loess

from Wikipedia, the free encyclopedia
Loess wall near Dirmstein (Palatinate)
Loess wall near Vicksburg (Mississippi)

Loess (main spelling) or loess is a homogeneous, non-stratified, light yellowish-gray sediment that consists mainly of silt . In addition, a certain carbonate content is often seen as an important criterion. Loess was mostly deposited by the wind , but mostly changed further after it was deposited in the ground.

Loess covers about ten percent of the earth's surface and is mainly found in the temperate climate zone. Most of it was formed in the Quaternary Cold Ages.

Loess is the starting substrate for the most arable soils worldwide. Furthermore, loosens and the fossil soils ( paleo soils ) incorporated in them are researched as archives for the reconstruction of Quaternary environmental changes .

Word origin, pronunciation and spelling

The term “loess” first appeared in 1821 in connection with loess deposits in the Rhine Valley and was introduced into geological and mineralogical literature in 1823 by Karl Caesar von Leonhard . He probably modified the dialect expression Lösch "loose ground" (from Alemannic delete "loose", "loose").

In the southern German- speaking area, loess [ løːs ] is pronounced with a long vowel, while in northern German the pronunciation loess [ lœs ] with a short vowel is common. The Heysesche rule , according to which the voiceless s According to a long vowel as beta and after a short vowel as ss is written, in this case leads to two different spellings, both of which are permitted. Both Duden. 24th edition. as well as the Wahrig dictionary of the German language , edition 2007, indicate loess as the main spelling and loess as the second spelling.

The term has entered many languages ​​as a technical term ( English loess , French lœss , Dutch löss ).

Research history

Loess was described by Ernst Heinrich von Dechen in the Siebengebirge in 1852 . In the 19th century there was still the predominant view that loess was a deposit of ice-edge reservoirs in the Ice Age (Felix Wahnschaffe, Gustav Steinmann, Friedrich Klockmann, August Leppla and others) or even earlier of river or sea deposits. The Aeolian origin as steppe dust was first represented by Ferdinand von Richthofen (1877) after observations on his trip to China (1868 to 1872) and then in 1888 by Adolf Sauer and Theodor Siegert after observing recent loess formation in Saxony. The realization of the Aeolian origin was slow to gain acceptance , for example Felix Wahnschaffe did not admit it until 1908. Albrecht Penck drew up a distribution map for the first time in 1884 and interpreted loess as repeatedly relocated, drifted ice age river clay. There was also a discussion about whether loess was formed interglacial or periglacial , but the periglacial formation was convincingly proven by Wolfgang Soergel , who also examined the temporal structure.

Controversial definitions

To this day, the term loess, its origin and its properties have been controversially discussed in the more than one hundred years of research. Considered in a simplistic way, there is a contrast between the sedimentological view, according to which loess as a deposit of mineral dust is an aeolian sediment , and the pedological perspective, according to which the typical structural change is the decisive element in loess formation regardless of the initial substrate.

In general, mineral dust was mobilized in the temperate zone mainly in dry and cold climates with sparse vegetation during the cold periods. The aggregation of minerals is often attributed to a tundra to cold steppe climate, which also prevailed in large areas of the temperate zone during cold periods. Since the formation conditions often overlap, both formation criteria are met for many deposits. Controversies arise over the question of whether non-Aeolian sediments with a typical loess structure or silty desert deposits can be called loess.

properties

Lösskindl ( oste logs )
Rheinaue south of Worms: alluvial loess used as arable land.

Loess consists mostly of silt . The clay content of fresh loess is very variable, 5 to over 20 percent, and can increase due to weathering . Loess is mixed with strongly changing proportions of fine sand ; its share in the mixture averages 20%, but it can also be higher. If the sand content is more than 50%, but the silt is still abundant, it is called sand loess , which is also known regionally as floating sand . Nevertheless, loess is considered a well-sorted sediment .

Mineralogically , loess consists for the most part (50 to 80 percent) of quartz grains with 8 to 20 percent calcareous fragments. Additions of iron hydroxides color loess yellowish to yellowish-red. Other minerals, such as feldspars , volcanic glasses or mafic minerals , are usually found in small quantities, but can also become dominant regionally, as in Argentina or on the North Island of New Zealand .

Loess is usually unlayered, unconsolidated and very porous. The porous structure of loess is often explained by former grass roots that formed in the deposited mineral dust. Concretions of lime in the loess caused by dissolution and reprecipitation of the finely divided limestone fragments and are referred to as Lösskindl or Lössmännchen designated. When loess is redistributed by water, the substrate can become stratified and is then called alluvial loess . In general, sediments that have arisen from relocation from loess, as well as decalcified loess, are called loess derivatives .

The individual particles in loess have a predominantly angular shape. Hence its high stability , which promotes the formation of loess walls on river banks, in sunken paths and the emergence of arable terraces .

Emergence

Loess is formed when silt and the finest sand are blown out when there is no vegetation cover and after a long transport (tens to several hundred kilometers) are deposited again in areas with denser vegetation. After the deposit, the individual minerals are usually cemented together by the easily mobilized lime or clay content. Loess formation is mainly restricted to the Pleistocene glacial periods, but it is still active in East Asia, for example. Loess can also form under warm, dry climatic conditions when dust is blown out of deserts and deposited in the peripheral areas where vegetation can flourish again.

In Central Europe , loess was formed during the cold ages, when there was hardly any vegetation due to the year-round low temperatures; the foreland areas of the glaciers and the river meadows were even largely free of vegetation . The largest deposits of loess in Central Europe are found in basins near larger rivers such as the Rhine, Main and Danube. Responsible for the mobilization of the mineral dust from the glacier forelands and floodplains were both the prevailing westerly winds and the frequent dry and cold winds from the Alpine ice sheet and the Nordic inland ice during the ice ages . While sand could only cover relatively short transport distances in the Aeolian transport and can often be found in the form of inland dunes in the immediate vicinity of the blow-out source, finer material was moved significantly further and deposited over a large area in the basin areas, with the greatest thickness on east-exposed slopes (on the leeward side ) . In the higher altitudes of the low mountain ranges, aeolian accumulated mineral dust is an important component of the periglacial layers .

Loess, loess derivatives and loess sediments

Already during, but also after the deposit, loess can be relocated by processes other than wind. Depending on the process, a distinction is made between creeping loess (slightly relocated on the slope), floating loess ( relocated by solifluction , usually contains a coarse skeleton of the material under the loess), flood (relocated by rinsing), landslide loess, infusion loess (deposited under aquatic conditions), cryoturbation loess ( mixed by frost change).

Furthermore, the weathering mainly attacks the upper areas of the loess. This loosens the lime and forms clay minerals. Loess loam is formed . All types of loess mentioned are called loess derivatives .

In 2002, the term loess sediments was introduced into the specialist literature as a superordinate term that includes both “fresh” loess and loess derivatives . The somewhat cumbersome terms “loess in the narrower sense” and “loess in the broader sense”, which were previously used, can therefore be dispensed with.

Distribution of the vegetation zones during the cold maximum of the last glacial period , in the period 24,500 to 18,000 BC In Europe
yellow hatching: loess desert; white: glaciation ; white dotted line: southern limit of the permafrost ground ; pink dashed line: southern limit of the tundra; green line: steppe / tree line.

distribution

Loess at the Kaiserstuhl

Loess is a very widespread sediment and - depending on the definition - occurs on all continents with the exception of Antarctica. About 10 percent of the earth's land surface is covered with loess. The main areas of distribution are the middle latitudes. In the tropics and high latitudes there are only a few occurrences of loess. The Central European Loess Zone extends from Belgium to western Ukraine. In the loess areas of Central Europe, loess occurs with an average thickness between 3 and 10 m, but it can be several tens of meters thick in favorable locations (slipstream) . Loess thicknesses of up to 400 m are known from the Chinese loess plateau .

Germany

Loess soil in Saxony (red) on a map from 1930

Loess areas are in southern Germany as Gäulandschaften , in northern Germany as Börden referred. Popular names for loess in southwest Germany (Ortenau, Kaiserstuhl) are glue or marl . Geologically speaking, however , marls are clayey carbonates.

The German loess areas north of the low mountain range belong to the Central European loess zone.

Austria

Loess wall near Großebersdorf

The Bavarian loess areas continue east into the Innviertel of Austria. 90 percent of Austria's loess deposits are in Lower Austria , especially in the Weinviertel . The deposits in the Krems area reach a thickness of up to 40 m. The Lower Austrian loos for Stone Age finds are world famous, such as the Venus of Willendorf , the Venus of Galgenberg or the twin graves of Krems-Wachtberg . The loess profiles of Paudorf , Furth bei Göttweig and Stillfried are well-known former type localities of loess research. Loess deposits can also be found in the south of Styria.

Switzerland

Only a few areas of Switzerland were not glaciated or crossed by large river systems. Loess deposits are only found in the far north around Basel , Baden and Schaffhausen .

Main distribution areas in Europe

Loess sediments are common over much of Eurasia, but focus on:

Loess landscape in China

Loess landscape near Hunyuan ( Datong city ) in the Chinese province of Shanxi

The most powerful loess blankets are found in East Asia , especially China. The solidified fly ash from the inner Asian steppes is yellow-brown and extremely rich in nutrients. Nowhere in the world has loess been deposited in large quantities. Along the Huang He ( Chinese: Yellow River ), which gets its name from the sediments it carries with it, it stands in ceilings of up to 400 meters. No river in the world has a higher sediment load, almost 40 kilograms per cubic meter of water. The entrained sludge is deposited in the river bed. Due to this sedimentation, the Yellow River increases its bed and has to be contained by ever higher dykes. At Kaifeng and Zhengzhou the water level is already ten meters above the surrounding area.

The loess layers in the mountainous regions of Henan , Shaanxi , Shanxi and Gansu are up to 300 m thick.

Loess in America

In both North and South America, the prairies and pampas are known all over the world and have been shaped by loess. They play a central role for the agriculture of the countries concerned (especially the USA and Argentina).

Today's meaning

nature

Loess wall, which has a large number of brood tubes
Loess dandelion ( Taraxacum serotinum )

Loess is of great importance for numerous plants and animals. For example, there is an enormous number of species of bees and wasps that make their nests in loess walls. Birds such as swifts and bee-eaters (the latter in isolated cases in southern Germany) also nest on or in loess walls. The loess dandelion ( Taraxacum serotinum ) needs loess subsoil and can take root on vertical loess walls.

Agriculture

Loess areas are very fertile and are part of the old settlements in Central Europe . The fertility comes from the small but not too fine grain size of the rock, which makes the mineral wealth easily accessible. The abundance of pores in loess, its good ventilation and its good properties as a water reservoir facilitate soil formation . Deep, easy to work and enormously powerful brown soils , parabrown soils and black soils are produced on loess . These soils and their distribution areas are particularly important for agriculture. With higher annual rainfall, loess can also develop pale earth and pseudogleye . It is estimated that around 80% of the world's grain grows on loess.

During the recultivation of the Rhenish lignite mining area , the separately cleared loess was either heaped up on the reclaimed agricultural land or in a polder process.

Loess favored the development of high Chinese culture. Its properties made agriculture easier, because loess is not only very fertile, but can also be worked with simple tools.

Danger

Agricultural use and in particular plowing destroy the soil structure in soils made from loess. Since it is easily washed away due to its small grain size (see Hjulström diagram ), the soil is endangered by erosion after the structure has been destroyed . Fallow or extensive cultivation of crops have the same effect. Heavy agricultural equipment also compacts the subsoil, so that water penetrates less easily and soil erosion increases with surface runoff .

In many places, old roads have cut deep into the loess, often creating loess hollow paths in loess regions .

Quaternary research

Loess offers an archive of Pleistocene climatic and environmental conditions and is researched with regard to their changes. The sequence of textural , structural and chemical changes allows conclusions to be drawn about past conditions and developments. Here are particularly important paleosols . In loess deposited under dry-cold conditions with little vegetation, soil formation shows longer phases of geomorphological stability; With higher precipitation and temperatures as well as denser vegetation, the sediment weather steadily and differentiates itself through further soil-forming processes. After covering with loess, the soil characteristics are archived and enable the reconstruction of the educational conditions. By examining many local manifestations of these so-called loess paleo-soil sequences, it is possible to determine which regional effects have caused global climate changes.

medicine

In natural medicine loess is used as a healing earth for internal and external use.

literature

  • IJ Smalley: Loess: Lithology and Genesis . John Wiley & Sons, Chichester 1976, ISBN 0-470-79901-3 .
  • IJ Smalley: Loess: A Partial Bibliography . Geo Abstracts, Norwich 1980, ISBN 0-86094-036-5 .
  • K. Pye: Aeolian dust and dust deposits . Academic Press, London 1987, ISBN 0-12-568690-0 .
  • C. Fleischer Mutel: Fragile Giants: A Natural History of the Loess Hills (A Bur Oak Original) . Univ. of Iowa Pr., 1989, ISBN 0-87745-257-1 .
  • L. Zöller: Würm and Rißlöß stratigraphy and thermoluminescence dating in southern Germany and neighboring areas. Habilitation thesis. Heidelberg 1995. (PDF; 3.9 MB)
  • M. Pecsi, G. Richter: Loess. Origin-Outline-Landscapes. In: Zeitschrift für Geomorphologie NF Supplementband 98, 1996.
  • R. Koch, H. Neumeister: For the classification of loess sediments according to genetic criteria. In: Journal of Geomorphology NF 49/2, 2005, pp. 183–203.
  • V. Gornitz (Ed.): Encyclopedia of paleoclimatology and ancient environments . Springer Verlag, Dordrecht 2009, ISBN 978-1-4020-4551-6 .
  • DR Muhs: Loess deposits, origins and properties. In: SA Elias (Ed.): Encyclopedia of Quaternary Science. Rotterdam 2007, pp. 1405-1418.

Web links

Wiktionary: Löss  - explanations of meanings, word origins, synonyms, translations
Wiktionary: Loess cover  - explanations of meanings, word origins, synonyms, translations
Commons : Löss  - collection of images, videos and audio files

Individual evidence

  1. ^ M. Pécsi: Loess is not just the accumulation of dust . In: Quaternary International . tape 7-8 , pp. 1–21 , doi : 10.1016 / 1040-6182 (90) 90034-2 ( elsevier.com [accessed April 13, 2017]).
  2. ↑ Loess. In: Encyclopaedia Britannica.
  3. ↑ Loess. In: Karl Caesar von Leonhard: Geology: or natural history of the earth. Volume 3, pp. 454 ff.
  4. ↑ Loess, loess. at: Duden online
  5. ↑ About the deposit of recent loess by the wind. In: Journal of the German Geological Society. 40, 1888, pp. 575-582.
  6. ^ Otfried Wagenbreth: History of Geology in Germany. Enke, 1999, ISBN 3-13-118361-6 , p. 125.
  7. ^ Albrecht Penck: About the loess in Germany. In: Journal of the German Geological Society. Volume 35, Issue 2, 1883, pp. 394-396.
  8. ↑ Wolfgang Soergel: Loess, Ice Ages and Paleolithic Cultures. G. Fischer, Jena 1919.
  9. ↑ Tobias Sprafke, Igor Obendet: Loess: Rock, sediment or soil - What is missing for its definition? In: Quaternary International (=  Kukla Loessfest ). tape 399 , April 18, 2016, p. 198–207 , doi : 10.1016 / j.quaint.2015.03.033 ( sciencedirect.com [accessed April 13, 2017]).
  10. ^ Ian Smalley, Slobodan B. Marković, Zorica Svirčev: Loess is [almost totally formed by] the accumulation of dust . In: Quaternary International (=  The Second Loessfest (2009) ). tape 240 , no. 1–2 , August 1, 2011, pp. 4–11 , doi : 10.1016 / j.quaint.2010.07.011 ( sciencedirect.com [accessed April 13, 2017]).
  11. ↑ P. Sümegi, K. Náfrádi, D. Molnár, Sz. Sávai: Results of paleoecological studies in the loess region of Szeged-Öthalom (SE Hungary) . In: Quaternary International (=  Loess and Dust: Contributions in honor of Ian Smalley ). tape 372 , June 22, 2015, p. 66–78 , doi : 10.1016 / j.quaint.2014.09.003 ( sciencedirect.com [accessed April 13, 2017]).
  12. ↑ Peter U. Clark, Arthur S. Dyke, Jeremy D. Shakun, Anders E. Carlson, Jorie Clark, Barbara Wohlfarth, Jerry X. Mitrovica, Steven W. Hostetler, A. Marshall McCabe: The Last Glacial Maximum . In: Science . tape 325 , no. 5941 , 2009, p. 710-714 .
  13. ↑ Sand wall for the sand martins near Giebenach Auf: Verein Naturnetz
  14. ↑ Investigations on Lössen in Northern Switzerland by Gouda Hassanein Gauda, ​​from 1958, on: Geographica Helvetica
  15. ↑ D. Haase, J. Fink, G. Haase, R. Ruske, M. Pécsi: Loess in Europe — its spatial distribution based on a European Loess Map, scale 1: 2,500,000 . In: Quaternary Science Reviews . tape 26 , no. 9-10 , May 1, 2007, pp. 1301-1312 , doi : 10.1016 / j.quascirev.2007.02.003 ( sciencedirect.com [accessed April 13, 2017]).
  16. ↑ Heiko Lindner, Frank Lehmkuhl, Christian Zeeden: Spatial loess distribution in the eastern Carpathian Basin: a novel approach based on geoscientific maps and data . In: Journal of Maps . tape 13 , no. 2 , February 6, 2017, p. 173-181 , doi : 10.1080 / 17445647.2017.1279083 .
  17. ↑ See naturopathy & naturopathic specialist portal: Heilerde heilpraxisnet.de