Hypothesis of a Decline in Global Sea Level at the Beginning of the Tertiary Period

L. A. Pukhlyakov

(Presented by Professor A. G. Sinyov)

The hypothesis that the level of the World Ocean was lower in the past was clearly formulated by F. Shepard [22, 23], who based it on the presence of submarine canyons. As is known, submarine canyons are depressions in the oceanic or marine floor resembling flooded riverbeds. Their similarity is expressed above all in the fact that the cross section of these formations, like the cross section of river channels, resembles a trapezoid with its short base turned downward, or a triangle with one of its angles directed downward. Further, the floor of submarine canyons is always inclined in one direction—toward the deep parts of the ocean. Many submarine canyons, like rivers, show a tendency to meander and have tributaries—other submarine canyons entering the main ones now from one side, now from the other.

As an example one may cite the Hudson Canyon, described by I. Tolstoy. This canyon begins near the mouth of the Hudson River, which flows into New York Bay. The floor of this canyon throughout its entire length is gently inclined toward the deep part of the Atlantic Ocean. Between the 3100 and 3800 m isobaths it cuts a gorge up to 550 m deep and up to 5.5 km wide, and on a comparatively level area, between the 4100 and 4200 m isobaths, it shows a tendency to meander. Finally, along almost its entire length, down to a depth of 4300 m, numerous tributaries flow into it.

In addition to the Hudson Canyon, on the eastern coast of North America there are the canyons Oceanographer, Baltimore, and others. On the floor of the Gulf of Mexico the Mississippi Canyon has been discovered, traced to a depth of 1646 m. Off the Californian coast of the USA the Monterey Canyon has been discovered, traced to a depth of 2743 m, where it enters a trough. North of the Aleutian Islands lies Bering Canyon, traced to a depth of 2378 m. Off the eastern coast of Asia the Tokyo and Pusan canyons have been discovered; they have been traced to depths of 1400 and 1473 m, respectively. In the Indian Ocean the Indus and Ganges canyons are known, the first of which has been traced to a depth of 1134 m. Off the western coast of Africa there is the Congo River canyon, which has been traced to a depth of 2266 m; off the shores of Portugal—the Nazaré Canyon to a depth of 2200 m [2; 22, pp. 221–226]. On the floor of the White and Barents Seas, extensions of the rivers Northern Dvina, Pechora, Mezen, and others have been discovered, which merge into a single

a channel ending between Scandinavia and Spitsbergen. In other seas of the Arctic Ocean, submarine extensions of the rivers Khatanga, Olenek, Lena, and Yana have been discovered; these extend far toward the basin of this ocean [6, pp. 31–33]. On the floor of the North Sea an extension of the Rhine channel has been discovered, into which the extension of the Thames channel flows [11, pp. 158–159]. Submarine canyons are also known in the Mediterranean Sea, where they have been traced to a depth of 2200 m [3, p. 21].

It is interesting to note that, according to B. Heezen et al. [21, p. 66], samples of Cretaceous rocks were recovered from the floors of some submarine canyons of the Atlantic Ocean. It follows from this that the main periods of their formation were the Paleocene and the lower Eocene. But, as J. Bourcart notes [3, p. 31], only F. Shepard acknowledges that the formation of submarine canyons began in the Cretaceous period. Most scholars attribute to them a younger age. Thus, O. K. Leontiev [7, p. 329], proceeding from the fact that the upper reaches of some submarine canyons are incised into Pliocene deposits, believes that they were formed in Quaternary time, although it would have been natural to suppose that in Quaternary time a repeated lowering of the level of the World Ocean occurred, which may not have reached the magnitude that it reached in the Paleocene and lower Eocene.

Many scholars do not regard submarine canyons as drowned river channels, since a lowering of the level of the World Ocean by 2–4 km in Quaternary time (and they, as noted above, date the formation of these features not to the Paleocene and lower Eocene, but precisely to the Quaternary period) could not have occurred. Further, they refer to the fact that, for example, in the Mediterranean Sea submarine canyons are traced to a depth of 2200 m, whereas the depth of the Strait of Gibraltar at its shallowest point is approximately 300 m. Hence, no matter how much the level of the World Ocean fell, in the Mediterranean Sea, in their opinion, it could not have dropped by more than 300 m.

Here, however, it should be noted that one cannot agree with these objections. In particular, with respect to the Strait of Gibraltar it is known that two currents pass through it: a surface current directed eastward and a deep current directed westward. The discharge of the first of these is 55.2 thousand km³ per year, and the discharge of the second is 51.9 thousand km³ per year. Thus, 3.3 thousand km³ of water enters the Mediterranean Sea annually from the Atlantic Ocean. Hence, if this sea were at present cut off from the Atlantic Ocean, its level would begin to fall and after a million years would be 2.2 km below the present level.

Incidentally, drilling on the floor of the Mediterranean Sea in the area of the Balearic Islands showed that at a depth of 400 m anhydrite was encountered there, which usually forms only in shallow-water lagoons with intense evaporation. Drilling in other places of this sea showed that in many places there, too, it had been dry land. In William Ryan’s opinion, the marine regime in the place of the Mediterranean Sea arose about 5–12 million years ago as a result of erosion in the mountain chain at Gibraltar [24].

Consistent with what has been set forth is the fact that among the land animals of Crete in the past there were bulls, but no horses. The main evolution of the horse family, as is known, took place in North America, whence they emigrated to Eurasia at the end of the Miocene or the beginning of the Pliocene, during the last period of existence of the isthmus at the site of the Bering Strait, that is, about 8–10 million years ago (see below). Before that, the territory of Crete apparently formed a single whole with the surrounding land, and bulls settled it. By the time horses appeared in Eurasia, however, the level of the Mediterranean Sea had already risen, and Crete had become an island.

But a new question arises. Why could the level of the world ocean have changed? The answer to this question should be sought in the hypothesis of a change in the amount of free water on Earth.

As is known, under conditions at the Earth’s surface many minerals are formed that contain bound water in their composition—for example, kaolin, brown iron ore, etc. In addition, a large amount of water is physically bound to various rocks (hygroscopic water, pore water, etc.). According to V. A. Sulin [18, p. 6] and other authors, the amount of bound water in the Earth’s crust down to a depth of 20 km reaches 8% by weight, which is equivalent to 21.5% by volume. Thus, if in the region of some continent one takes a column of rock 20 km high and subjects it to all methods of action capable of taking away water, then this column of rock will yield a column of water 4.3 km high.

Between the middle Eocene and the Pliocene—that is, during almost the entire Tertiary period—powerful folding movements took place on the surface of the Earth, as a result of which the Cordilleras, the Andes, the Himalayas, the Caucasus, the Alps, and many other mountain structures were formed. In the process of the formation of these mountains, enormous masses of sedimentary rocks found themselves in the deep zones of the Earth. An even larger quantity of sedimentary rocks at the end of the Upper Cretaceous found itself in the deep zones of the Earth as a consequence of the attachment to the Earth of a second satellite. The rocks of this satellite initially formed on the Earth a gigantic accumulation, which subsequently subsided, and in its place the Pacific Ocean arose [14, 15, 15-a]. Having undergone metamorphism, these rocks must have released the bound water contained in them—transferring it into the free state. Once on the surface of the Earth, this water must have significantly raised the level of the world ocean.

But if many scientists doubt the erosional origin of submarine canyons, no one doubts that other submarine formations—guyots—were formed owing to the activity of water, that is, practically under subaerial conditions. As is known, guyots are submarine mountains with flat summits, located at depths from one thousand to three thousand meters. They occur in all oceans and by their existence indicate a lowering of the level of the entire world ocean.

Further, I. Tolstoy and M. Ewing, in studying the floor of the Atlantic Ocean on the summit of the submarine mountain Muir (230 km northeast of the Bermuda Islands), at a depth of 1400 m discovered calcareous clays containing Eocene foraminifers. Moreover, on these clays clear traces of subaerial weathering were observed. On the summit of another submarine mountain, which was located at a depth of approximately 3000 m, these researchers found disk-shaped pieces of limestone with remains of early Tertiary pteropods, in the shaping (eroding) of which surface agents had taken part.

An analogous picture is also observed in the Pacific Ocean. Thus, during drilling on Eniwetok Atoll (Marshall Islands), H. Ladd and others [26] established that at the base of this atoll, in the depth interval 1300–900 m, lies organogenic limestone of Eocene age; above it, in the interval 900–200 m, lies analogous limestone of Miocene age, and above 200 m the same rocks of Pliocene age. Rocks of Oligocene age are entirely absent here. These scientists further note that such limestone could not have formed at a depth of more than 50 m.

E. Hamilton [25], studying several guyots on the submarine rise between Necker Island (Hawaiian Islands) and Wake, that is, in

in the central part of the Pacific Ocean; in two of them, at a depth of 1200–1400 m, he discovered shallow-water fauna of Eocene age, and in three others, at depths of 1600–1900 m, fauna of Upper Cretaceous age.

From the material cited it follows once again that in the past, at the end of the Upper Cretaceous and the beginning of the Paleogene, there occurred a gigantic lowering of the level of the world ocean.

One of the problems of contemporary Earth science at the present time is that of land connections between the continents at the beginning of the Tertiary period. Thus, according to F. Van Houten [28], the lower Eocene faunas of North America and Western Europe had a very great similarity. They contained 95% identical families and 45% identical genera. Moreover, some genera appeared and disappeared in these regions simultaneously. This similarity is especially striking against the background of those differences between the faunas of these continents that are observed in the middle Eocene and in later deposits. In the opinion of F. Van Houten, these facts can be explained by assuming the existence of a land bridge in the place of the northern part of the present Atlantic Ocean, which at the beginning of the middle Eocene was flooded by the waters of this ocean.

The similarity between the faunas of the landmasses under consideration, which is observed between them in the middle Eocene and in some later epochs, in the opinion of F. Van Houten [28], can be explained by the existence of a land isthmus in the place of the present Bering Strait. This isthmus, as is known, continued to exist until the end of the lower Oligocene. Then it also disappeared and reappeared in the upper Miocene. But this time its existence was briefer—at the end of the lower Pliocene it disappeared. The first period of its existence is associated with the migration from North America to Eurasia and back of brontotheres and other animals, and the second with the migration from Eurasia to North America of proboscideans (mastodons), rhinoceroses, and carnivores, and from North America to Eurasia of horses [17, p. 253].

The fauna of Madagascar is very distinctive. Here, as is known, there occur animals very close to Malayan and South American forms, while many animals widely distributed in Africa are absent. In particular, there are no monkeys, cats, dogs, bears, proboscideans, or ruminants here [10, p. 64], and the ungulates are represented only by the dwarf hippopotamus, which has by now become extinct, and by the water hog—both animals are semiaquatic. On the other hand, 36 species of lemurs live here, of which 35 belong to the true lemurs and one to the lemuroids, or lorises. Other representatives of the loris family inhabit Ceylon and southern India (the slender loris), Kalimantan, Indochina, and India (the slow loris), as well as Africa (the galago) [12]. In the fossil state, lemurs have been discovered in the most ancient terrestrial deposits of the Cenozoic of Europe (Thanetian stage) [1, p. 342] and in deposits of approximately the same age in North America.

Among insectivores, Madagascar is inhabited by the family of tenrecs (bristle hedgehogs), which are close to the solenodons of the Greater Antilles, and by shrews. There are no venomous snakes here, but boas and iguana lizards close to South American forms do occur [4, pp. 268–270]. In the past, moreover, here, as well as in Egypt, France, and Wyoming, lived giant tortoises similar to the modern tortoises of the Galápagos, Seychelles, and Mascarene Islands [19, pp. 158–159]. Finally, in Madagascar a true edentate (braditherium) has been found in the fossil state, which, moreover, is known only in South America [10, pp. 64–65]. Of the modern carnivores, Madagascar is inhabited by viverrids, which, moreover, occur in Africa and southern Asia.

The best explanation of these facts is the assumption that at the beginning of the Tertiary period the level of the world ocean was much lower than at present. Owing to this, in the place of the present Mozambique Strait there was an isthmus. A similar isthmus extended to the present submarine rise within which the Seychelles and Mascarene Islands are now situated. And this rise, which at that time apparently was land, had a land connection with southern India. The southern spurs of the Mid-Atlantic Ridge likewise constituted land. Evidently, the Galápagos Islands also then had a land connection with South America. Thus, all the enumerated land masses were at that time connected with one another and had an approximately identical terrestrial fauna. This fauna included edentates, gigantic flightless birds (ancestors of ostriches, epiornises, etc.), giant tortoises, boas, iguana lizards, lemurs, tenrecs, and others.

There are no lemurs in South America, nor have they been found there in the fossil state. On this basis G. Simpson [27] and certain other scholars doubt that in the Paleocene this continent was connected with Africa and Madagascar (they date this connection to the Upper Cretaceous); however, the similarity of the Malagasy tenrecs to the solenodons of the Greater Antilles and the presence on Madagascar of a living edentate indicate that these regions were connected already after the appearance of mammals, i.e., at least in the Paleocene. As for lemurs, they may possibly have inhabited South America, but their bones have not been preserved. Arboreal animals in general are very poorly preserved in the fossil state.

Consistent with this is the fact noted by F. Van Houten that one representative of the notoungulates, Palaeostylops, has been discovered in the late Paleocene of Mongolia, and another, Arctostylops, in the earliest Eocene deposits of North America. Meanwhile, until now it had been considered that notoungulates were typical representatives of the South American mammalian fauna.

At the end (and possibly in the middle) of the Paleocene, owing to a rise in the level of the world ocean, the isthmuses that connected South America with other continents disappeared. From that time onward, and until the middle (or end) of the Pliocene, when the Panamanian Isthmus arose, the fauna of this continent developed in isolation from the faunas of the other continents. Around the middle of the Eocene the Mozambique Isthmus was flooded. At approximately the same time, the isthmuses connecting Madagascar with the Seychelles–Mascarene rise and India ceased to exist. Thus, already in the second half of the Eocene all land masses of the Southern Hemisphere proved to be separated from one another, and representatives of the newly developing fauna of Arctogea that began to penetrate into Africa and India—including monkeys, ungulates, cats, dogs, proboscideans, and others—could not reach Madagascar [10].

Objecting to this assumption, S. V. Maksimova [8] notes that it is contradicted by the absence on Madagascar of Eocene ungulates and creodonts, and, moreover, in her opinion it cannot explain the presence here of viverrids, rodents, the hippopotamus, and the bushpig, which appeared in Africa in the Oligocene, Miocene, Pliocene, and Pleistocene, respectively. In fact, there is nothing inexplicable here, since lemurs, tenrecs, and edentates apparently appeared in this region somewhat earlier than Eocene ungulates and creodonts. As for the later animals, viverrids and rodents could have crossed the Mozambique Strait on tree trunks, while the hippopotamus and the bushpig—simply by swimming.

Thus, in accordance with the hypothesis of a gigantic lowering of the level of the world ocean, several groups of facts are found. The first is the presence of submarine canyons in the most diverse regions of the floor of the world ocean. The second is traces of subaerial weathering and the presence of remains of shallow-water fauna at depths from 1–2 km in the Pacific to 3–4 km in the Atlantic oceans. The third is the strengthening of land connections between continents at the beginning of the Tertiary period (especially in the Paleocene). If one proceeds from this assumption, it should be expected that at some period in the past (in the Paleocene) the world ocean did not constitute a single whole, but was divided by isthmuses into several parts. One part combined the Arctic Ocean and the adjacent part of the Atlantic. The second corresponded to the central part of the Atlantic. The third part included the southern part of the Atlantic and the southwestern part of the Indian oceans. Finally, the fourth part combined the entire Pacific Ocean and the eastern part of the Indian. In the east this part was bounded by an isthmus in the area of the Drake Passage, and in the west by the Kerguelen–Indostan, or Central Indian Ocean, ridge projecting above the water. Within this basin lay Australia and New Zealand, isolated from one another and from other land masses. In accordance with this is the fact that neither Australia nor New Zealand has native placentals, which settled other land masses at the beginning of the Paleocene. In one way or another, almost complete isolation of the Australian and New Zealand faunal regions throughout the entire Cenozoic is acknowledged by all biogeographers without exception.

In light of the foregoing, the problem of mammalian evolution is resolved in a new way. In his time this problem was posed by I. V. Mushketov [9, p. 344], who noted that Mesozoic mammal fossils are extremely rare and belong to one order, the multituberculates. Other authors also speak of the presence among them of marsupials and insectivores, but the exceptional rarity of mammals among Mesozoic fossils remains generally recognized. However, in the Cenozoic, beginning with the very base of the Paleocene, in addition to multituberculates, marsupials, and insectivores, creodonts (ancient carnivores), condylarths (ancient ungulates, which had many features of carnivores), amblypods (another order of ungulates), lemurs, dermopterans, tillodonts, and rodents are encountered in the most diverse regions of the land. Edentates and ferungulates are known from the Paleocene as well; somewhat later from these arose litopterns, notoungulates, and other ungulates of South America. Somewhat later (in the Eocene) cetaceans and other orders of placentals appeared.

Such diversity of animals of a single group could not have arisen without prior evolution. In this connection J. T. Gregory [5, p. 625] suggests that mammals underwent a long evolution during the Mesozoic somewhere outside the well-studied continents. One gains the impression that they underwent this evolution in Antarctica. In fact, as R. W. Fairbridge [20], as well as E. Plumstead and D. White [13], note, during the Upper Paleozoic, Mesozoic, and the first half of the Cenozoic, rather luxuriant vegetation grew here and conditions existed for the normal life of many animals.

Furthermore, if one takes into account that the climate here was nevertheless colder than on the rest of the land, and that mammals have a number of adaptations to life in a cold climate (constant body temperature, hair covering, etc.), it is not difficult to imagine that, finding themselves isolated in Antarctica, they must have occupied the dominant—

a dominant position among other animals and to produce the diversity of forms that we observe at the beginning of the Cenozoic. When the level of the world ocean fell and, from Antarctica toward India and South America, the two above-mentioned intercontinental bridges, or isthmuses, extended, the former inhabitants of Antarctica penetrated into all corners of the land, except Australia and New Zealand.

In accordance with the foregoing is the fact noted by É. Reclus [16, p. 121], that on one of the islands situated within the Kerguelen–Hindustan rise, or the Mid-Indian Ocean ridge, Rodrigues (the Mascarene Islands), there live (or at least in the recent past lived) one species of lemurs and one species of tenrecs.

Thus, the hypothesis of a gigantic lowering of the level of the world ocean at the beginning of the Tertiary period is entirely acceptable at the present stage in the development of the science of the Earth.

REFERENCES

1. A. A. Borisyak. Course of Historical Geology. Gornefteizdat, 1934.

2. J. Bourcart. Relief of the Oceans and Seas (trans. from French). IL, 1953.

3. J. Bourcart. Morphology of the precontinent from the Pyrenees to Sardinia. In: Relief and Geology of the Ocean Floor (trans. from French). Progress Publishers, 1964.

4. A. G. Voronov. Biogeography. Moscow University Press, 1963.

5. J. T. Gregory. Vertebrate Animals in the Geochronological Scale. In: The Earth’s Crust (trans. from English). IL, 1957.

6. V. P. Zenkovich. The Sea Floor. State Publishing House of Technical-Theoretical Literature, 1956.

7. O. K. Leont’ev. Geomorphology of Sea Coasts and the Sea Floor. Moscow University Press, 1955.

8. S. V. Maksimova. The hypothesis of continental displacement and zoogeography. Priroda, No. 5, 1958.

9. I. V. Mushketov. Physical Geology, vol. 1. Gosizdat, 1924.

10. É. Haug. Geology, vol. 1 (trans. from French). Moscow, 1914.

11. D. G. Panov. Morphology of the Floor of the World Ocean. Publishing House of the Academy of Sciences of the USSR, 1963.

12. J. J. Petter. Our Relatives the Lemurs (abridged trans. from French). Nauka i zhizn’, No. 5, 1964.

13. E. Plumstead, D. Uitz. Review of Paleobotanical Investigations in Antarctica. In: Recent Geological Investigations in Antarctica (trans. from English). Mir Publishers, 1966.

14. L. A. Pukhlyakov. On the question of the origin of mountains and oceans (the hypothesis of an increase in the speed of the Earth’s rotation). Izv. TPI, vol. 120, 1961 (1962).

15. L. A. Pukhlyakov. On the question of the origin of the Pacific Ocean. Izv. TPI, vol. 127, issue 2, 1965.

15-a. L. A. Pukhlyakov. Review of Geotectonic Hypotheses. Tomsk University Press, 1970.

1. É. Reclus. The Earth and Its Inhabitants. Universal Geography. XIV. Ocean and Oceanic Lands. Madagascar, the Mascarene Islands, Insulinde, the Philippine Islands, Melanesia, Australia, Polynesia. St. Petersburg, 1895.

2. N. M. Strakhov. Fundamentals of Historical Geology, part II. Gostgeolizdat, 1948.

3. V. A. Sulin. Hydrogeology of Oil Fields. Gostoptekhizdat, 1948.

4. P. V. Terent’ev. Herpetology. Vysshaya shkola Publishers, 1961.

5. R. W. Fairbridge. Geology of Antarctica. In: Contemporary Antarctica (trans. from English). IL, 1957.

6. B. Heezen, M. Tharp, M. Ewing. The Floor of the Atlantic Ocean, part I (trans. from English). IL, 1962.

7. F. Shepard. Marine Geology (trans. from English). IL, 1951.

8. F. Shepard. The Earth Beneath the Sea (trans. from English). Mir Publishers, 1964.

1. “Is the Mediterranean Becoming a Desert?” Newspaper Soviet Russia, No. 239 (4358), October 13, 1970.

2. E. L. Hamilton. Sunken islands of the Mid-Pacific Mountains. The Geological Society of America. Memoir 64, 1956.

3. H. S. Ladd, L. Ingerson, R. C. Townsend, M. Russell and H. Stephenson. Drilling on Eniwetok atoll, Marshall islands. Bull. Amer. Ass. Petrol. Geol., vol. 37, No. 10, 1953.

4. G. G. Simpson. History of the fauna of Latin America. American Scientist, vol. 38, No. 3, 1950.

5. F. Van Houten. Review of latest Paleocene and early Eocene mammalian faunas. Journal of Paleontology, vol. 19, No. 5, 1945.