shelf

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Schematic profile of a continental margin including the shelf area and the beginning of the abyssal plains on the right margin. Small black arrows indicate the shear movement along the fault lines of the continental basement .
  • Shelf areas of the earth in turquoise blue

    • Shelf , continental shelf , continental shelf , continental shelf are names for the most marginal area of a continent , which is covered by sea. Such a sea is called a shelf sea . If this shelf sea extends relatively far into the interior of the continent, it is also referred to as an epicontinental sea ("sea located on the continent").

    Origin of term and name

    The term and its name Schelf were introduced into German-language specialist literature by the geographer Otto Krümmel at the latest in 1902 . The name is the Germanization of the English word shelf , which, according to Krümmel, was first used in 1887 by the Scottish geographer and meteorologist Hugh Robert Mill in the combination continental shelf . Krümmel himself described the shelf as "cornice on the sea edge of the continental shelf".

    oceanography

    In the morphological-oceanographic sense, a shelf is a platform that is slightly inclined towards the sea ​​and is up to 200 meters below sea ​​level . Since this is very little compared to the mean depth of the oceans, as well as to delimit it from the deep sea , the shelf sea is sometimes also referred to as shallow sea or shallow sea . The biologically highly productive shelf sea is the "nursery" of many fish species and also very rich in various animal and plant species.

    Depending on the geological conditions, the shelf can be a narrow hem or a wide, frayed belt. With 1500 kilometers, the Siberian Shelf has the largest seaward extent. On a global average, the shelf area is approximately 74 kilometers wide.

    On the landward side, the shelf is bounded by the chute , on the seaward side it is the shelf edge - a line from which the inclination of the seabed increases significantly. This area with a relatively steep gradient that follows the shelf edge towards the sea is the continental slope . It goes seaward into the continental foot, the slope of which is less than that of the continental slope, but greater than that of the shelf. The abyssal plains join the continental foot towards the sea .

    geology

    Slab of sandstone from the Cambrian of Spain with trace fossil Cruziana  i. e. S. This trace genus was very likely created by a trilobite and is generally typical of shelf sediments of the Paleozoic Era .

    In general geology, a shelf is defined as part of a continental block that is covered by the sea, that is, the subsurface of a shelf always consists of continental crust . In this sense, Zealandia can also be referred to as a shelf area, even if the sea there is well over 200 meters deep (in such cases one speaks of a sunken shelf ). Large parts of a continental shelf can dry out during periods of global sea level depression caused, among other things, by the Ice Age . The total area of ​​the shelf seas is then reduced in some cases drastically, which often leads to species extinction among the shelf inhabitants. In the opposite case, when the sea level rises worldwide due to the melting of the ice sheets at the poles, many rather narrow shelves expand into so-called epicontinental seas.

    The shelf term in sedimentology , on the other hand, is based on bathymetric criteria and the distance of the deposit area from the coast. "Shelf" where denotes a marine facies area ( neritic facies ), which all rocks comprises relatively close to the coast and at depths below the fair-weather shaft base (about 10 meters) to about 150 meters (± 50 meters) came to the deposit. On so-called clastic shelves, the neritic facies are characterized, among other things, by sandy storm deposits with characteristic sediment structures (hummocky stratification) and by the presence of silt grains in the finer-grained sediments. In the tropics and subtropics in particular, the predominantly biological formation of carbonates occurs on the shelf . The “carbonate factory”, d. H. the entirety of the marine organisms that produce carbonate, either directly or indirectly, reach their highest productivity there. With relatively little input of nutrients and turbidity, typical carbonate platforms with reefs form on tropical shelves . The mean sedimentation rate (thickness of deposited sediment per unit of time) is generally significantly higher on the shelf with several 100 meters per million years than in a deep ocean basin with a few meters per million years.

    All sedimentary rocks that were deposited at depths of more than 150 meters (± 50 meters) and relatively remote from the coast are summarized as deep-sea sediments or pelagic sediments (pelagic facies), regardless of whether the deposit area was underlain by continental crust or oceanic crust, that is, not every sediment in a shelf or epicontinental sea is necessarily a neritic sediment. In fact, the sediments stored today in the deep ocean basins have only a small chance of being geologically transmitted in the long term (several 100 million years) because they are largely subducted at the plate edges together with the underlying oceanic lithosphere . The pelagic sedimentary rocks that can be found on the mainland today were actually deposited on the continents (i.e. on shelves in the general geological sense), including in the coastal regions of passive continental margins. Pelagic sediments in the sediment lore of the continental platforms provide evidence of a particularly high global sea level at the time of their deposition.

    Both the current shelf platforms and the shelves of the geological past that are located on the mainland today are areas with significant oil and gas deposits. Examples of natural gas deposits in recent shelves are z. B. the North Sea or the northern Gulf of Mexico. The oil and natural gas deposits in Texas and on the Arabian Peninsula go back to the shelf seas of the Permian Basin and the Tethys Ocean , which no longer exist .

    Legal system

    Zones under international law according to the Convention on the Law of the Sea

    The continental shelf ( english shelf continental ) is legal, or to put it in maritime law meaning one of the Convention defined (UNCLOS) United Nations maritime zones. The continental shelf does not belong to the territory of the coastal state, the territorial sea ; however, the coastal state exercises sovereign rights over the continental shelf for the purpose of its exploration and the exploitation of its natural resources (Art. 77 para. 1 UNCLOS). Nobody may explore or exploit the continental shelf without the express consent of the coastal state. In practice, this particularly applies to marine mining .

    Demarcation

    In terms of demarcation, the continental shelf differs in two respects from the Exclusive Economic Zone according to the Convention on the Law of the Sea: First, the term continental shelf refers exclusively to the seabed and subsoil, not to the column of water or air above it. Second, the continental shelf can protrude beyond the extent of the Exclusive Economic Zone, which is limited to 200 nm from the baseline ( 200 mile zone ), if the geomorphological conditions justify this (Art. 76 para. 4 UNCLOS). This possibility is based on the idea that the continental shelf is the submarine continuation of the mainland. The deep-sea floor outside the continental shelf and the resources located there are, however, permanently withdrawn from the sovereignty claims of individual coastal states under UNCLOS and are regarded as the common heritage of all humanity (Art. 136 UNCLOS).

    History, politics

    In contrast to the Exclusive Economic Zone , which is a construct of UNCLOS from 1982, the continental shelf goes back historically to the so-called “ Truman Proclamation”, with which the USA was the first state in 1945 to claim the economic use of its continental shelf. In the following decades this idea quickly became customary international law and led to the Geneva Convention on the continental shelf of April 29, 1958. Subsequently, a number of states, including the Federal Republic of Germany (on January 20, 1964), proclaimed a continental shelf. In the course of technological development (possibilities for marine mining) and the simultaneous shortage of raw materials, the use of the continental shelf is becoming increasingly political and is leading to increased use of areas as continental shelf by states, such as the People's Republic of China in the case of the Senkaku Islands .

    Continental shelf of the Federal Republic of Germany

    The establishment of a continental shelf in the Baltic Sea and North Sea is difficult because of the proximity of the other neighboring countries and because both are shallow seas with depths of less than 200 m. The delimitation in the North Sea was a matter of dispute for a long time. It finally took place through bilateral agreements based on the judgment of the International Court of Justice on the so-called North Sea Continental Shelf Case (1969). The result of these determinations is the so-called "duck's bill" , which was established in 1972 through bilateral agreements with Great Britain, the Netherlands and Denmark. In the Baltic Sea, the delimitation of the German continental shelf is based on bilateral agreements with Denmark and Sweden based on the principle of equidistance.

    The exploration, extraction and processing of mineral resources in the area of ​​the continental shelf is regulated in the continental shelf mining ordinance .

    List of contracts for the continental shelf of the FRG

    • United Nations Convention on the Law of the Sea of ​​December 10, 1982 German version z. B. in: Shipping law: Maritime Laws, Ordinances, Convention MAP Handelsgesellschaft mbH, Hamburg, 1998, ISBN 978-3980122214
    • Treaty on the delimitation of the continental shelf under the North Sea between the Federal Republic of Germany and the Kingdom of Denmark (Federal Law Gazette 1972 II p. 882)
    • Treaty on the delimitation of the continental shelf under the North Sea (Federal Law Gazette 1972 II p. 882 ff.) Between the Federal Republic of Germany and the Kingdom of the Netherlands (Federal Law Gazette 1972 II p. 889 ff.)
    • Treaty on the delimitation of the continental shelf under the North Sea between the Federal Republic of Germany and the United Kingdom of Great Britain and Northern Ireland of 25 November 1971 (Federal Law Gazette 1972 II p. 897 ff.)
    • Diplomatic exchange of notes on May 26th / 28th December 1976 between the Federal Republic of Germany and the Kingdom of Denmark on the delimitation of the continental shelf in the Baltic Sea
    • Proclamation by the government of the GDR on the continental shelf on the Baltic Sea coast. From May 26, 1964. Journal of Laws of 1964 Part I No. 6, p. 99
    • Treaty and protocol between the GDR and the Kingdom of Sweden on the delimitation of the continental shelf of June 22, 1978 (Journal of Laws of 1979 II. P. 39)
    • Treaty and protocol between the GDR and the Kingdom of Denmark on the delimitation of the continental shelf and the fishing zones of September 14, 1988 (Journal of Laws of 1989 II. P. 147)

    See also

    literature

    • IN McCave: Sedimentary Settings on Continental Margins - an Overview. Pp. 1-14 in G. Wefer, D. Billett, D. Hebbeln, BB Jørgensen, M. Schlueter, T. van Weering (Eds.): Ocean Margin Systems. Springer, Berlin, Heidelberg, New York, 2003 ISBN 3-540-43921-8
    • F. Press and R. Siever: General geology . Spectrum Akademischer Verlag GmbH, Heidelberg-Berlin-Oxford 1995

    Web links

    Remarks

    1. ↑ Krümmel uses the term continental shelf here synonymously with continent and not, as here in the article, synonymous with shelf .

    Individual evidence

    1. ↑ Section after Otto Krümmel: The ocean. 1st edition, Salzwasser, Paderborn, 2012 (reprint of the original edition from 1902) ISBN 978-3-86444-478-4 .
    2. ↑ Geological Makeup of Marine Environments marinebio.org (marine biological website, English).