Loess
Loess is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of the Earth's land area is covered by loess or similar deposits.
Loess is an aeolian sediment being an accumulation of: twenty percent or less clay and the balance mainly equal parts sand and silt typically from 20 to 50 micrometers per particle often loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs.
The word loess, with connotations of origin by wind-deposited accumulation, came into English from German Löss, which can be traced back to Swiss German and is cognate with the English word loose and the German word los. It was first applied to Rhine River valley loess about 1821.
Properties
Loess is, porous, friable, pale yellow or buff, slightly, typically non-stratified and often calcareous. Loess grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar, mica and other minerals. Loess can be described as a rich, dust-like soil.Loess deposits may become very thick, more than a hundred meters in areas of China and tens of meters in parts of the Midwestern United States. It generally occurs as a blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick.
Loess often stands in either steep or vertical faces. Because the grains are angular, loess will often stand in banks for many years without slumping. This soil has a characteristic called vertical cleavage which makes it easily excavated to form cave dwellings, a popular method of making human habitations in some parts of China. Loess will erode very readily.
In several areas of the world, loess ridges have formed that are aligned with the prevailing winds during the last glacial maximum. These are called "paha ridges" in America and "greda ridges" in Europe. The form of these loess dunes has been explained by a combination of wind and tundra conditions.
Etymology
Loess comes from the German Löss or Löß, and ultimately from Alemannic lösch meaning drop as named by peasants and masons along the Rhine Valley.History of research
The term "Löß" was first described in Central Europe by Karl Cäsar von Leonhard who reported yellowish brown, silty deposits along the Rhine valley near Heidelberg. Charles Lyell brought this term into widespread usage by observing similarities between loess and loess derivatives along the loess bluffs in the Rhine and Mississippi. At that time it was thought that the yellowish brown silt-rich sediment was of fluvial origin being deposited by the large rivers. It wasn't until the end of the 19th century that the aeolian origin of loess was recognized, especially the convincing observations of loess in China by Ferdinand von Richthofen. A tremendous number of papers have been published since then, focusing on the formation of loess and on loess/palaeosol sequences as archives of climate and environment change. These water conservation works were carried out extensively in China and the research of Loess in China has been continued since 1954.Much effort was put into the setting up of regional and local loess stratigraphies and their correlation. But even the chronostratigraphical position of the last interglacial soil correlating to marine isotope substage 5e has been a matter of debate, owing to the lack of robust and reliable numerical dating, as summarized for example in Zöller et al. and Frechen, Horváth & Gábris for the Austrian and Hungarian loess stratigraphy, respectively.
Since the 1980s, thermoluminescence, optically stimulated luminescence and infrared stimulated luminescence dating are available providing the possibility for dating the time of loess deposition, i.e. the time elapsed since the last exposure of the mineral grains to daylight. During the past decade, luminescence dating has significantly improved by new methodological improvements, especially the development of single regenerative protocols resulting in reliable ages with an accuracy of up to 5 and 10% for the last glacial record. More recently, luminescence dating has also become a robust dating technique for penultimate and antepenultimate glacial loess allowing for a reliable correlation of loess/palaeosol sequences for at least the last two interglacial/glacial cycles throughout Europe and the Northern Hemisphere. Furthermore, the numerical dating provides the basis for quantitative loess research applying more sophisticated methods to determine and understand high-resolution proxy data, such as the palaeodust content of the atmosphere, variations of the atmospheric circulation patterns and wind systems, palaeoprecipitation and palaeotemperature.
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 enabled 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.
Formation
According to Pye, 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.Periglacial loess
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 icesheets and mountain icecaps during the spring and summer. During the autumn and winter, when melting of the icesheets and icecaps 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 these 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.During the Quaternary, loess and loess-like sediments were formed in periglacial environments on mid-continental shield areas in Europe and Siberia, on the margins of high mountain ranges like in Tajikistan and on semi-arid margins of some lowland deserts like in China.
In England, periglacial loess is also known as brickearth.
Non-glacial
Non-glacial loess can originate from deserts, dune fields, playa lakes, and volcanic ash.Some types of nonglacial loess are:
- Desert loess produced by aeolian attrition of quartz grains
- 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.
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.Soils underlain by loess tend to be excessively drained. The fine grains weather rapidly due to their large surface area, making soils derived from loess rich. One theory states that the fertility of loess soils is due largely to cation exchange capacity and porosity. 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 farmland can experience dramatic erosion of well over 2.5 kg /m2 per year. In China the loess deposits which give the Yellow River its color have been farmed and have produced phenomenal yields for over one thousand years. Winds pick up loess particles, contributing to the Asian Dust pollution problem. The largest deposit of loess in the United States, the Loess Hills along the border of Iowa and Nebraska, has survived intensive farming and poor farming practices. For almost 150 years, this loess deposit was farmed with mouldboard ploughs and fall tilled, both intensely erosive. At times it suffered erosion rates of over 10 kilograms per square meter per year. Today this loess deposit is worked as low till or no till in all areas and is aggressively terraced.
Large areas of loess deposits and soils
Argentina
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.The neotropical loess is made of silt or silty clay. Relative to the pampean loess the neotropical loess is poor in quartz and calcium carbonate. The source region for this loess is thought by some scientists to be areas of fluvio-glacial deposits the Andean foothills formed by the Patagonian Ice Sheet. Other researchers stress the importance of volcanic material in the neotropical loess.
The pampean loess is sandy or made of silty sand.
Central Asia
From Southern Tajikistan up to Almaty, Kazakhstan, spans an area of multiple loess deposits.China
The Loess Plateau, also known as the Huangtu Plateau, is a plateau that covers an area of some 640,000 km2 in the upper and middle of China's Yellow River and China proper. The Yellow River was so named because the loess forming its banks gave a yellowish tint to the water. The soil of this region has been called the "most highly erodible soil on earth". The Loess Plateau and its dusty soil cover almost all of Shanxi, Shaanxi, and Gansu provinces, the Ningxia Hui Autonomous Region, and parts of others.New Zealand
Extensive areas of loess occur in New Zealand including the Canterbury Plains and on the Banks Peninsula The basis of loess stratigraphy was introduced by John Hardcastle in 1890United States
The Loess Hills of Iowa owe their fertility to the prairie topsoils built by 10,000 years of post-glacial accumulation of organic-rich humus as a consequence of a persistent grassland biome. When the valuable 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 Mississippi River near Vicksburg, Mississippi, consist of three layers. The Peoria Loess, Sicily Island Loess, and Crowley's Ridge Loess accumulated at different periods of time during the Pleistocene. Ancient soils, called paleosols, have developed in the top of the Sicily Island Loess and Crowley's Ridge Loess. The lowermost loess, the Crowley's Ridge Loess, accumulated during the late Illinoian Stage. The middle loess, Sicily Island Loess, accumulated during early Wisconsin Stage. The uppermost loess, the Peoria Loess, in which the modern soil has developed, accumulated during the late Wisconsin Stage. Animal remains include terrestrial gastropods and mastodons.