it was the first case of usage of the bioturbations for the System boundary definition. This decision has created enormous difficulties for the stratigraphers (Rozanov et al., 1997). The point is that the etiological (behavioral) and paleoecological variations of the bioturbations make their systematics difficult and loose (Jensen et al., 2000). Beside, in the most cases an ichnological taxon can not be assigned to any of the known body fossil. Organisms that belong to different taxa often produce similar or identical bioturbations related to their basic functions. Most of ichno-taxa have a long time span and the highest degree of the paleoenvironmental control. As to the Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky because of occurrence of very similar trace fossils belonging to the Treptichnids group well below the T. pedum in Namibia, Spain and Newfoundland (Gehling et al., 2001), and possibly, in the west of USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain. The return back to the Siberian standard of the Vendian/Cambrian Boundary based on the stratigraphic succession of the distinct assemblages of the mineralized skeletal remains (small shelly fossils) seems necessary and inevitable (see Khomentovsky and Karlova, 2005).
Lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites (for references see Sokolov, 1997). Here below we would like to consider the correlation of the Vendian and Ediacaran, in what degree their geobiological contents overlap or coincide. This comparison may help to develop the strategy planning in terms of research and international collaboration. However, there are some obstacles related to the Ediacaran concept.
The formal recognition of the Ediacaran System and Period and designation of the Lower boundary GSSP for this system at the base of the cap carbonate (Nuccaleena Formation), immediately above the Elatina diamictite in the Enorama Creek section, Flinders Ranges, South Australia was approved on February 16th, 2004 by the International Commission on Stratigraphy and ratified by I.U.G.S. on 19 March (IUGS E-Bulletin, March 2004). However, in spite of the democratic vote, this decision contradicts to the fundamental principles and clauses of the International Stratigraphic Guide, to the recommendations of the International Geological Congress in respect of the stratigraphy (Montreal. 1972), as well as to the stratigraphic tradition that has proven to be effective for more that two centuries of the practical work.
1) The definitions of the lower and upper boundaries of the Ediacaran are based on different principles: biostratigraphic (though non-classical, paleoichnoiogical) substantiation of the upper boundary, and non-biological (basically, lithostratigraphic!) approach to the lower boundary.
2) The Ediacaran does not have a hierarchical structure of its internal subdivision (series, stage, zone) of which any Geological System must consist.
3) The procedure of the Ediacaran substantiation was right the opposite to the recommendation of the International Stratigraphic Guide (1994, Second Edition, p. 82): "A primary step in refining the definition of a system is to decide just what stages and series are to be included into in the system. The definition of these component stages and series then automatically defines the system and its boundaries". The reason why the proponents of the Ediacaran did not follow this rule may be explained by the fact that the reference section of the Ediacaran is not subdivided into stages and, probably, does not have a prospective for that so far.
4) The substantiation of the Ediacaran System via GSSP of its lower boundary based on non-biostratigraphic approach, contradicts the recommendations made by the international Geological Congress in Montreal, 1972. The IGC recommended the biostratigraphic principle of definition of the stratigraphic boundaries between and within the Geological Systems (see Martinsson A. (Ed.). The Silurian-Devonian boundary. IUGS, Series A, No. 3. 1977).
5) International Stratigraphic Guide strongly recommends that the boundary stratotypes of standard global chronostratigraphic units should be located within marine homofacial fossiliferous sections where the faunal or floristic elements are as diverse as possible (Murphy & Salvadore, 1999). From that widely accepted point of view, the sharp facial change at the contact between Elatina diamictite and Nuccaleena cap carbonates makes this level a "worst possible choice" for any stratigraphic boundary as it is indicated in the International Stratigraphic Guide (Salvador 1994, pp. 90-91). The base of the Nuccaleena cap carbonate is a bad choice for a number of other reasons.
6) Like any kinds of rock, cap carbonates cannot serve as a time marker for it is not unique in the geological record. The number of Neoproterozoic glacial events is in question, and there is always a danger of misidentifying the base of Ediacaran in other regions.
7) Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually the siliciclastic sediments replace laterally the cap carbonates in a rather short distance.
8) Cap carbonates do not occur above every tillite elsewhere in the world. Very often the same kind of carbonates occur inside the glacial deposits (like the beds of dolomite inside the marino-giacial Tanin Formation, Middle Urals, according to Chumakov, 1992) so the presence of the cap carbonates does not necessary mean the end of the glacial period.
9) Intercalation of dolomites with the marino-glacial deposits (such as Koiva Formation) indicates a few episodes of the carbonate precipitation in the coldwater basins, not a single-act event during and after glaciation (Chumakov, 1992).
10) Cap carbonates are normally non-fossiliferous and, thus, unsuitable object for biostratigraphic dating and correlation.
11) Any of the geochemical markers such as an isotope ratio can not be considered a reliable and time-relevant signal. The Sr- and C-isotope chemostratigraphic characteristics obtained for contemporaneous cap carbonates in different parts of the world may be variable in a wide range owing to different degree of secondary alteration of carbonates, dissimilar criteria used for selection of the least altered samples, and, as far as the C-isotope data are concerned, due to primary lateral variations of δ l3Ccarb in the upper layer of the ocean.
12) There are not any prospective for the radiometric dating of the Nuccaleena cap carbonates because of absence of suitable minerals such as the zircon form the volcanic ash beds. Without a direct radiometric dating on the Nuccaleena cap carbonate, no real certainty as to which of the several Neoproterozoic glacial events the Marinoan represents.
The time interval for the Marinoan glaciation is determined so far very loosely between 660 and 600 Ma (Kendall et al., 2004; Calver et al., 2004; Zhou et al., 2004). The radiometric age of zircons from the lower part of the Doushantuo (628.3±5.8 Ma) in Southern China is often considered as the end time of Marinoan glaciation (Yin Chongyu et al., 2005), however this opinion is hard to prove. Global correlation of the Neoproterozoic glacial periods (MacGabhann, 2005) shows four distinct glaciations but their duration is determined with a wide range of uncertainty (RU indicated in Ma): Gaskiers - 585-582 Ma (RU 3), Marinoan - 660-635 Ma (RU 25), Sturtian - 715-680 Ma (RU 35), Kaigas - 770-735 Ma (RU 35). In the same paper the author suggests to consider the Vendian as the Upper Ediacaran, and the lower boundary of the Vendian to be put at the level that marks the end of the Gaskiers glaciations. However, the precise time correlation of the Gaskiers tillites and the Waranger (Laplandian) glacial deposits is yet to be established.
Neither International Stratigraphic Guide nor the scientific traditions require the global stratotype for a System. And there are no such stratotypes for the Phanerozoic systems. Nevertheless, the type section for the Ediacaran in the Flinders Range, South Australia is considered now to be the standard section (or the major reference succession) for the Terminal Proterozoic System (Period). From the point of view of the global stratigraphy it is the worst of any possible choices.
The type section of the Ediacaran consists of about three kilometers of practically non-fossiliferous quartzite between the cap carbonate below and the base of the famous Ediacara Member (about 100 meters thick) at the very top of the section. So the most part of the Ediacaran type section is non-informative in terms of biostratigraphy, but not only in this respect. This section doesn't contain extensive carbonate succession or ash beds that would provide additional means for C-isotope correlation and radiometric dating.
The Ediacara Member, rich in the soft-bodied metazoans, represents very short time interval of a long history of animal life. This is contrasting to the extensive fossiliferous sections in Russia, Ukraine, Canada and even Namibia. Long experience of the detailed sedimentological, stratigraphic and paleontological research of the Ediacaran, made by prominent Australian geologists and by many foreign specialists leaves no much hope that the situation may be improved in terms of new fossiliferous levels and biostratigraphy in the future. Thus, the Ediacaran succession in Flinders Ranges has no distinct time-markers (except the topmost Ediacara Member) and has no prospective for detailed subdivision of the Ediacara System, for the dating and correlation. The section seems to be hopeless in terms of further progress in the Neoproterozoic stratigraphy.
Conclusions
All the above suggests that it was premature to approve the Ediacaran as a new Geological System and Period. Formally the Ediacaran does not fit to any of the stratigraphic categories recommended by the International Stratigraphic Guide in terms of the stratigraphic nomenclature. The Ediacaran has no internal structure of the subordinate unites. Its upper boundary (defined paleoichnologically) is blurred or, rather, uncertain. The identification and correlation of the Ediacaran lower boundary beyond its GCCP can not be realized because of absence of the time-relevant characters in the Nuccaleena cap carbonates. The approval Ediacaran puts the Proterozoic stratigraphy in the state of crisis that directly affects many areas of activity in Earth sciences and applied geology (from the stratigraphy and geological mapping to the paleotectonic and paleogeographic reconstructions).
Type section of the Ediacaran makes sharp contrast to the extensively fossiliferous successions of the same age in Newfoundland, Namibia, Russia etc. These sections being extremely rich in diverse and uniformly distributed fossils provide opportunities for the biostratigraphic substantiation of the Terminal Proterozoic System via its subordinate units (subdivisions). Progress in the multidisciplinary study of the Neoproterozoic during past few decades demonstrates that principles of the Phanerozoic chronostratigraphy can be effectively applied to the late Neoproterozoic marine deposits, and this experience has to be considered in the substantiation and definition of Terminal Proterozoic System and its subdivisions. Extensive paleobiological data from the Terminal Proterozoic that were obtained in Russia, Namibia, Australia, Canada and elsewhere opened a possibility to expand the realm of the biostratigraphic method down to the uppermost Proterozoic. It is at least non-practical that these data have been ignored in the process of the Ediacaran substantiation. In part it may be related to the characteristics of the type section of the Ediacaran.
Over two decades ago, the Redkino, Kotlin and Rovno regional stages have been substantiated in the type area of the Vendian on the basis of the abundant organic-walled microfossils, megascopic algae, metazoan body fossils and ichnofossils (see English version of 2-volume monograph "The Vendian System" edited by Sokolov, Iwanowski, and Fedonkin, 1990). Since then abundant paleontological and stratigraphic data has been collected and a series of radiometric dating has been obtained (some are presented in this volume). These data let us put on agenda the substantiation of the detailed biostratigraphy of the Vendian (Fedonkin et al., 2003, 2007 and this volume) with the globally traceable subdivisions and their boundaries, including its lower one. The lower boundary of the Vendian could have a biostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acantomorph acritarchs (some of which can be metazoan eggs or egg cases) documented in Central Australia, north-east parts of the Eastern-European Platform, central Siberia, in China and other regions (see the papers in this volume). The time range of the Pertalataka microfossil assemblage as well as the age of the Doushantuo phosphatised metazoan embryos and eukaryotic algae certainly predate the typical Ediacara fauna and associated microfossils.
We suggest that the study of the geobiohistorical content of the Upper Proterozoic should be further developed on the international basis, including the IGCP and ICS, in order to identify the globally recognizable hierarchy of the biostratigraphic unites. Results of this work will make a firm ground for definition of the time range and the boundaries of the system. The Vendian sequences of the Eastern-European Platform (purely siliciclastic paleobasin) and of the Siberian Platform (carbonate paleobasin) offer the best possibility for characterizing biochronologically, chemostratigraphically (C- and Sr isotope methods) and radiometrically the terminal part of the Proterozoic record, thus providing a framework for global correlation and further detailed elaboration of the Neoproterozoic chronostratigraphic scale with the growing arsenal of other methods.
The study is supported by the Russian Fund for Basic Research (Grant We 05-05-64825), The President Program "Scientific Schools of the Russian Federation" Grant ¹ 2899.2006.5), Program 18 of the RAS Presidium and IGCP Project 493 (UNESCO).
