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1993 | Buch | 2. Auflage

The Sea Floor

An Introduction to Marine Geology

verfasst von: Prof. Dr. Dr. h.c. mult. Eugen Seibold, Prof. Wolfgang H. Berger, Ph.D.

Verlag: Springer Berlin Heidelberg

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Über dieses Buch

Man's understanding of how this planet is put together and how it evolved has changed radically during the last 30 years. This great revolution in geology - now usually subsumed under the concept of Plate Tectonics - brought the realization that convection within the Earth is responsible for the origin of today's ocean basins and conti­ nents, and that the grand features of the Earth's surface are the product of ongoing large-scale horizontal motions. Some of these notions were put forward earlier in this century (by A. Wegener, in 1912, and by A. Holmes, in 1929), but most of the new ideas were an outgrowth of the study of the ocean floor after World War II. In its impact on the earth sciences, the plate tectonics revolution is comparable to the upheaval wrought by the ideas of Charles Darwin (1809-1882), which started the intense discussion on the evolution of the biospere that has recently heated up again. Darwin drew his inspiration from observations on island life made during the voyage of the Beagle (1831-1836), and his work gave strong impetus to the first global oceanographic expedition, the voyage of HMS Challenger (1872- 1876). Ever since, oceanographic research has been intimately associ­ ated with fundamental advances in the knowledge of Earth. This should come as no surprise. After all, our planet's surface is mostly ocean.

Inhaltsverzeichnis

Frontmatter
Introduction
Abstract
Marine geology is a young offshoot of geology, a branch of science which begins with James Hutton (1726–1797) and his Theory of the Earth (Edinburgh, 1795). Among other things, Hutton studied marine rocks on land. Changes of sea level (“encroachment of the ocean”, and “the placing of materials accumulated at the bottom of the sea in the atmosphere above the surface of the sea”) was a central tenet of his “Theory”. Thus, the question of what happens at the bottom of the sea was raised at the beginning of systematic geologic investigation. This question had to be attacked if the marine deposits on land were to be understood. Hutton was not alone in these concerns. A few years before the Theory of the Earth appeared, the great chemist Antoine Laurent Lavoisier (1743–1794) distinguished two kinds of marine sedimentary layers, that is, those formed in the open sea at great depth, which he called pelagic beds, and those formed along the coast, which he termed littoral beds. “Great depth” for Lavoisier was everything beyond wave base. Lavoisier supposed that sediment particles would settle quietly in deep water and reworking would be much less in evidence here than near the shore.
Eugen Seibold, Wolfgang H. Berger
1. Origin and Morphology of Ocean Basins
Abstract
The obvious question to ask about the sea floor is how deep it is and why. The overall depth distribution first became known through the voyage of H. M. S. Challenger (Fig. 1.1). We see that there are two most common depths: a shallow one near sea level (shelf seas), and a deep one between 1 and 5 km (normal deep ocean). The sea floor connecting shelves and deep ocean is of intermediate depths and makes up the continental slopes and rises. There is a portion of sea floor which is twice as deep as normal: such depths occur only in narrow trenches, mainly in a ring around the Pacific Ocean (Table 2.1).
Eugen Seibold, Wolfgang H. Berger
2. Origin and Morphology of Ocean Margins
Abstract
The continents are very old: they contain rocks aged thousands of millions of years. Ultimately, they are the product of selective accumulation of low density mantle material. Because of this low density they float on the mantle. The ocean floor, on the other hand, is geologically young, as we have seen. The basaltic rock which forms the ocean floor basement is rather close in composition to the mantle rock it came from. It is slightly heavier than continental rock (largely due to its high iron content, Appendix A6). The light-weight continental mass protrudes above the surrounding sea floor (Fig. 2.1a). Thick sediment piles accumulate at the boundary between continent and ocean, which build the actual margin (Fig. 2.1c). These sediments may be well layered or strongly deformed, depending on the tectonic forces active at the margin.
Eugen Seibold, Wolfgang H. Berger
3. Sources and Composition of Marine Sediments
Abstract
Marine sediments show great variety: there ist the debris from the wearing down of continents, shells, and organic matter derived from organisms, salts precipitated from seawater, and volcanic products such as ash and pumice (Fig. 3.1).
Eugen Seibold, Wolfgang H. Berger
4. Effects of Waves and Currents
Abstract
We have seen that the morphology of the ocean margins is largely determined by tectonics and sediment supply (Chap. 2), and we have reviewed the various types of sediments which are involved (Chap. 3). We now turn to the all-important role of water motion in determining the distribution of sediments on the sea floor. A first impression of this role can be gained from contemplating Fig. 4.1 which illustrates the redistribution of sediment supplied by a river in a setting typical for southern California. The water motions indicated are not the only ones that need to be considered, as we shall see.
Eugen Seibold, Wolfgang H. Berger
5. Sea Level Processes and Effects of Sea Level Change
Abstract
When studying sedimentary rocks on land, the first question a geologist will ask is whether the sediment was laid down above or below sea level, that is, whether or not it is of marine origin. For marine sediments, the next question usually is about the depth of deposition, that is, about the position of sea level relative to the sedimentary environment. On the present sea floor, depth of deposition rather dominates the major facies patterns of the material accumulating on it: the size distributions of clastic sediments, the chemistry of biogenous and authigenic matter, the distribution of benthic organisms. For the past, sea level fluctuations, on scales between thousands and millions of years, dominate the calendar of geologic history (Fig. 5.1).
Eugen Seibold, Wolfgang H. Berger
6. Productivity and Benthic Organisms — Distribution, Activity, and Environmental Reconstruction
Abstract
In the enormous living space provided by the sea, there are plankton (drifters), nekton (swimmers), and benthos (bottom-living organisms). Much of the benthos releases eggs and larvae into the plankton — this is the meroplankton, abundant in coastal waters. The larvae are dispersed by currents; they settle when their time has come, and start growing on the appropriate solid subtrate. The meroplankton feeds planktonic and nektonic predators. Conversely, the plankton feeds the benthos. Thus there is an intimate ecologic relationship between free-swimming and bottom-living organisms. Ultimately, of course, benthic organisms rely on food produced in surface waters in the sunlit zone (Fig. 6.1). The notable exception is the deep-sea benthic community at the hot vents of the Mid-Ocean Ridge (see Sect. 6.9).
Eugen Seibold, Wolfgang H. Berger
7. Imprint of Climatic Zonation on Marine Sediments
Abstract
The chief factor in producing climatic zonation is the amount of energy received from the sun — it is high in the tropics, low at the poles (Fig. 7.1). A coarsely latitudinal zonation of the oceans generally employs the categories tropical, subtropical, temperate, and polar, whereby the poleward part of temperate and the more temperate part of polar could be distinguished as subpolar (Fig. 7.2).
Eugen Seibold, Wolfgang H. Berger
8. Deep-Sea Sediments — Patterns, Processes, and Stratigraphic Methods
Abstract
As mentioned in the introduction deep-sea deposits were first explored in a comprehensive fashion during the British Challenger Expedition (1873–1876). Many thousands of samples were subsequently studied by John Murray (1841–1914), naturalist on the Challenger. He and his co-worker A. F. Renard published a weighty report on the results, which laid the foundation for all later work in this field of research. The first great step beyond Murray’s work were the results of the German Meteor Expedition, almost half a century later (1927–1929). A new branch of oceanography started with the recovery of long cores by the Swedish Albatross Expedition (1947–1949), that is, Pleistocene oceanography. It revolutionized our understanding of the great Ice Ages. Another great step came in 1968 with GLOMAR Challenger and the Deep Sea Drilling Project, which provided the samples for Tertiary and Cretaceous ocean history.
Eugen Seibold, Wolfgang H. Berger
9. Paleoceanography — the Deep-Sea Record
Abstract
Paleoceanography, the study of ocean history, emerged in the 1930s and 1940s, when cores became available that provided data from which history could be reconstructed. Initial efforts by W. Schott (1935) using short cores taken by the German research vessel Meteor have been mentioned (Sect. 8.2.3). In essence, the Swedish Deep Sea Expedition (1947–1948) played the same role in launching the new science of historical oceanography that the Challenger Expedition had played 70 years earlier, for physical and biological oceanography. The research vessel Albatross set out from Gothenburg in 1947, to begin the circumnavigation of the world’s tropical environment, under the leadership of Hans Petterson. The expedition used a new device, the piston corer, developed by B. Kullenberg in Copenhagen. This technique typically recovered cores of a length of 7 m or so, with the oldest sediment being from 0.3 to 1 million years in age. Kullenberg’s device, with modifications, is still used today (Fig. 9.1).
Eugen Seibold, Wolfgang H. Berger
10. Resources from the Ocean Floor
Abstract
The ocean floor contains energy sources (petroleum and gas) and raw materials (sand and gravel, phosphorite, corals and other biogenic carbonates, heavy metal ores). Also, the sea floor is used as a dump site for waste, which represents a considerable economic value. In terms of dollars and cents, energy (hydrocarbons) is the most important resource, while (at present) raw materials are of regional importance only. Nothing as yet has been gained from deep sea ores, although they are of great scientific interest and are potentially valuable. The various resources are summarized in Table 10.1. Many of the figures given are rather crude guesses: resources within the ground are difficult to quantrify. We shall briefly treat the geologic background for seafloor resources here, with some mention of the economic and the political problems associated with the use of the sea floor.
Eugen Seibold, Wolfgang H. Berger
Epilog
Abstract
We have attempted, in this brief survey of sea floor studies, to show where we have been and where we are now. But where are we going? An answer can only be tentative, of course. Just like other fields in the earth sciences, marine geology has experienced an exponential expansion of knowledge in the last two decades. This knowledge explosion is largely driven by technological developments: satellites, submarines, deep-sea drilling, all sorts of remote sensing devices, highly sophisticated laboratory equipment, and electronic information handling. Also, large-scale integration of geology with physics, chemistry, and biology has continued at a rapid pace, with new specialities arising from this cross-fertilization in quick succession, and recombining across disciplines to attack common targets: messages from the mantle, ridge-crest processes, fluid circulation in the margins, large-scale extinction, global environmental change.
Eugen Seibold, Wolfgang H. Berger
Backmatter
Metadaten
Titel
The Sea Floor
verfasst von
Prof. Dr. Dr. h.c. mult. Eugen Seibold
Prof. Wolfgang H. Berger, Ph.D.
Copyright-Jahr
1993
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-662-22519-6
Print ISBN
978-3-662-22521-9
DOI
https://doi.org/10.1007/978-3-662-22519-6