by Yuri AVSYUK, RAS Corresponding Member; Yuri GENSCHAFT, Dr. Sc. (Geol. & Mineral.); and Zinaida SVETLOSANOVA, research scientist, Otto Schmidt Institute of Physics of the Earth, Russian Academy of Sciences

...About 65 mln years ago some mysterious force wiped out half of the living matter on earth. Such were the opening lines of an article which two American scientists, W. Alvarez and F. Azaro, wrote for the magazine IN THE WORLD OF SCIENCE in 1990. The authors concluded: 65 mln years ago a giant body - an asteroid or a comet - swooped down from the heavens and collided with the earth at a speed of over 10 km/s. Such a thrilling scenario of biota's annihilation at one fell swoop made quite a hit. But is it valid scientifically?

Articles in this rubric reflect the opinion of the author. - Ed.

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CHANCE OR NORMALITY?

The chief argument of the authors for their collision theory and for the subsequent extinction of the then widespread dinosaurs and ammonites (conchs related to the present-day nautilus shells) was an indium anomaly discovered in deposits of 65,000,000 years ago: a huge amount of this metal, indium, brought to the earth by an asteroid or some other celestial body, killed those beings. Other scientists, however, begged to differ, for similar phenomena, registered in deposits of different ages, are related to the activation of volcanism at different stages of geological history Besides, most of the paleontologists think that at the end of the Cretaceous (or 65 mln years ago) not all of the biological species died out suddenly. In North America, for instance, this process of extinction took at least 12 mln years when the number of dinosaurian taxa kept dwindling gradually. In addition, by stratigraphic data available today the extinction of ammonites could be due to the protracted regression (lowering) of the ocean level in the Late Cretaceous. Thus the scenario of an "instantaneous" disappearance of a vast number of biological species can hardly tally with the actual course of events.

This could be explained with the help of historical geology. First, we should model natural conditions when this or that species made its appearance in a biological community. The time boundary is just as important as the boundary of degradation and extinction; that is why it is essential to trace the evolution of processes which were instrumental in changing the habitation conditions. Proceeding accordingly, we can identify the periods of efflorescence, degeneration and subsequent extinction of particular animal species (we know of several periods of such upheavals in the history of the earth, and one of the worst occurred about 250 mln years ago, when our planet lost a great number of living organisms). Needless to say, such kind of reconstruction, or modeling, of the response of the biosphere to a changed course of natural processes will be ambiguous and disputable if we are not in the clear about the prime causes of what has happened. If these causes are cogently explained in keeping with the laws of physics and are confirmed by factual time-related evidence, then it will become possible to reconstruct the evolution of the plant and animal kingdoms as a function of climatic and geological changes.

In the present article we would like to draw attention to the general regularities underlying the revolution of the planets of the solar system along their orbits and, simultaneously, we would like to point out the differences in these movements which predetermine dissimilarity of the course of natural processes in these celestial bodies. For example, the development of the earth, which has a massive natural satellite, the moon (its mass equal to 1/81th of the earth's), should have a different history than that of Mars with its small satellites, or of Venus, which has none.

TIDAL EVOLUTION OF THE EARTH-MOON-SUN SYSTEM

When studying long-period fluctuations in the course of natural processes, we should not lapse into simplification in describing the characteristics of an object in the focus of our attention. Therefore let us stress at the very beginning: our planet, the earth, belongs to a system "planet plus a massive satellite, the moon"; the notion of annual cyclicity stands for a period of revolution of the center of the masses of the earth-moon (barycenter) around the sun; and ecliptic applies to the plane of the barycenter's orbital travel. Both our planet and its natural satellite, the moon, obey a menstrual cyclicity in their movement along an elliptical orbit tipped to the ecliptic at 5; their movement, mind you, is perturbed motion amenable to registration with astronomical instruments.

According to the laws of mechanics, a heavenly body's orbital and rotational motions are interconnected. Consequently, the time- related characteristics of the earth's rotation should be studied by taking due account of its orbital motion. The interdependence of these two kinds of motion is effected through tidal forces engendered by the gravitational interaction of celestial bodies.

A world ocean's area maximum.

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Quick overview of Kelvin's model for the mechanism of mass redistribution in the solar system.

Changes: 1 - perturbations from the sun; 2 - nonperturbed component of lunar tidal effect on the earth; 3 - sum total of the perturbed and nonperturbed components of tidal effect as a function of the changing distance between the earth and the moon, in terrestrial radii.

Now we know: a change in the mode of the earth's revolution is caused by the variations in the magnitude and orientation of its axial rotation. An increase in the rate of axial rotation entails a global movement of water contained in the hydrosphere from high- latitude to pre-equatorial regions; the resultant rise of the ocean level in the tropics will cause an overflow of rivers and large zones of shallow waters. Paleontologists have good reason in noting this phenomenon: it is not by chance that the greatest mass of fossilized dinosaurian remains ever found occurs in deposits of a vast lowland area in the western United States, mostly on the territory of what are now the states of Utah, Wyoming and Colorado.

But back to the tidal forces. The moon, which always faces us with its one side only, is a graphic example of their regulatory effect. Today our satellite persists in a regime when the periods of its axial rotation and orbital motion are equal (what astronomers call resonance 1:1). Under these conditions the power of the tidal impact of the earth and the sun on the moon's interior is down to a minimum, with volcanic activity actually arrested there, and the magnetic field, which the moon had 2.5 bln years ago, gone. A similar pattern of synchronization of orbital and rotational motions has been registered in the satellites of Jupiter and Saturn as well. The tidal effect thus operates as a mechanism uniting these two main motions of the planets and their satellites within the solar system.

Now more about the earth-moon twain. Changes in the rotation rate of the earth and in the orbital velocity of the moon are usually determined from the longitude of the sun and of the planets closest to it. Then, eliminating the value of short-period variations in the earth's rotational velocity, we find its tidal change value. We also register variations in the angular velocity of the motion of its satellite as well as the rate at which the distance between our planet and the moon is changing. From these data we make a quantitative estimate of the moment of force which kind of "controls" the course of the tidal evolution of the earth-moon system-a natural process the present-day phase of which lends itself to instrumental registration.

At this point a few words will be in place about a model often used for reconstructing the orbital-rotational motion of the earth-moon system on a geological time scale. The first ever explanation of the mechanism behind the tidal evolution was suggested in the 19th century by the British scientist William Thomson (who merited for his services the title of Baron Kelvin). He thought that the hydrosphere covering the homogeneous substance of the earth takes the form of an ellipsoid (in a rough approximation) under the action of a tidal force. The semi-major axis of this ellipsoid- because of the friction and the rotational velocity of our planet being higher than the orbital velocity of the moon-shifts at some angle in respect of the line connecting the centers of these two celestial bodies. But since in this very model the angle of shift does not change sign, the tidal evolution course as well as change in the force impact should be unidirectional. In Kelvin's model (explaining the mechanism of mass distribution within the solar system), the influence of the sun was considered to be something of secondary and little importance. Even though this was but a first approximation only, it explained the physical relevance of the mechanism underlying the tidal evolution.

While keeping to the essential points of this model, let us ascertain

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the structural composition of our planet.

It will be appropriate to recall the discovery which the German woman scientist I. Lehmann made in 1936. She postulated there was a cavity within our planet filled with a liquid, and with a solid core in the center. Under the tidal effect of the sun, the core kept shifting; coming as indirect proof of that are variations in the rotational axis of the earth (which have been registered by astronomers) and seismological data on the passage of elastic waves through its core.

From the tidal force working in the earth's center and from some other characteristics of the inner core it has become possible to make an estimate of such shifts for earth-moon distances in a range of 20 to 80 terrestrial radii, and evaluate corresponding variations of the gravitational force on the terrestrial surface. The shifts of distances reach 3 to 20 meters, and the variations of the gravitational pull - 20-100x10 ^-6 cm/s ² . Changes of the gravitational force caused by shifts of the inner core are of the same order in value as the tidal effect of the moon. That is to say, both have the same impact on the formation of flows in the hydrosphere and on its dynamic pattern. This means that the angle of shift with respect to the line linking the centers of the earth and the moon changes sign (it does not in Kelvin's model, as we remember). And this, in turn, means that the tidal evolution course can be oscillatory, not unidirectional. The subject of oscillatory motion has been studied in detail in the works of one of the authors of the present article, Yuri Avsyuk.

Before turning to materials on the biological stratigraphy and the tidal evolution of the earth-moon-sun system, let us discuss how it measures up with the "heat engine" model used to explain processes taking place in the bowels of the earth.

PERIODICITY IN GEOLOGICAL HISTORY

Late in the 19th century Marcel Bertrand, a French scientist and foreign member of the St. Petersburg Academy of Sciences, identified, empirically, three geological cycles: the Caledonian, Hercynian and Alpine. They embrace three major epochs of orogeny (mountain making) over the last 550 mln years that led to the formation of three independent systems of fold structures on earth. Each of these Bertrand cycles took about 170 mln years; subsequently the German geologist Hans Stille singled out tectonomagmatic phases within each of these cycles.

In our days. Academician Yevgeny Milanovsky, while looking into the evolution of tectonic structures and crustal movements as well as eustatic (caused by pelagic processes) fluctuations in the level of the world ocean and in volcanicity, has discovered

Shorthand of the earth-moon tidal evolution on a geochronological scale. Changes in geological time: 1 - rotational velocity of the earth; 2 - equator slope, earth-moon distance; 3 - distance between the earth and the moon; 4 - boundaries of geological periods; 5 - rapid change intervals of the rotational axis of the earth.

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planetary cycles of this activity, each 40 to 50 min years long. Another our contemporary. Academician Nikolai Dobretsov, has found the recurrence interval of geological phenomena over this period to be 30 mln years.

Nor constant either are the intensity and polarity of the earth's geomagnetic field: there are long intervals when it does not change sign (so-called superchrons, about 30 to 40 mln years long), and intervals when its polarity has been changing rapidly (excursions, every 1 to 2 mln years). We might as well note that though in the course of time the magnetic field of the earth has been changing but irregularly, the periods of its major cycles concur with the cycles of eustatic fluctuations of the ocean level and also with the global variations of tectomagmatic activity within the earth.

Meanwhile the history of our planet has seen cold climatic periods followed by warmer spells, such climatic cycles correlating with geological ones-volcanicity, orogeny and the like.

The periodicity of natural processes is an objective fact. Now what is its original cause? There are several conceptual theories to this effect, and one is the "heat engine" model. It postulates that the heat released with the decay of radioactive elements in the interior part of the earth causes a thermal and gravitational convection of mass distribution. Geologically, the gravitational convection is manifest on the terrestrial surface; and geological processes, in their turn, affect the climate and the habitation conditions of the organic world.

Academician Nikolai Dobretsov sees the cause of periodicity of the endogenic (internal) activity of the earth in the

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instability of flows of matter in the lower mantle. The "overheating" of the external core gives rise to thermal fluctuations at its boundary with the mantle; as a consequence, large plumes or groups of plumes (giant "drops" of molten matter) arc off. Rising, they interact with the upper shells of the mantle and cause tectomagmatic activity It is pertinent to ask here: why are these heat fluctuations so ordered in time?

Now, from a standpoint of the "heat engine" model one need not discuss the orbital motion of our planet; it keeps immobile, as it were. This invites an analogy with the geocentric theory of the earth, well in use among astronomers before it was replaced by the heliocentric theory The same may also be said about attempts to apply this model to the plate tectonics theory which has gained much currency now. It is meant, above all, to prove such things as "natant continents", their collisions and overthrusts - that is what relates to surface tectonics. But the "heat engine" model does not explain the problem of global change of climate. Neither does it make us wiser on the mechanism of geomagnetic field generation, and on the causes of cyclicity of various parameters of this field. The point is that this model totally ignores the effect of the earth- moon system.

TIDAL EVOLUTION AND NATURAL PROCESSES

The tidal evolution model, applied to the system earth-moon-sun, not only agrees with the general regularities of motion relative to bodies of the solar system-it also explains the cyclicity of processes occurring on the surface of the earth and in its interior. Consequently, the parameters of the

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earth-moon orbit change in a cyclic mode, which means that in definite time intervals the natural satellite of our planet is either farther or closer to it. If the moon gets closer, with the slope of the equator to the ecliptic being smaller than it is today, then comes a cold spell; if it is farther away, with the slope being larger accordingly, the climate will become warmer then.

Ancient glaciations belong to the most dramatic events in the history of the earth as nodal points of climate fluctuations. Therefore changes of the equator's slope under the gravitational pull of the moon may be regarded as a plausible explanation of cooling periods. Climatologists view at least three epochs as glacial periods: the transition from the Algonkian to the Cambrian (ca. 540 mln years ago), from the Carboniferous period to the Permian (ca. 295 mln years ago), and in the Quaternary period (the recent 2 mln years). Periods in between successive glacial ages average 200 to 250 mln years. If we call this value a period of one cycle (and knowing the earth's rotational velocity and the orbital velocity of the moon and also, the value of angular kinetic energy equal to 2 x 10 ¹² W), we can make an estimate of variations within the following range: for the distance between the earth and the moon, +/-8R (its present value being 60R, where R is the radius of the earth); for the slope of the equator to the ecliptic, +/-6 ⁰ ; for the astronomic day period, +/-10 min. And so on.

Using these estimates, we can trace the pattern of change for each of the parameters, beginning from the Late Permian to the Recent period. This time interval, studied by geologists in great detail, fits squarely with the complete period of tidal evolution.

Permian was the time when the moon and the earth came closest to each other. In keeping with available data, the slope of the equator to the ecliptic stood at 17 ⁰ then. Climatologists say that small slopes are conducive to land glaciation because, as they tend to zero, the temperature difference between the poles and the equator attains to a maximum. Simultaneously, seasonal differences phase out. Insolation (exposure of terrestrial surface to solar radiation), as a function of slope, keeps increasing from the Permian on, reaching a maximum in the Late Jurassic (slope =30 ⁰ , with all latitudes exposed to the same average amount of solar radiation annually). Then again comes a period when the slope of the equator to the ecliptic decreases. The next slope minimum after the Permian occurs in the Late Tertiary and Early Quaternary Period.

The internal structure of the bowels of the earth is likewise connected with

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cyclic changes in the rotational velocity of the earth and in the slope to the equator. Slope changes should cause inhomogeneous flows at the mantle core and external/internal core interfaces, and also affect the state of the earth crust and stimulate volcanism. In the tidal evolution scenario, we can determine time intervals of a rapid and slow shifting of the axis. Rapid shifts will correlate with periods of volcanicity activation, of fold and fault formation; while slow shifts will concur with phases of the sluggish repacking of crustal blocks, manifest in the seismic "cracking" of the crust.

The characteristic time separating periods of active volcanicity and fold formation is in a range of 40 to 60 mln years by the tidal evolution model. These periods are close to those which Academicians Nikolai Dobretsov and Yevgeny Milanovsky have obtained empirically.

There are grounds to view the recent stage in the evolution of the earth-moon system as the time when the natural satellite of our planet, after a period of its maximal approach to us (late in the Tertiary and early in the Quaternary), has started moving away.

BIOGRAPHY OF DINOSAURS

As an independent class they, the dinosaurs, made their appearance in the Middle Triassic (ca. 225 mln years ago) and grew to a heyday of efflorescence in the Jurassic (200 to 135 mln years ago). They vanished, however, between the Cretaceous and the Tertiary (65 mln years ago). Now let us try to collate the history of their development with the tidal evolution model of the earth- moon system on a geochronological scale. The Middle Triassic witnessed an increase in the rotational velocity of the earth and, consequently, a runoff of water from high-latitude regions toward the equator. The equator's slope to the ecliptic increased thereby too, a process that made for a broader zone of the tropics and a larger volume of warm hydrosphere. The rising ocean level in the near-equatorial regions caused the inflowing rivers to burst their banks, a

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phenomenon that gave rise to extensive shallow waters. The plant kingdom changed as a result, and so did the animal kingdom.

As it is evident from fossilized remains, dinosaurs changed their body size. Initially those were small animals for the most part. But they grew much larger in the Jurassic and in the Cretaceous. So far no cogent explanation has been offered for this tendency. Yet the answer will be clear in the context of the tidal effects on the rotational velocity of the earth: that period saw changes in the area and depth of fresh-water bodies of water, and so plant and animal life had to accommodate. Such kind of situation was quite real, as shown by a map of flooded land in the Recent Cretaceous epoch or as it follows from a paleographic overview of what is now Mexico in the Recent Jurassic.

In keeping with the tidal evolution model, the living conditions of the plant and animal kingdoms were changing but insignificantly during the Jurassic (that is 60 mln years ago). Yet this was a rather long epoch - long enough to give rise to narrow-specialized species. Biological giants must have appeared at that time - such as the plant-eating diplodocus (dipolodocus), one of the pangolins (scaly anteaters): it measured more than 20 meters in length. Another giant, brontosaur (brontosaurus) weighed more than five present-day elephants. The largest of that breed lived in shallow waters where the buoyance force eased somewhat the terrific overloads of body weight on the limbs. Those monsters had no trouble getting food from water; besides, they were well protected against predators who could not steal up on their prey-that was next to impossible in large water shallows.

During the Mesozoic (230-65 mln years ago) both the species composition of dinosaurs and their habitats changed many a time, with some groups dying out to make room for others.

With a decrease in the rotational velocity of the earth the hydrosphere started migrating in the opposite direction, from low to high latitudes. The once bare shelves came to be inundated, and strips of dry land (like one linking Asia and America) disappeared. In turn, the shallow waters of the tropics were contracting slowly, and the shoreline of the ocean receding from the coast. As a result, the narrow-specialized animal species - the dinosaurs among other reptiles, started dying out. First water giants, the sauropods, disappeared and next, the land animals were gone. In the Mesozoic, from the Middle Jurassic or so, the rotation of the earth and the slope of the equator to the ecliptic started decreasing little by little. This brought about a change in climatic and hydrological conditions with a trend toward cooling and shrinking of water areas.

The end of the Cretaceous concurred with many major changes in the habitation media of various faunistic groups. For one, the sea level was more than 100 meters down (compared with the present), which meant a dramatic contraction of the marine ecological space. The pattern of oceanic streams likewise changed; the temperature of sea water and, possibly, its chemical composition were changing rapidly, with the concentration of oxygen in the benthic layers and all through the water mass going down. Most of the dinosaurs, however, were amphibians living both on land and in water. Throughout the Late Mesozoic, mind you, the climate was warm and humid for the most part, something that lowered the adaptive capability of various faunistic groups, dinosaurs among them.

Evidence on natural processes related to the tide-effected changes of the rotational velocity of the earth as well as other scientific findings on changed currents in the hydrosphere (transgressions and regressions of the ocean), on climate fluctuations and on activation of mountain formation and volcanicity - all these data enable us to speak of biota's adaptiveness to changing conditions. Those were not cataclysms but sluggish changes of the habitation medium. This means that critical conditions in biota's life may come not from some sudden dramatic events, but rather from a long period of slow change when organisms, used to "comfortable living", have to adjust to new conditions. Some species are unable to do that because of the narrow specialization of their morphology. But others, whose organism is "flexible" enough, cope well, without big losses.

Epochs of mass extinctions usually occur at the turn of transgression/ regression cycles. Dinosaurs had it "comfy" in the Cretaceous: a warm and humid climate, rather mild even in high latitudes (fossilized dinosaurian remains have been found as far north as Spitsbergen), abundance of plant food, extensive shallow waters. But then the situation changed much for the worse: flooding of high-latitude zones, sharp continental climate, distinct latitudinal zonality, intensive eruptive activity (the iridium anomalies in deposits could well correlate with it). It was too bad for the dinosaurs and other groups of animals. But these conditions that led to their extinction were anything but unique or extraordinary, they belonged in the scheme of things. In fact, many other species have disappeared from the face of the earth in the past 540 mln years. So we should not look for any cataclysms here, or else we shall not see the sun for meteoric explosions.

Pondering over the extinction of ammonites and dinosaurs, N. Newell of the United States said in the 1960s it was essential to find common causes of events before making correct conclusions. In keeping with the principle of parsimony of arguments, he continued, a hypothesis is considered correct if it satisfies a maximal amount of observations given a minimal amount of assumptions. As it appears to us, the tidal evolution model of the earth-moon-sun system fully conforms to the principle of parsimony of arguments.

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