Loess

Source: Wikipedia, the free encyclopedia.
This article is about the geologic material. For the statistical technique, see local regression.
Loess in Vicksburg, Mississippi, United States

A loess (US: /ˈlɛs, ˈlʌs, ˈloʊ.əs/, UK: /ˈloʊ.əs, ˈlɜːs/; from German: Löss [lœs]) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust.[1] Ten percent of Earth's land area is covered by loesses or similar deposits.[2]

A loess is a

periglacial or aeolian (windborne) sediment, defined as an accumulation of 20% or less of clay with a balance of roughly equal parts sand and silt (with a typical grain size from 20 to 50 micrometers),[3][4] often loosely cemented by calcium carbonate. Usually, they are homogeneous and highly porous and have vertical capillaries that permit the sediment to fracture and form vertical bluffs
.

Properties

Hunyuan, Datong, Shanxi
, China
Loess from the Rhine lowlands near Mannheim with calcareous concretions

Loesses are

angular, with little polishing or rounding, and composed of quartz, feldspar, mica, or other mineral crystals. Loesses have been described as rich, dust-like soil.[5]

Loess deposits may become very thick: at more than a hundred meters in areas of Northwestern China and tens of meters in parts of the Midwestern United States. Loesses generally occur as blanket deposits that cover hundreds of square kilometers. The deposits are often tens of meters thick. Loesses often have steep or vertical faces.[6] Because the grains are angular, loesses will often stand in banks for many years without slumping. This type of soil has "vertical cleavage", and thus, it can be easily excavated to form cave dwellings, which is a popular method of making human habitations in some parts of China. However, loesses can readily erode.

In several areas of the world, loess

glacial maximum. These are called "paha ridges" in America and "greda ridges" in Europe. The formation of these loess dunes has been explained as a combination of wind and tundra
conditions.

Etymology

The word loess, with connotations of origin by wind-deposited accumulation, was introduced into English from the German Löss, which can be traced back to Swiss German and is cognate with the English word loose and the German word los.[7] It was first applied to the Rhine River valley loesses around 1821.[8][9]

History of research

The term "Löß" was first described in

fluvial origin and had been deposited by large rivers.[1] The aeolian origin of the loesses was recognized later (Virlet D'Aoust 1857),[11] particularly due to the convincing observations of loesses in China by Ferdinand von Richthofen (1878).[1][12] A tremendous number of papers have been published since then, focusing on the formation of loesses and on loess/paleosol (older soil buried under deposits) sequences as the archives of climate and environment change.[1]
These water conservation works have been carried out extensively in China, and the research of loesses in China has been ongoing since 1954. [33]

Much effort was put into setting up regional and local loess stratigraphies and their correlations (Kukla 1970, 1975, 1977).[13][14][15] However, even the chronostratigraphical position of the last interglacial soil correlating with marine isotope substage 5e was a matter of debate, due to the lack of robust and reliable numerical dating, as summarized, for example, by Zöller et al. (1994)[16] and Frechen et al. (1997)[17] for the Austrian and Hungarian loess stratigraphy, respectively.[1]

Since the 1980s,

last glacial record.[1] More recently, luminescence dating has also become a robust dating technique for penultimate and antepenultimate glacial loess (e.g. Thiel et al. 2011,[19] Schmidt et al. 2011)[20] allowing for a reliable correlation of loess/palaeosol sequences for at least the last two interglacial/glacial cycles throughout Europe and the Northern Hemisphere (Frechen 2011).[1][21] Furthermore, the numerical dating provides the basis for quantitative loess research applying more sophisticated methods to determine and understand high-resolution proxy data including the palaeodust content of the atmosphere, variations of the atmospheric circulation patterns and wind systems, palaeoprecipitation, and palaeotemperature.[1]

Besides luminescence dating methods, the use of radiocarbon dating in loess has increased during the past decades. Advances in methods of analyses, instrumentation, and refinements to the radiocarbon calibration curve have made it possible to obtain reliable ages from loess deposits for the last 40–45 ka. However, the use of this method relies on finding suitable in situ organic material in deposits such as charcoal, seeds, earthworm granules, or snail shells.[22][23][24]

Formation

Medicinal clay produced by Luvos. The clay is composed of loess with a fineness grade of 1.

According to Pye (1995),[25] four fundamental requirements are necessary for the formation of loess: a dust source, adequate wind energy to transport the dust, a suitable accumulation area, and a sufficient amount of time.[1]

Periglacial loess

Periglacial (glacial) loess is derived from the floodplains of glacial braided rivers that carried large volumes of glacial meltwater and sediments from the annual melting of continental ice sheets and mountain ice caps during the spring and summer. During the autumn and winter, when the melting of the ice sheets and ice caps ceased, the flow of meltwater down these rivers either ceased or was greatly reduced. As a consequence, large parts of the formerly submerged and unvegetated floodplains of these braided rivers dried out and were exposed to the wind. Because the floodplains consist of sediment containing a high content of glacially ground flour-like silt and clay, they were highly susceptible to winnowing of their silts and clays by the wind. Once entrained by the wind, particles were then deposited downwind. The loess deposits found along both sides of the Mississippi River alluvial valley are a classic example of periglacial loess.[26][27]

During the Quaternary, loess and loess-like sediments were formed in periglacial environments on mid-continental shield areas in Europe and Siberia as well as on the margins of high mountain ranges like in Tajikistan and on semi-arid margins of some lowland deserts as in China.[1]

In England, periglacial loess is also known as brickearth.

Non-glacial

Non-glacial loess can originate from

playa lakes, and volcanic ash
.

Some types of nonglacial loess are:[28]

  • Desert loess produced by aeolian attrition of quartz grains;[29]
  • Volcanic
    loess in Ecuador and Argentina;
  • Tropical
    loess in Argentina, Brazil and Uruguay;
  • Gypsum loess in Spain;
  • Trade wind
    loess in Venezuela and Brazil;
  • Anticyclonic
    loess in Argentina.

The thick Chinese loess deposits are non-glacial loess having been blown in from deserts in northern China.[30] The loess covering the Great Plains of Nebraska, Kansas, and Colorado is considered to be non-glacial desert loess.[26] Non-glacial desert loess is also found in Australia[31] and Africa.[27]

Fertility

Loess tends to develop into very rich soils. Under appropriate climatic conditions, it is some of the most agriculturally productive terrain in the world.[32]

Soils underlain by loess tend to be excessively drained. The fine grains

cation exchange capacity (the ability of plants to absorb nutrients from the soil) and porosity
(the air-filled space in the soil). The fertility of loess is not due to organic matter content, which tends to be rather low, unlike tropical soils which derive their fertility almost wholly from organic matter.

Even well managed loess

terraced.[citation needed
]

Large areas of loess deposits and soils

Central Asia

An area of multiple loess deposits spans from southern Tajikistan up to Almaty, Kazakhstan.[33]

East Asia

China

The

Ningxia Hui Autonomous Region, and parts of others.[citation needed
]

Europe

Loess deposits of varying thickness (decimeter to several tens of meters) are widely distributed over the European continent.

Ebro Valley and central Spain.[40][41]

North America

United States

Map showing the distribution of loess in the United States[42]

The

A-horizon topsoil is eroded or degraded, the underlying loess soil is infertile, and requires the addition of fertilizer in order to support agriculture
.

The loess along the

gastropods and mastodons.[43]

Oceania

New Zealand

Extensive areas of loess occur in New Zealand including the Canterbury Plains[44] and on the Banks Peninsula.[45][46] The basis of loess stratigraphy was introduced by John Hardcastle in 1890.[47][non-primary source needed]

South America

Argentina

An outcrop of loess in Patagonia

Much of Argentina is covered by loess. Two areas of loess are usually distinguished in Argentina: the neotropical loess north of latitude 30° S and the pampean loess.[48]

The neotropical loess is made of silt or silty clay. Relative to the pampean loess the neotropical loess is poor in

fluvio-glacial deposits the Andean foothills formed by the Patagonian Ice Sheet. Other researchers stress the importance of volcanic material in the neotropical loess.[48]

The pampean loess is sandy or made of silty sand.[48]

See also

  • Börde – Fertile plain – North German loess regions
  • Gäue – landscape type – South German loess regions
  • Loam – Soil composed of similar proportions of sand and silt, and somewhat less clay

References

Attribution

This article incorporates CC-BY-3.0 text from the reference "Loess in Europe: Guest Editorial".[1]

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  3. ^ Smalley, I. J.; Derbyshire, E. (1990). "The definition of 'ice-sheet' and 'mountain' loess". Area 22. pp. 300–01.
  4. ^ Donahue, Miller, Shickluna (1977). Soils: An Introduction to Soils and Plant Growth (4th ed.). Prentice Hall.
  5. ^ Pearson Prentice Hall - World Studies - Europe and Russia
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  7. ^ "loess". Online Etymology Dictionary.
  8. ^ "loess (sedimentary deposit)". Encyclopædia Britannica. Retrieved 2013-12-22.
  9. ^ "DWDS | Suchergebnisse". Dwds.de. Retrieved 2013-12-22.
  10. ^ Leonhard K. C. von (1823–1824). Charakteristik der Felsarten. 3 Vols., J. Engelmann Verlag Heidelberg, pp. 772.
  11. ^ Virlet D'Aoust P.T. (1857). "Observations sur un terrain d`origine météorique ou de transport aerien qui existe au Mexique et sur le phénomène des trombes de poussière auquel il doit principalement son origine". Geol. Soc. France, Full., 2d, Ser. 2, 129–139.
  12. ^ Richthofen F. von (1878). "Bemerkungen zur Lößbildung". Verh Geol Reichsanst, Berlin, pp 1–13.
  13. ^ Kukla G. (1970). "Correlation between loesses and deep-sea sediments". Geologiske Foreningen Foerhandlingar 92: 148–180. Stockholm.
  14. ^ Kukla G. J. (1975). "Loess stratigraphy of Central Europe". In: Butzer K. W. & Isaac G. L. (eds.) After the Australopithecus, pp. 99–188. Mouton, The Hague.
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    ., Fig. 1 (b) showing the distribution of loess, deserts, and mountains in Central Asia (adopted from [T.S. Liu, Loess and the Environment, China Ocean Press, Beijing, 1985.]). The locality of the Chashmanigar loess section is indicated by the solid arrow.
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  47. ^ *Hardcastle, J. 1890. On the Timaru loess as a climate register. Transcations & Proceedings of the New Zealand Institute 23, 324-332 (on line: Royal Society of New Zealand http://rsnz.natlib.govt.nz; reproduced in Loess Letter supplement 23, November 1988).
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Further reading

External links

Look up loess in Wiktionary, the free dictionary.
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Wikisource has the text of the 1920 Encyclopedia Americana article Loess.
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