Importance of studies of the ocean depths

LETTERS

TO

THE

EDITORS

Importance of studies of the ocean depths THE surface of the seas and oceans extends over 361,000,000 km 2 or 70'7 % of the terrestrial globe. The total volume of this immense mass of salt water amounts to 137,000,000 km 3. Eighty-eight and one tenth per cent of the total surface of the ocean (or 61-5 % of the total earth's surface) covers depths greater than 1000 m. In the atmosphere the biosphere or living space extends vertically up to a hundred metres (with the exception of temporary sojourns of birds, insects, and some other organisms, as well as of their sporocysts, in higher layers). The oceanic part of the biosphere, populated from surface to bottom, has an average depth of 3800 m (accessory seas included). Consequently the living space of the deep-water zones of seas and oceans is at least 30-35 times as great as that of the atmosphere. Or, in other words, the deep region of the ocean is, spatially, the greatest existing biotope, immeasurably wider than all the other biotopes put together. Until recently our knowledge of the depths of the world's oceans was very restricted. It was generally confined to data bearing on the distribution of depths and superficial layers o f the sea floor, and on the hydrological, hydrochcmical, and biological characteristics of the water masses, and their dynamics. But studies of oceanic depths - bottom relief, bottom sediments, composition, and distribution of abyssal f a u n a s - are meant to provide correct answers, to aid in elucidating many important problems of this history of the world's oceans, palaeogeograpby in general, geological chronology, geotectonics, history of development of marine faunas, palaeoclimatology and other associated fields of science. Our insufficient knowledge of the ocean depths has impeded the progress of many related disciplines. During the last decade a growing interest in the study of the ocean depths has been observed in many countries. Within these years three around-the-world deep-sea expeditions have followed one another : the Swedish expedition on the Albatross, the Danish on the Galathea, and the British

Challenger 11. For many years systematic research work has been carried on by American research vessels, notably by the Albatross and Atlantis. For seven years the Vitjaz has been carrying out complex oceanographic investigations of the deep-water zone in the Okhotsk, Bering, and Japan seas, and the adjacent part of the Pacific Ocean. ICSU established a special committee on deep-sea investigations and DeepSea Research was initiated four years ago as an associated periodical. All these facts are evidence of the very great interest in deep-sea investigations which has grown up in recent years ; hitherto this branch of the science of the earth and its past, has been very backward. What are the causes of this new interest ? One of the most important problems of geology is the question of the antiquity of the oceans, the relative youth of the Atlantic and diffierent parts of the world's oceans. Modern seismographic methods (perfected and refined) permit estimates of the total thickness of oceanic bottom sediments. If such a map could be obtained it would undoubtedly put an end to the long discussion on the relative antiquity of the oceans and their different parts. It would also be possible to reconstruct more reliably than before, the outlines of oceanic basins of past-geological periods. The determination of the thickness of bottom sediments underlying the water masses of the oceans will permit us to pose a question, which so far has not been approached. An accumulation of deposits during billions of years, even at an extremely slow rate of deposition (some few m m per 1,000 years), should assuredly raise the revel of the world's oceans, such a rise being a matter of thousands, rather than of hundreds metres. Allowing for the moistness of bottom sediments (about 70%) and the presumed length of existence of the world ocean this rise of the ocean level may be assumed to reach 2-3 km, i.e. the whole horizon of the bathyal. Furthermore the rate of this process may have been far from uniform. 67

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Letters to the Editors

Apart from this, the study of deep-sea depressions (more than 7000 m deep) is of great interest, though their total area covers merely 0.1% of the whole area of the ocean. These depressions Conceal some of the most important elements of the geotectonic relief of the earth's crust. They are, together with the adjoining island arcs, geosynclines in their early phases of formation and are accompanied by a complex of characteristic phenomena, including earthquakes and volcanic activity. The deep-sea depressions attract the attention of investigators by their structure, geographical distribution, and fauna. There are nineteen such depressions in the world ocean, and the far greater majority of them (15) are situated in the Pacific ocean. The deep-water depressions, extending over many thousand miles on the western side of the Pacific, from the Aleutian to the Philippine and Indonesian islands, follow the great line of rupture of the earth's crust, along the so-called andesite line, where the sial shell of the continent joins the sima shell of the floor of the Pacific. The Kuril Trench, which lies to the east of the Kuril island arc, is one of the deepest of the Pacific depressions (10382 m) and nearly the youngest of them at that. Here are well expressed all the elements of an island arc, such as a deep-water marine basin (Sea of Okhotsk) adjoining it from the side of the continent, and a depression from the side of the ocean. The great importance of studies of the island arcs, as an early phase of geotectonic processes, has been repeatedly stressed by Academician A. N. ZAVARITZKY,who believes studies of the Pacific ocean, with the widest possible scope, to be " one of the most important problems that confront the science of the earth "" and considers th~ problem of island arcs its most fundamental part. " No insight," says ZAVAmTZKV " c a n be obtained into the formation of the earth's surface in general, its continents, and its s e a s . . . without elucidating the question of the origin of the Pacific o c e a n . . , and to this problem are in their turn closely r e l a t e d . . , a whole chain of the deepest and basic questions of geophysics, geology, c o s m o g o n y . . . " " The tasks confronting the sciences of the earth's pertinent to the study of the margins of the Pacific ocean, are indeed unlimited "* This appreciation of the problem given by ZAVARITZKYfrom a purely geological position, cannot be underrated; on the contrary it could be greatly stressed and developed from the standpoint of many related sciences, such as palaeogeography, palaeoclimatology, '~'biochemistry, biology, etc. Studies of the great depths of the far-eastern seas and adjacent parts of the Pacific were initiated in the U.S.S.R. in 1932 by Professor K. M. D•RJUGIN, but these investigations were confined to depths of some 4000-4300 m. In 1949 the R/V Vitjaz of the Institute of Oceanology of the Academy of Sciences of the U.S.S.R. started its investigations involving the vast regions of the north western part of the Pacific ocean and all the depths. Extensive material was collected by the Vitjaz on the Kuril Trench, as well as on the Aleutian, Japan, and Bonin tren:hes. A comparison of the deep-sea depressions on the western side of the Pacific reveals a similarity of depth between the deepest of them, the depths ranging from 10300 to 10800 m. There is also a striking resemblance of the configuration of the depressions and their profiles. Their lateral slopes are very steep, whereas the bottom proves to be utterly flat. These similar features seem to imply the existence of some definite structural feature in the surface layer of the earth's crust underlying the. oceans, which accounts for this likeness. The narrowness of the Kuril Trench is one of the most characteristic features of its bottom relief. The trench outlined by the 9000 m isobath has a length of 550 km by a width of 5 km. The 6500-m isobath connects three depressions- the Kuril, the Japan, and the Bonin. To the south of the latter the f l o o r of the ocean rises considerably. The isobath of 5000 m links these three depressions to the northwestern part of the Aleutian Trench. The northern part of the Kuril Trench is characterized by an extensive development of tectonic forms of relief. Faults, s!ides, and denudations of ancient bedrocks often present a very variegated picture. Another noteworthy feature of the macrorelief of the Pacific ocean floor is a meridionally directed elevation in its northern part. This elevation divides the depths of" the ocean into an eastern and a western basin. It is especially well expressed in the region of the Hawaiian islands and northwards up to the Aleutians. It can be also traced southwards down to the Antarctic. However it will be the task of future investigations to determine its extent and character for the whole Pacific ocean and to attempt to draw an analogy with the meridional underwater elevation of the Atlantic. Many sedimentary rocks on the land were formed on the bottom of oceans and seas, but very little is known about how they are formed at present. We can m~rely state that the formation is controlled chemically, physically, biologically, and geologically by the overlying thickness of oceanic waters. *Zavaritzky A. N. 0952) Ostrovniye dugi, Moscow (in Russian).

Letters to the Editors

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Cores several meters long have already been obtained (the Vitjaz has taken cores of 34 m), and cores of 100 and 150 m will probably be obtained in the near future. This will permit an insight into the processes of diagenesis, i.e. the transformation of marine sediments into sedimentary rocks in the course of many millions of years. A comparison of the results of stratigraphic and biologic analyses (especially for diatoms and foraminifera) by the method of carbon-isotope and analogous techniques allows very precise assessment of the climatic variations of the Glacial and Post-Glacial period. It is hardly possible to overrate the possibility of palaeoclimatologic reconstructions for many million years, for the whole terrestrial globe. It seems quite probable that it will be possible to reconstruct the chronology of palaeontologic time for even greater periods. Oceanic bottom sediments in most cases preserve their regular stratification a condition which together with a considerable moistness, facilitate a most thorough analysis. It may be reasonably assumed that sediment solutions preserve some chemical characteristics from the time of their deposition. If so it will be possible to reconstruct the salinity of seas and oceans for the geological time corresponding to the length of the core obtained. The history of the salinity of the world's oceans still remains one of the most enigmatic problems of oceanology. Many problems are hidden too in the dynamics and chemigtry of the oceanic bottom-water layers. Data so far available seem to indicate a rather uniform saline, gaseous, and even thermal regime of the ocean depths. Thus in the immense expanse bounded by the 40 ° parallels the temperature of the bottom waters varies between 1-2°C, salinity within 34.8-34.9~o. The oxygen content is about 5 ml/l and the active reaction varies from 8.0 to 8'1. We might infer an extreme uniformity of the deep water in respect of all major indices. But is it really so ? Do our scales of uniformity and our methods of physical and chemical analyses used in oceanography, apply to this case ? We know that the deep-sea red clay is very rich in radium. Holothurians of the elasipodid group, echiuroids, pogonophores, siliceous sponges, and other forms that predominate in the deep-water fauna, are characterized by a peculiar chemical composition. Are there some peculiar biochemical provinces in the deepwater zones which determine to a considerable degree the distribution of organisms ? Do the deep-water organisms, which exist with seemingly very small differences of environmental factors, possess a special sensitivity to the smallest variations, not detected by our methods of analysis ? Thus the fauna may be used as a very fine indicator of the homogeneity (or heterogeneity) of environmental factors in the deep-water zones of the ocean. Investigations of the deep-sea fauna, including that of the ultra-abyssal (beyond depths of 6000-7000 m) carried out during the last few years have proved the incorrectness of our notion of a geographical and vertical homogeneity of the pelagic and benthic deep-sea fauna. On the contrary it can be divided vertically, into well-defined zones. The resemblance between the deep-sea fauna of the Pacific, Indian, and Atlantic ocean, their southern and northern parts, their eastern and western depressions (where delimitedby meridional elevations) is very relative and insignificant. Furthermore different groups of deep-water fauna endemic to one or another part of the world ocean are often groups of a high taxonomic order. The vertical and horizontal dissimilarity in the distribution of deep-sea fauna, persisting through periods of geologic time, cannot be explained by impediments in the dispersion of populations. The cause must be sought also in a heterogeneity of their environment despite its apparent homogeneity. Some peculiarities of different regions of the sea bottom, particularly the chemical ones, may still remain unknown. Microvariations of chemical and physical factors of the deep water layers that cannot be precisely determined by our analytical instruments and methods are perceptible to the deep-water organisms which have adapted themselves precisely to these peculiar conditions of life. It is also probable that the dynamics of those layers of oceanic waters which are deepest and nearest to the bottom, are still hidden from the investigators. The relief of the sea floor affects considerably the circulation of the water mass adjacent to the bottom, controlling the system of currents and cyclonic and anticyclonic circulations, of perhaps no lesser complexity than that of the surface layers. The historical aspect must evidently also be taken into consideration. The geographical distribution of the fauna of the upper layers of the oceans reflects generally the present orography of seas and oceans or that of the nearest geological periods the Post-Glacial, Glacial, a n d -, to a lesser degree-, the Tertiary.

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Letters to the Editor

Not a few data are now available which suggest that at great ocean depths the geographic distribution of fauna may still bear the imprint of former geological periods with a different orography of oceans. The deeper the horizon the more ancient is the orography of the oceans where the causes of the peculiarities of the recent distribution are to be sought, A detailed discussion of several problems pertinent to the study of the deep-sea fauna is given in another paper which will be published in this journal. The author could not approach many questions of no lesser interest than those discussed in this paper. There are undoubtedly many problems of equal importance in the domain of physical and chemical oceanography, which also require a thorough discussion. U A. ZENKEVICH

Institute of Oceanology Academy of Sciences of U.S.S.R.

" Oceanology"

versus " O c e a n o g r a p h y "

THE discussion aroused in Deep-Sea Research concerning the terms oceanology and oceanography is of fundamental interest. It has become desirable to attain some agreed point of view and avoid confusing these two terms. There is no doubt as to this confusion. In 1931 there appeared a fine book written by the well-known American sea biologist H. B. BIGELOW. He called the book Oceanography, and it included physics, chemistry, geology, and biology. BIGELOW concluded his book with an interesting chapter (VII) named "Physical, chemical, geological, and biologic unity in the sea." In other words, H. BIGELOW together with many others attaches to the term oceanography a sense which includes our whole sea knowledge and all aspects of these sciences. The sixth chapter of this book is named " E c o n o m i c value of oceanographic investigations." The author of the well-known book International Aspects of Oceanography, I. W. VAUGrtAN (1937), evidently treats the term in the same way. On the other hand, many authors do not understand by the term " oceanography," the whole of sea science but only a part, and primarily without biology. So, for instance, there appeared in 1955 a fine book Papers in Marine Biology and Oceanography dedicated to B1GELOW,the heading of which separates sea biology from oceanography. 1 fully subscribe to the opinion of BIGELOW (1931) about the unity of processes, performed in the ocean, and this ought to be reflected in the name of the science_ studying the ocean in this unity (physical, chemical, geological, and biological). But there arises a second question - what will be the name of this science ? Shall we use the old term, oceanography, or the new one, coming into broad use, oceanology ? It seems to me that it is not worth violating the term oceanography since there are many people who understand in it primarily physical oceanography the bottom relief and sediments- and think frankly that biology is not a part and parcel of oceanography. On the other hand the term oceanology is very convenient in linguistics and nobody will h~tve any doubt that it embraces all the science divisions of the sea hydrosphere (halosphere). Thus, I think (together with Dr. BRUUN and Commander HALL) that the term oceanology has all the rights of citizenship. If the term oceanography appears to be mol'e suitable to someone in the sense in which we use the term oceanology we may express a wish to indicate the sense in which it is used. L. ZENKEVICH

?~stitute of Oceanology Academy of Sciences of U.S.S.R.