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Chapter 12.1 Geophysical Inferences from Stromatolite Lamination
12. CONTRIBUTIONS TO THE HISTORY OF THE EARTH-MOON SYSTEM
Chapter 12.1
GEOPHYSICAL INFERENCES FROM STROMATOLITE LAMINATION Giorgio Pannella
INTRODUCTION
In the preceding pages different aspects and uses of stromatolites have been emphasized. Potentially, if not practically at the moment, the use of stromatolites as recorders of geophysical and astronomical conditions of the Earth during its evolution could be even more important. Since growth of stromato!ites is a direct outcome of environmental dynamics, it has been suggested that growth structures could be used to check geological changes in the speed of the Earth’s rotation (Runcom, 1966; McGugan, 1967; Pannella et al., 1968; Pannella, 1972a) and to determine the amplitude of paleotides and polar shifts (Cloud, 1968a; Nordeng, 1963; Vologdin, 1963, 1964; Cloud, 1968a). Fossil organic structures which may provide the first type of information have been called paleontological clocks (Runcorn, 1966). In order to separate the effects of changes in the Earth’s moment of inertia from those due to tidal dissipation, a paleontological clock must provide, from continuous sequences, two sets of data: one on the length of the sidereal day, the other on the length of the synodic month (Runcom, 1964). The crucial and controversial question with regard to the use of stromatolites as clocks is with what precision and continuity stromatolites record, during growth, the astronomically driven environmental fluctuations and how much of this record is left for us to decipher. For several reasons, which will be mentioned in the following pages, stromatolites, in general, are not as reliable paleontological clocks as other fossils. Hofmann (1973) attributes to them either zero or a very low degree of reliability. In this paper it is not intended to take an opposite and over-optimistic view of these applications because stromatolitic growth records in most instances are so incomplete as to justify Hofmann’s conclusion. However, in exceptional instances and with the help of statistical techniques, stromatdites are going to provide precious insights into the largely unknown Precambrian. The strongest reservation to these applications of stromatolites depends on the danger of extrapolating from one localized situation to world-wide generalizations. Another reservation is the uncertainty in respect to determining the absolute age of Precambrian material.
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G. PANNELLA
Notwithstanding the many difficulties and the rather tenuous results of the first naive attempts, there is ample room for improvement and, perhaps, the ultimate goal of tracing in broad lines the Precambrian evolution of the Earth-Moon system, is not too far in the future. The geologist and the paleontologist with the help of stromatolites will undoubtedly contribute.
STROMATOLITES AS RECORDERS OF PERIODICAL ENVIRONMENTAL CHANGES
The modem analogs of stromatolites provide insight and models of their growth. Light stimulation, by controlling metabolism, plays an essential role in the growth of stromatolites. Short-term experiments, extending from a few days t o two weeks, have shown that laminae of many modem stromatolites form with a daily frequency because of light stimulation of algae or bacteria. Monty’s experiments, in which Bahamian algal oncolites formed laminae consisting of organic-rich (deposited during the day) and inorganicrich lamellae (deposited at night), suggest daily growth is a basic feature of some stromatolites, but add little to the question of the accuracy and continuity of stromatolitic sequences (Monty, 1965b, 1967). Monty noted that there is size limitation of growth for the oncolites which are carried away after about two months growth. Gebelein (1969) described daily layers of about 1mm thick from subtidal algal oncolites from Bermuda and attributed them to the day-night algal growth cycle. He also emphasized the role of currents, and of velocity and rate of sediment movement in the building of stromatolitic structures. Currents over 20 cm/sec d o not permit the formation of algal mats, and only bottom sediment movement of less than 60-80 g/h per foot allows mats and domes t o accrete. The thickness of lamination is inversely proportional t o current velocity. Tidal fluctuations, which control current velocity and direction, can thus leave a record in sequences of laminae. In Bermuda, for example, more sediments are transported at flood tides than at ebb tides (Gebelein, 1969). In intertidal algal mats high-tide flooding creates sedimentrich lamellae, the thickness of which is proportional to the period of flooding, whereas organic-rich lamellae are related to low tides (Gebelein and Hoffman, 1968). The relationships between tidal fluctuations and stromatolitic deposition in the intertidal zone could produce, in areas of semidaily tides, two laminae per day. The possibility that this could reduce the accuracy of stromatolites by producing more laminae than days is not relevant here since intertidal stromatolites, limited and discontinuous in their growth, are unlikely candidates for geochronometry. But the tidal mechanism of stromatolitic growth must be kept in mind when interpreting sequences of laminae. Whereas flooding and air exposure are the controlling parameters of
HISTORY OF THE EARTH-MOON SYSTEM
675
TABLE I Hypothetical frequencies of periodical growth sequences in modem stromatolites Growth-pattern frequencies Tidal bands/year
2 variable Y 0
U
I
d
Daily increments/tidal band 1- 3 (only exceptionally higher)
7 8 - Extreme - - - -high - -water - - of spring - - - tides --- ---
m--
a
< 53-7
the frequencies are highly variable depending on tidal types, diurnal inequality
< 20
4-10
columnar stromatolites and algal mats are common forms
;
z1 rn
laminae are rarely daily; discontinuous sequences ; algal mats are common forms
< 12
a - - - Extreme - - - -low - -water - - of- spring - - -tides - -- -
2
Remarks
<25
< 14
--
laminae in most instances are deposited with daily frequency ;continuous sequences may form; large dome and digitate stromatolites are the most characteristic forms
lamina formation in the intertidal zone, subtidal stromatolites, unfortunately still poorly known, are affected by both light-dark cycle and tidal effects on sediment supply and are the only promising paleontological clocks. In addition the relatively deep-water ones are less subject to erosion. Other works show that not all laminations are daily. Supratidal algal mats grow only during floodings, the frequency of which depends on many factors: tidal amplitude, height of the algal mats above mean low water, and storms (Hardie and Ginsburg, 1971). Generally the higher is the position of the mats the more discontinuous is the record. Theoretically, and apart from any post-depositional disruptive action, the number of laminae per tidal cycle (Pannella, 1972b) is a direct function of tidal floodings, which, in turn, are a function, at any given point, of tidal amplitude and tidal type. Table I represents the theoretical frequency ranges of periodical growth patterns, based on these assumptions: (1)a lamina records a 24-h dark-light cycle, or an episode of tidal flooding (which can occur with frequencies ranging from 12.42 h up to six lunar months), or a mixture of both tidal and solar cycles; (2) deposition occurs when algal mats are covered with water, when there is a proper sediment supply (in allochemical growth), and during metabolic
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G . PANNELLA
activity of algae or bacteria (in allochemical and orthochemical growth). The Table has obvious limitations because it applies only to instances of a constant accretion, which are extremely rare in nature, and does not include the very common random noise and disruptions, but it provides consistency to the apparently contradictory and spotty data on lamina formation. One of the weaknesses of any generalized model of stromatolitic growth lies in the fact that, in the shallow-water marine environment the same parameters involved in growth patterns may also be the cause of their destruction. This fact, together with the tendency, in the intertidal zone, to form incomplete sequences, led to the conclusion-at the time when stromatclites were thought to be only intertidal - of the unsuitability of stromatolites as paleontological clocks. From Table I it is clear that ideal growth sequences can only form in a subtidal environment, where the light-dark cycle is still effective and where physical disruption is less likely. In the modem situation, however, even a subtidal environment cannot produce ideal sequences because of biological disruption. One has to look for a special habitat where metazoan life is impossible. Hence the importance of data on growth patterns of hot-spring stromatolites and stromatolite-like deposits. From the experimental data obtained by marking stromatolites in the Yellowstone hot springs it appears that algal and bacterial stromatolites there grow by daily addition of laminae as in the Bermudan stromatolites. The growth record, however, does not appear to be continuous or similar in all columns even in the same spring. The change of flow patterns gives to each column an individual flood and, thus, growth history. In conical forms the laminations appear to be “crudely diurnal” (Walter et al., Ch. 6.2). Laminations in nonbiogenic “stromatolites” (geyserites) have periodicities that appear to be related more to geyser activity than to diurnal environmental variations. Their number is highly variable as shown by Walter’s figures in this book (p. 106). It will be interesting to confront these figures with continuously monitored variations in the pools and t o compare lamination periodicities in stratiform geyserites in the same pools. All these works on modem stromatolites demonstrate that while diurnal laminae are common the continuity of the growth record is restricted to short time intervals. For the formation of ideal sequences an environment must meet the following conditions: (1)be aqueous, quiet, and with some physicochemical parameters oscillating regularly and periodically with daily fixed frequencies; (2) be affected by tides but not subjected t o strong storm waves; (3) be populated by algae or bacteria with diurnal growth and which cause precipitation of a preservable layer; (4) be lacking in other biological systems which cause disruptions. Such an environment containing stromatoIites, with perhaps the exception of hot-spring pools, is not found in today’s world. In the Precambrian, however, this favourable environment may have existed.
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FOSSIL STROMATOLITES
Considering that we are living in unusual geologic-climatic conditions, extrapolations from modem analogs to Phanerozoic stromatolites are dangerous. From modem analogs to their Precambrian ancestors, however, there is a “Quantum jump”, because of the time involved and of the biological and geological evolution of our planet. The Precambrian geologic record with its impressive sequences of rhythmites and stromatolites creates the inevitable feeling that in pre-metazoan time environmental rhythms were written undisturbed in the rocks, that the Precambrian is the ideal locus of rhythmometric studies. Strangely enough, no in-depth study like that of Sander (1951) exists for the Precambrian.
Paleontological clocks
A survey of many fossil stromatolites has led to the conclusion that most of them, in fact all of those studied except one, could not provide reliable data for reconstructing the length of the day and of the synodic month in the Precambrian (Pannella, 1972a, and unpublished data). Sequences are incomplete and show unmistakable signs of interruptions and erosion (Fig. 1).In their general morphology most of the studied stromatolites are similar to modern Shark Bay stromatolites. Pannella (1972a) has pointed out, and further observations have confirmed, that small, digitate columnar stromatolites show generally longer sequences and a higher number of laminae per periodical band than large columnar ones. Much evidence strongly suggests that they were formed in subtidal environments and they are similar to others reported as subtidal (Playford and Cockbain, 1969; MacGregor et al., 1974). Very small digitate types are present in stromatolitic beds of the 2,000 m.y. old Biwabik and Gunflint formations. One type, found on the western flank of Mink Mountain in the Thunder Bay area in Ontario, has yielded the most promising sequences of laminae, and if further study confirms the finding, is going to provide the first clues on the length of the synodic month and the year of Huronian times (Pannella, in prep.) Using morphological criteria Walter (1972b) has suggested that some Mink stromatolites are nonbiogenic geyserite-like deposits very close to those found in Yellowstone. This hypothesis does not contradict my results nor change the interpretation of the periodicities in the lamination sequences. Harmonic analysis of the lamination spacing indicates strong tidal control of the lamina deposition. The possibility of a tidal effect on geyser activities is mentioned by Walter (Ch. 3.3 herein). Earth tides are more strongly felt nearshore because of the direct contribution of marine tides (Melchior, 1966). In the nearshore setting where the Gunflint stromatolites grew tides must have represented the strongest periodical force.
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G. PANNELLA
Fig. 1. Polished surface of Great Slave Lake stromatolites showing distinct seasonal changes in color and composition of growth bands. Light bands contain thicker laminae than dark bands and were deposited during periods of fast growth (possibly warm and/or dry season). Dark bands show high organic content and are preferentially deposited over micro-unconformities indicating erosion (due, perhaps, to “equinoctial” storms) of the light bands. The filling between columns contains large fragments of light-band laminae further supporting the idea that the energy in the environment increased after the fastgrowing periods. The numerous interruptions in the sequences make this type of stromatolite (similar to the modem ones found in Shark Bay) unsuitable for geochronometry. w = slow-growth bands; s = fast-growth bands; d = major discontinuities in growth sequences. Scale 1 cm.
While the analytical procedures and the detailed discussion of the sequences will be the object of another work, some results must be mentioned here for they justify the optimism in the use of stromatolites for deriving geophysical information. Preliminary results of the study of the Mink stromatolites and of the Biwabik Formation which contains stromatolites simjlar in morphology but quite different in microstructure have been published elsewhere (Pannella, 1972% b). The method used to obtain the periodicity of recurrent growth bands and zones consisted simply of counting laminae between repetitive patterns. There are, at least, two objections to this method: it relies only on the observer to solve ambiguities of the record
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SILICA
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nicn
Fig. 2. a. Tidal amplitude modulations at Barnstaple Harbor, Massachusetts, from September 1965 to April 1967. b. Curve obtained by using running mean of lamina thicknesses in Mink Mountain stromatolites (Gun flint Iron Formation). The dashed line in A is related to the lunar changes in declination, in B is only a hypothetical reconstruction since the noisy record does not allow a resolution of the ambiguities. The dotted area coincides with the silica-rich, fast growing band. The use of several curves of this type and coherence tests will possibly resolve the ambiguities. The important fact remains that the two curves are comparable, supporting the conclusion that, besides changes in periodicities, the Earth-Moon system has not changed much in the last 2,000 million years.
and it erases many useful details recorded in the sequences. The method of measuring the thickness of the laminae, though by no means foolproof, is better in that it provides quantitative figures that can be subjected to mathematical analysis. Both methods were applied to the study of Mink stromatolites. But, while the former supplied values for the frequency of what have been interpreted as monthly and annual bands, of 39 laminae (the highest count) and 448 laminae, respectively, the latter allows the harmonic analysis of the variations in spacing, providing a more precise way to evaluate the frequency of the bands and other additional data that may help the interpretation of their meaning. Fig. 2 compares the amplitude modulations of modern tides in Boston Harbor (U.S.A.) (uniformly semidiumal tides with the principal variations following the changes in the Moon's distance and phase) with the spacing modulations of the laminae of the Mink stromatolites, obtained by using the running mean to smooth the noise (Dolman, 1975). The similarity of the two curves is striking and implies the same shaping cause. We know from data of modem analogs that tidal amplitude can effect stromatolitic growth and thus the lamina spacing. The most noticeable matching is between the modern six-month amplitude changes and the lamina thickness and the alternation of iron-rich and silica-rich bands. These bands, characteristic of the iron formation and the associated stromatolites, have been attributed to seasonal chemical changes in the environment of deposition. The Mink periodogram appears t o be the result of lunar
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G . PANNELLA
(spring-neap periodical changes) and solar (six-month chemical changes reflected in the Si/Fe ratio) effects. There are some differences between the modem curve and the Huronian peridogram. Basically the modem curve, being a calculated prediction, is much more regular than the real one which records (as stromatolites do) all the noise spectra of the environment, but the Mink spectrum seems to contain not only more noise but some, perhaps fundamental, differences in tidal modulations. The similarity of the two curves supports the original interpretation of the tidal bands and of the annual band (Pannella, 197213). Still open is the question whether the laminae are daily or semidaily (in which case the length of the synodic month would not have changed in the last 2,000 m.y.). But statistical and coherence tests, hopefully, will answer the question. It must be pointed out that stromatolites can only provide minimum values and thus any recorded number of the days per year and per synodic month in the Precambrian higher than that at present indicates a definite deceleration in the rotation of the Earth around its axis, but leaves open the question of the rate. Moreover, only negative accelerations could be inferred from stromatolite data. Qualitatively, if not quantitatively, stromatolites have already provided the information that the Moon has been associated with the Earth for at least three aeons (the age of the oldest stromatolite with developed tidal bands) (Pannella, 197213). This information obviously eliminates all hypotheses implying a late lunar capture (Olson, 1966, 1968; Niini, 1969). Laminae can also be used to determine the amplitude of paleotides, as discussed in the following pages. Stromatolites are tidal gauges The discovery of Shark Bay stromatolites, in many aspects similar to those found in great abundance in Precambrian rocks, has influenced many of the current ideas and speculations on stromatolites. One of them, that the height of stromatolites could be a reflection of tidal amplitude, is based on the highly suggestive coincidence of the height of the Shark Bay stromatolites with the tidal amplitude of the area. Logan stated that their height is controlled “by the tidal range and the position of the structure in the intertidal zone” (Logan, 1961, p. 531). The way stromatolites grow forbids upward extension above the highest water level and it is likely that in a system in which this level remains unchanged for a long time, stromatolites will grow up to that level. Thus, Cloud’s (1968a) suggestion of using the height of Precambrian stromatolites to infer the variations of tidal amplitude through time, and thus the moment of closest approach of the Moon to the Earth, is logical. By checking the literature and his notebooks, Cloud found that stromatolites supported the idea of a maximum tidal amplitude, and thus of a closest approach of the Moon, in the Middle Precambrian. Cloud’s data
HISTORY O F THE EARTH-MOON
SYSTEM
681
were by no means complete or even representative and his argument was rather tenuous. Walter (1970a) has stressed several points that make Cloud’s conclusion untenable. First, not all stromatolites with relief grew in intertidal areas; many were probably subtidal. The trend of increasing stromatolite height with increasing age is not at all evident from the stromatolites reported in the literature. If anything, they seem to indicate that the tallest stromatolites formed during the Paleozoic. Also tidal amplitude is the result, at any one locality, of the complicated interaction of many variables and cannot be used in any way even qualitatively, to estimate the distance from the Earth to the Moon. What amplitude could we use today as indicative of this distance: the 60-cm amplitude of Shark Bay or the 14m of the Bay of Fundy?. The hopelessness of the task has been pointed out by H o h a n n (1973). To all these difficulties one can also add Monty’s suggestion that Precambrian gigantism may be related to a sequence of environmental circumstances and intrinsic algal factors rather than to any change in the amplitude of the tides (Monty, 197313). While Cloud’s generalization is untenable, the fact that stromatolite growth is governed by tidal fluctuations remains and. can be used as a method to infer tidal amplitude, at least locally. One way of doing it is to check the periodicity, in terms of laminae, of the tidal bands. As suggested in Table I, the number of laminae per band should change progressively from subtidal through intertidal to supratidal. Along the same bioherm, and perpendicularly to the shoreline, it is possible to check these variations and determine quite accurately the extent of the intertidal zone, and the tidal amplitude. So far stromatolites have been studied individually rather than collectively; studies that consider lateral as well as vertical morphological variations are needed and could provide useful insights and information. Along the same synchronous bed from deep to shallow water the number of laminae per periodical band should decrease. Vertical variations are often easier to study than lateral ones. Examples of morphological vertical evolution from columnar types to stratiform types are well developed in the Nash Formation, Medicine Bow Mountains (Wyoming). Many bioherms are capped by a finely laminated dolomitic bed. The rhythms: columnar stromatolites-stratiform stromatolites-dolomite are repeated several times and many indicate a change from intertidal (or slightly subtidal) to supratidal environment. The thickness of this cycle is consistently 1.5 m and represents, assuming the sea level was not changing significantly, the amount of growth necessary to reach a supratidal environment, that is, the tidal amplitude of the locality. Bioherm evolution from columns to stratiform types lasted from 150 to 400 2.5 cm thick bands (Fig. 3). The preservation of the laminae is so poor that study of the vertical frequency changes is not possible. The thickness of the bands and the number of laminations still recognizable within each band decreases upward.
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G . PANNELLA
In one type of the Mink stromatolites, the upward evolution of the lamina frequencies has been checked. Fig. 4 shows vertical morphological variations in the columns. As the number of laminae in the tidal bands decreases so also does the convexity, and the shape of the columns changes from erect t o inclined, uniform t o coalescent and linked. The evolution is also accompanied by the increasing abundance upwards of oolites between the columns. All seems to indicate an upward increase in the energy of the environment. The number of laminae per tidal band follows the trend and decreases from 14-17 to 3-9. This change occurs in a vertical distance of 15 cm. It is possible that this was the tidal amplitude. At least locally stromatolites may be able to provide accurate records of tidal range.
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Fig. 3. A. Early Proterozoic stromatolites from the Nash Formation, Medicine Bow Mountains, Wyoming. Morphologically the bioherms show a vertical change from large columns to stratifonn stromatolites, to dentate laminae. The upper part is capped by laminated dolomite. These cyclic variations are successively repeated several times. Scale in inches. B. Lower part of a bioherm showing large columns and seasonal bands consisting of light dolomite and dark silica zones. Scale in inches.
CONCLUSIONS
The use of stromatolites as paleontological clocks presents many difficulties, but before concluding that these are always insuperable much more work is needed. The odds of determining the number of days per year with any
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G . PANNELLA
Fig. 4. Periodical growth bands in Mink Mountain stromatolites (Gunflint Formation). Photographic print from a thin section used as negative. The number of laminae per tidal band ( t ) below the level A ranges from 14 to 1 7 , above the level from 3 to a maximum of 9. Boundaries between annual bands are marked by X. Scale 1 cm.
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degree of accuracy are low indeed. But if not quantitatively, then qualitatively stromatolites can be useful. Figures should always be considered to represent minima and thus can only suggest decelerations (if indeed slowing down has occurred). Reconstruction of paleotide types and amplitude is a promising application. It could provide constraints on the evolutionary models of the Earth-Moon system. Not all laminae have the same chronological meaning: they represent episodes of organic and inorganic deposition occurring with different frequencies ranging from tidal (12.4 h, 24.8 h, fortnight) to solar (24 hr, for one organic and one inorganic lamella) to random (i.e., storm laminae). These frequencies are characteristic of different environments from subtidal t o supratidal. Using the criteria for the recognition of stromatolites from these different environments one can select the most promising samples for geophysical analysis. This selection is critically important. Subtidal stromatolites are more promising than intertidal ones. Only in an absolutely calm environment, where post-depositional erosion is unlikely to occur, however, may long and uninterrupted sequences of laminations survive. The environment most likely to provide the ideal sequences for reliable data must have the following characteristics: be absolutely calm, and be affected by seasonal, fortnightly, tidal and daily changes. On a pure numerological basis, when seasonal tidal bands are present, it is possible to recognize whether the sequence is continuous or interrupted (Pannella, 1972a, b). The degree of incompleteness can be used for separating subtidal from intertidal stromatolites. Most columnar stromatolites, morphologically similar to Shark Bay analogs, cannot be used as paleontological clocks because of their discontinuous growth. Small columnar types appear to contain longer periodical sequences. Harmonic analysis of the variations in thickness of the laminae should be used t o support the interpretation of the periodical bands. Keeping in mind that one or even several attributes (Hofmann, 1969a) do not provide unambiguous clues, the data from stromatolitic sequences should be critically evaluated and corroborated by as many other sources of information as possible. Their validity will be a direct outcome of a total and still faraway understanding of the physico-chemical and biological parameters at the basis of each attribute. ACKNOWLEDGEMENTS
While assuming the full responsibility for the ideas expressed here, the writer wishes t o thank those who have kindly helped with their comments, suggestions and materials, namely: J.D. Weaver, M.R. Walter, H.J. Hofmann, P.F. Hoffman, S.K. Runcom, G. Rosenberg, J. Dolman and Cristian Jones. The research of this paper was made possible by the financial support of Minna-James-Heineman Foundation of Hanover (Germany).
Chapter 12.1
GEOPHYSICAL INFERENCES FROM STROMATOLITE LAMINATION Giorgio Pannella
INTRODUCTION
In the preceding pages different aspects and uses of stromatolites have been emphasized. Potentially, if not practically at the moment, the use of stromatolites as recorders of geophysical and astronomical conditions of the Earth during its evolution could be even more important. Since growth of stromato!ites is a direct outcome of environmental dynamics, it has been suggested that growth structures could be used to check geological changes in the speed of the Earth’s rotation (Runcom, 1966; McGugan, 1967; Pannella et al., 1968; Pannella, 1972a) and to determine the amplitude of paleotides and polar shifts (Cloud, 1968a; Nordeng, 1963; Vologdin, 1963, 1964; Cloud, 1968a). Fossil organic structures which may provide the first type of information have been called paleontological clocks (Runcorn, 1966). In order to separate the effects of changes in the Earth’s moment of inertia from those due to tidal dissipation, a paleontological clock must provide, from continuous sequences, two sets of data: one on the length of the sidereal day, the other on the length of the synodic month (Runcom, 1964). The crucial and controversial question with regard to the use of stromatolites as clocks is with what precision and continuity stromatolites record, during growth, the astronomically driven environmental fluctuations and how much of this record is left for us to decipher. For several reasons, which will be mentioned in the following pages, stromatolites, in general, are not as reliable paleontological clocks as other fossils. Hofmann (1973) attributes to them either zero or a very low degree of reliability. In this paper it is not intended to take an opposite and over-optimistic view of these applications because stromatolitic growth records in most instances are so incomplete as to justify Hofmann’s conclusion. However, in exceptional instances and with the help of statistical techniques, stromatdites are going to provide precious insights into the largely unknown Precambrian. The strongest reservation to these applications of stromatolites depends on the danger of extrapolating from one localized situation to world-wide generalizations. Another reservation is the uncertainty in respect to determining the absolute age of Precambrian material.
674
G. PANNELLA
Notwithstanding the many difficulties and the rather tenuous results of the first naive attempts, there is ample room for improvement and, perhaps, the ultimate goal of tracing in broad lines the Precambrian evolution of the Earth-Moon system, is not too far in the future. The geologist and the paleontologist with the help of stromatolites will undoubtedly contribute.
STROMATOLITES AS RECORDERS OF PERIODICAL ENVIRONMENTAL CHANGES
The modem analogs of stromatolites provide insight and models of their growth. Light stimulation, by controlling metabolism, plays an essential role in the growth of stromatolites. Short-term experiments, extending from a few days t o two weeks, have shown that laminae of many modem stromatolites form with a daily frequency because of light stimulation of algae or bacteria. Monty’s experiments, in which Bahamian algal oncolites formed laminae consisting of organic-rich (deposited during the day) and inorganicrich lamellae (deposited at night), suggest daily growth is a basic feature of some stromatolites, but add little to the question of the accuracy and continuity of stromatolitic sequences (Monty, 1965b, 1967). Monty noted that there is size limitation of growth for the oncolites which are carried away after about two months growth. Gebelein (1969) described daily layers of about 1mm thick from subtidal algal oncolites from Bermuda and attributed them to the day-night algal growth cycle. He also emphasized the role of currents, and of velocity and rate of sediment movement in the building of stromatolitic structures. Currents over 20 cm/sec d o not permit the formation of algal mats, and only bottom sediment movement of less than 60-80 g/h per foot allows mats and domes t o accrete. The thickness of lamination is inversely proportional t o current velocity. Tidal fluctuations, which control current velocity and direction, can thus leave a record in sequences of laminae. In Bermuda, for example, more sediments are transported at flood tides than at ebb tides (Gebelein, 1969). In intertidal algal mats high-tide flooding creates sedimentrich lamellae, the thickness of which is proportional to the period of flooding, whereas organic-rich lamellae are related to low tides (Gebelein and Hoffman, 1968). The relationships between tidal fluctuations and stromatolitic deposition in the intertidal zone could produce, in areas of semidaily tides, two laminae per day. The possibility that this could reduce the accuracy of stromatolites by producing more laminae than days is not relevant here since intertidal stromatolites, limited and discontinuous in their growth, are unlikely candidates for geochronometry. But the tidal mechanism of stromatolitic growth must be kept in mind when interpreting sequences of laminae. Whereas flooding and air exposure are the controlling parameters of
HISTORY OF THE EARTH-MOON SYSTEM
675
TABLE I Hypothetical frequencies of periodical growth sequences in modem stromatolites Growth-pattern frequencies Tidal bands/year
2 variable Y 0
U
I
d
Daily increments/tidal band 1- 3 (only exceptionally higher)
7 8 - Extreme - - - -high - -water - - of spring - - - tides --- ---
m--
a
< 53-7
the frequencies are highly variable depending on tidal types, diurnal inequality
< 20
4-10
columnar stromatolites and algal mats are common forms
;
z1 rn
laminae are rarely daily; discontinuous sequences ; algal mats are common forms
< 12
a - - - Extreme - - - -low - -water - - of- spring - - -tides - -- -
2
Remarks
<25
< 14
--
laminae in most instances are deposited with daily frequency ;continuous sequences may form; large dome and digitate stromatolites are the most characteristic forms
lamina formation in the intertidal zone, subtidal stromatolites, unfortunately still poorly known, are affected by both light-dark cycle and tidal effects on sediment supply and are the only promising paleontological clocks. In addition the relatively deep-water ones are less subject to erosion. Other works show that not all laminations are daily. Supratidal algal mats grow only during floodings, the frequency of which depends on many factors: tidal amplitude, height of the algal mats above mean low water, and storms (Hardie and Ginsburg, 1971). Generally the higher is the position of the mats the more discontinuous is the record. Theoretically, and apart from any post-depositional disruptive action, the number of laminae per tidal cycle (Pannella, 1972b) is a direct function of tidal floodings, which, in turn, are a function, at any given point, of tidal amplitude and tidal type. Table I represents the theoretical frequency ranges of periodical growth patterns, based on these assumptions: (1)a lamina records a 24-h dark-light cycle, or an episode of tidal flooding (which can occur with frequencies ranging from 12.42 h up to six lunar months), or a mixture of both tidal and solar cycles; (2) deposition occurs when algal mats are covered with water, when there is a proper sediment supply (in allochemical growth), and during metabolic
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activity of algae or bacteria (in allochemical and orthochemical growth). The Table has obvious limitations because it applies only to instances of a constant accretion, which are extremely rare in nature, and does not include the very common random noise and disruptions, but it provides consistency to the apparently contradictory and spotty data on lamina formation. One of the weaknesses of any generalized model of stromatolitic growth lies in the fact that, in the shallow-water marine environment the same parameters involved in growth patterns may also be the cause of their destruction. This fact, together with the tendency, in the intertidal zone, to form incomplete sequences, led to the conclusion-at the time when stromatclites were thought to be only intertidal - of the unsuitability of stromatolites as paleontological clocks. From Table I it is clear that ideal growth sequences can only form in a subtidal environment, where the light-dark cycle is still effective and where physical disruption is less likely. In the modem situation, however, even a subtidal environment cannot produce ideal sequences because of biological disruption. One has to look for a special habitat where metazoan life is impossible. Hence the importance of data on growth patterns of hot-spring stromatolites and stromatolite-like deposits. From the experimental data obtained by marking stromatolites in the Yellowstone hot springs it appears that algal and bacterial stromatolites there grow by daily addition of laminae as in the Bermudan stromatolites. The growth record, however, does not appear to be continuous or similar in all columns even in the same spring. The change of flow patterns gives to each column an individual flood and, thus, growth history. In conical forms the laminations appear to be “crudely diurnal” (Walter et al., Ch. 6.2). Laminations in nonbiogenic “stromatolites” (geyserites) have periodicities that appear to be related more to geyser activity than to diurnal environmental variations. Their number is highly variable as shown by Walter’s figures in this book (p. 106). It will be interesting to confront these figures with continuously monitored variations in the pools and t o compare lamination periodicities in stratiform geyserites in the same pools. All these works on modem stromatolites demonstrate that while diurnal laminae are common the continuity of the growth record is restricted to short time intervals. For the formation of ideal sequences an environment must meet the following conditions: (1)be aqueous, quiet, and with some physicochemical parameters oscillating regularly and periodically with daily fixed frequencies; (2) be affected by tides but not subjected t o strong storm waves; (3) be populated by algae or bacteria with diurnal growth and which cause precipitation of a preservable layer; (4) be lacking in other biological systems which cause disruptions. Such an environment containing stromatoIites, with perhaps the exception of hot-spring pools, is not found in today’s world. In the Precambrian, however, this favourable environment may have existed.
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FOSSIL STROMATOLITES
Considering that we are living in unusual geologic-climatic conditions, extrapolations from modem analogs to Phanerozoic stromatolites are dangerous. From modem analogs to their Precambrian ancestors, however, there is a “Quantum jump”, because of the time involved and of the biological and geological evolution of our planet. The Precambrian geologic record with its impressive sequences of rhythmites and stromatolites creates the inevitable feeling that in pre-metazoan time environmental rhythms were written undisturbed in the rocks, that the Precambrian is the ideal locus of rhythmometric studies. Strangely enough, no in-depth study like that of Sander (1951) exists for the Precambrian.
Paleontological clocks
A survey of many fossil stromatolites has led to the conclusion that most of them, in fact all of those studied except one, could not provide reliable data for reconstructing the length of the day and of the synodic month in the Precambrian (Pannella, 1972a, and unpublished data). Sequences are incomplete and show unmistakable signs of interruptions and erosion (Fig. 1).In their general morphology most of the studied stromatolites are similar to modern Shark Bay stromatolites. Pannella (1972a) has pointed out, and further observations have confirmed, that small, digitate columnar stromatolites show generally longer sequences and a higher number of laminae per periodical band than large columnar ones. Much evidence strongly suggests that they were formed in subtidal environments and they are similar to others reported as subtidal (Playford and Cockbain, 1969; MacGregor et al., 1974). Very small digitate types are present in stromatolitic beds of the 2,000 m.y. old Biwabik and Gunflint formations. One type, found on the western flank of Mink Mountain in the Thunder Bay area in Ontario, has yielded the most promising sequences of laminae, and if further study confirms the finding, is going to provide the first clues on the length of the synodic month and the year of Huronian times (Pannella, in prep.) Using morphological criteria Walter (1972b) has suggested that some Mink stromatolites are nonbiogenic geyserite-like deposits very close to those found in Yellowstone. This hypothesis does not contradict my results nor change the interpretation of the periodicities in the lamination sequences. Harmonic analysis of the lamination spacing indicates strong tidal control of the lamina deposition. The possibility of a tidal effect on geyser activities is mentioned by Walter (Ch. 3.3 herein). Earth tides are more strongly felt nearshore because of the direct contribution of marine tides (Melchior, 1966). In the nearshore setting where the Gunflint stromatolites grew tides must have represented the strongest periodical force.
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Fig. 1. Polished surface of Great Slave Lake stromatolites showing distinct seasonal changes in color and composition of growth bands. Light bands contain thicker laminae than dark bands and were deposited during periods of fast growth (possibly warm and/or dry season). Dark bands show high organic content and are preferentially deposited over micro-unconformities indicating erosion (due, perhaps, to “equinoctial” storms) of the light bands. The filling between columns contains large fragments of light-band laminae further supporting the idea that the energy in the environment increased after the fastgrowing periods. The numerous interruptions in the sequences make this type of stromatolite (similar to the modem ones found in Shark Bay) unsuitable for geochronometry. w = slow-growth bands; s = fast-growth bands; d = major discontinuities in growth sequences. Scale 1 cm.
While the analytical procedures and the detailed discussion of the sequences will be the object of another work, some results must be mentioned here for they justify the optimism in the use of stromatolites for deriving geophysical information. Preliminary results of the study of the Mink stromatolites and of the Biwabik Formation which contains stromatolites simjlar in morphology but quite different in microstructure have been published elsewhere (Pannella, 1972% b). The method used to obtain the periodicity of recurrent growth bands and zones consisted simply of counting laminae between repetitive patterns. There are, at least, two objections to this method: it relies only on the observer to solve ambiguities of the record
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nicn
Fig. 2. a. Tidal amplitude modulations at Barnstaple Harbor, Massachusetts, from September 1965 to April 1967. b. Curve obtained by using running mean of lamina thicknesses in Mink Mountain stromatolites (Gun flint Iron Formation). The dashed line in A is related to the lunar changes in declination, in B is only a hypothetical reconstruction since the noisy record does not allow a resolution of the ambiguities. The dotted area coincides with the silica-rich, fast growing band. The use of several curves of this type and coherence tests will possibly resolve the ambiguities. The important fact remains that the two curves are comparable, supporting the conclusion that, besides changes in periodicities, the Earth-Moon system has not changed much in the last 2,000 million years.
and it erases many useful details recorded in the sequences. The method of measuring the thickness of the laminae, though by no means foolproof, is better in that it provides quantitative figures that can be subjected to mathematical analysis. Both methods were applied to the study of Mink stromatolites. But, while the former supplied values for the frequency of what have been interpreted as monthly and annual bands, of 39 laminae (the highest count) and 448 laminae, respectively, the latter allows the harmonic analysis of the variations in spacing, providing a more precise way to evaluate the frequency of the bands and other additional data that may help the interpretation of their meaning. Fig. 2 compares the amplitude modulations of modern tides in Boston Harbor (U.S.A.) (uniformly semidiumal tides with the principal variations following the changes in the Moon's distance and phase) with the spacing modulations of the laminae of the Mink stromatolites, obtained by using the running mean to smooth the noise (Dolman, 1975). The similarity of the two curves is striking and implies the same shaping cause. We know from data of modem analogs that tidal amplitude can effect stromatolitic growth and thus the lamina spacing. The most noticeable matching is between the modern six-month amplitude changes and the lamina thickness and the alternation of iron-rich and silica-rich bands. These bands, characteristic of the iron formation and the associated stromatolites, have been attributed to seasonal chemical changes in the environment of deposition. The Mink periodogram appears t o be the result of lunar
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(spring-neap periodical changes) and solar (six-month chemical changes reflected in the Si/Fe ratio) effects. There are some differences between the modem curve and the Huronian peridogram. Basically the modem curve, being a calculated prediction, is much more regular than the real one which records (as stromatolites do) all the noise spectra of the environment, but the Mink spectrum seems to contain not only more noise but some, perhaps fundamental, differences in tidal modulations. The similarity of the two curves supports the original interpretation of the tidal bands and of the annual band (Pannella, 197213). Still open is the question whether the laminae are daily or semidaily (in which case the length of the synodic month would not have changed in the last 2,000 m.y.). But statistical and coherence tests, hopefully, will answer the question. It must be pointed out that stromatolites can only provide minimum values and thus any recorded number of the days per year and per synodic month in the Precambrian higher than that at present indicates a definite deceleration in the rotation of the Earth around its axis, but leaves open the question of the rate. Moreover, only negative accelerations could be inferred from stromatolite data. Qualitatively, if not quantitatively, stromatolites have already provided the information that the Moon has been associated with the Earth for at least three aeons (the age of the oldest stromatolite with developed tidal bands) (Pannella, 197213). This information obviously eliminates all hypotheses implying a late lunar capture (Olson, 1966, 1968; Niini, 1969). Laminae can also be used to determine the amplitude of paleotides, as discussed in the following pages. Stromatolites are tidal gauges The discovery of Shark Bay stromatolites, in many aspects similar to those found in great abundance in Precambrian rocks, has influenced many of the current ideas and speculations on stromatolites. One of them, that the height of stromatolites could be a reflection of tidal amplitude, is based on the highly suggestive coincidence of the height of the Shark Bay stromatolites with the tidal amplitude of the area. Logan stated that their height is controlled “by the tidal range and the position of the structure in the intertidal zone” (Logan, 1961, p. 531). The way stromatolites grow forbids upward extension above the highest water level and it is likely that in a system in which this level remains unchanged for a long time, stromatolites will grow up to that level. Thus, Cloud’s (1968a) suggestion of using the height of Precambrian stromatolites to infer the variations of tidal amplitude through time, and thus the moment of closest approach of the Moon to the Earth, is logical. By checking the literature and his notebooks, Cloud found that stromatolites supported the idea of a maximum tidal amplitude, and thus of a closest approach of the Moon, in the Middle Precambrian. Cloud’s data
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were by no means complete or even representative and his argument was rather tenuous. Walter (1970a) has stressed several points that make Cloud’s conclusion untenable. First, not all stromatolites with relief grew in intertidal areas; many were probably subtidal. The trend of increasing stromatolite height with increasing age is not at all evident from the stromatolites reported in the literature. If anything, they seem to indicate that the tallest stromatolites formed during the Paleozoic. Also tidal amplitude is the result, at any one locality, of the complicated interaction of many variables and cannot be used in any way even qualitatively, to estimate the distance from the Earth to the Moon. What amplitude could we use today as indicative of this distance: the 60-cm amplitude of Shark Bay or the 14m of the Bay of Fundy?. The hopelessness of the task has been pointed out by H o h a n n (1973). To all these difficulties one can also add Monty’s suggestion that Precambrian gigantism may be related to a sequence of environmental circumstances and intrinsic algal factors rather than to any change in the amplitude of the tides (Monty, 197313). While Cloud’s generalization is untenable, the fact that stromatolite growth is governed by tidal fluctuations remains and. can be used as a method to infer tidal amplitude, at least locally. One way of doing it is to check the periodicity, in terms of laminae, of the tidal bands. As suggested in Table I, the number of laminae per band should change progressively from subtidal through intertidal to supratidal. Along the same bioherm, and perpendicularly to the shoreline, it is possible to check these variations and determine quite accurately the extent of the intertidal zone, and the tidal amplitude. So far stromatolites have been studied individually rather than collectively; studies that consider lateral as well as vertical morphological variations are needed and could provide useful insights and information. Along the same synchronous bed from deep to shallow water the number of laminae per periodical band should decrease. Vertical variations are often easier to study than lateral ones. Examples of morphological vertical evolution from columnar types to stratiform types are well developed in the Nash Formation, Medicine Bow Mountains (Wyoming). Many bioherms are capped by a finely laminated dolomitic bed. The rhythms: columnar stromatolites-stratiform stromatolites-dolomite are repeated several times and many indicate a change from intertidal (or slightly subtidal) to supratidal environment. The thickness of this cycle is consistently 1.5 m and represents, assuming the sea level was not changing significantly, the amount of growth necessary to reach a supratidal environment, that is, the tidal amplitude of the locality. Bioherm evolution from columns to stratiform types lasted from 150 to 400 2.5 cm thick bands (Fig. 3). The preservation of the laminae is so poor that study of the vertical frequency changes is not possible. The thickness of the bands and the number of laminations still recognizable within each band decreases upward.
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In one type of the Mink stromatolites, the upward evolution of the lamina frequencies has been checked. Fig. 4 shows vertical morphological variations in the columns. As the number of laminae in the tidal bands decreases so also does the convexity, and the shape of the columns changes from erect t o inclined, uniform t o coalescent and linked. The evolution is also accompanied by the increasing abundance upwards of oolites between the columns. All seems to indicate an upward increase in the energy of the environment. The number of laminae per tidal band follows the trend and decreases from 14-17 to 3-9. This change occurs in a vertical distance of 15 cm. It is possible that this was the tidal amplitude. At least locally stromatolites may be able to provide accurate records of tidal range.
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Fig. 3. A. Early Proterozoic stromatolites from the Nash Formation, Medicine Bow Mountains, Wyoming. Morphologically the bioherms show a vertical change from large columns to stratifonn stromatolites, to dentate laminae. The upper part is capped by laminated dolomite. These cyclic variations are successively repeated several times. Scale in inches. B. Lower part of a bioherm showing large columns and seasonal bands consisting of light dolomite and dark silica zones. Scale in inches.
CONCLUSIONS
The use of stromatolites as paleontological clocks presents many difficulties, but before concluding that these are always insuperable much more work is needed. The odds of determining the number of days per year with any
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Fig. 4. Periodical growth bands in Mink Mountain stromatolites (Gunflint Formation). Photographic print from a thin section used as negative. The number of laminae per tidal band ( t ) below the level A ranges from 14 to 1 7 , above the level from 3 to a maximum of 9. Boundaries between annual bands are marked by X. Scale 1 cm.
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degree of accuracy are low indeed. But if not quantitatively, then qualitatively stromatolites can be useful. Figures should always be considered to represent minima and thus can only suggest decelerations (if indeed slowing down has occurred). Reconstruction of paleotide types and amplitude is a promising application. It could provide constraints on the evolutionary models of the Earth-Moon system. Not all laminae have the same chronological meaning: they represent episodes of organic and inorganic deposition occurring with different frequencies ranging from tidal (12.4 h, 24.8 h, fortnight) to solar (24 hr, for one organic and one inorganic lamella) to random (i.e., storm laminae). These frequencies are characteristic of different environments from subtidal t o supratidal. Using the criteria for the recognition of stromatolites from these different environments one can select the most promising samples for geophysical analysis. This selection is critically important. Subtidal stromatolites are more promising than intertidal ones. Only in an absolutely calm environment, where post-depositional erosion is unlikely to occur, however, may long and uninterrupted sequences of laminations survive. The environment most likely to provide the ideal sequences for reliable data must have the following characteristics: be absolutely calm, and be affected by seasonal, fortnightly, tidal and daily changes. On a pure numerological basis, when seasonal tidal bands are present, it is possible to recognize whether the sequence is continuous or interrupted (Pannella, 1972a, b). The degree of incompleteness can be used for separating subtidal from intertidal stromatolites. Most columnar stromatolites, morphologically similar to Shark Bay analogs, cannot be used as paleontological clocks because of their discontinuous growth. Small columnar types appear to contain longer periodical sequences. Harmonic analysis of the variations in thickness of the laminae should be used t o support the interpretation of the periodical bands. Keeping in mind that one or even several attributes (Hofmann, 1969a) do not provide unambiguous clues, the data from stromatolitic sequences should be critically evaluated and corroborated by as many other sources of information as possible. Their validity will be a direct outcome of a total and still faraway understanding of the physico-chemical and biological parameters at the basis of each attribute. ACKNOWLEDGEMENTS
While assuming the full responsibility for the ideas expressed here, the writer wishes t o thank those who have kindly helped with their comments, suggestions and materials, namely: J.D. Weaver, M.R. Walter, H.J. Hofmann, P.F. Hoffman, S.K. Runcom, G. Rosenberg, J. Dolman and Cristian Jones. The research of this paper was made possible by the financial support of Minna-James-Heineman Foundation of Hanover (Germany).