Loess is an aeolian sediment formed by the accumulation of wind-blown silt,[3] typically in the 20–50 micrometer size range, twenty percent or less clay and the balance equal parts sand and silt[4] that are 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.[5] It was first applied to Rhine River valley loess about 1821
Properties
Loess is homogeneous, porous, friable, pale yellow or buff, slightly coherent, 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.[8]
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.[9] 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.[clarification needed]
History of research
The term "Löß" was first described in Central Europe by Karl Cäsar von Leonhard (1823–1824)[10] who reported yellowish brown, silty deposits along the Rhine valley near Heidelberg.[1] Charles Lyell (1834) brought this term into widespread usage by observing similarities between loess and loess derivatives along the loess bluffs in the Rhine and Mississippi.[1] At that time it was thought that the yellowish brown silt-rich sediment was of fluvial origin being deposited by the large rivers.[1] It wasn't until the end of the 19th century that the aeolian origin of loess was recognized (Virlet D'Aoust 1857),[11] especially the convincing observations of loess in China by Ferdinand von Richthofen (1878).[1][12] A tremendous number of papers have been published since then, focusing on the formation of loess and on loess/palaeosol (older soil buried under deposits) sequences as archives of climate and environment change.[1] These water conservation works were carried out extensively in China and the research of Loess in China has been continued since 1954. (Liu TS, Loess and the environment)
Much effort was put into the setting up of regional and local loess stratigraphies and their correlation (Kukla 1970, 1975, 1977).[13][14][15] 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. (1994)[16] and Frechen, Horváth & Gábris (1997)[17] for the Austrian and Hungarian loess stratigraphy, respectively.[1]
Since the 1980s, thermoluminescence (TL), optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) dating are available providing the possibility for dating the time of loess (dust) deposition, i.e. the time elapsed since the last exposure of the mineral grains to daylight.[1] During the past decade, luminescence dating has significantly improved by new methodological improvements, especially the development of single aliquot regenerative (SAR) protocols (Murray & Wintle 2000)[18] resulting in reliable ages (or age estimates) with an accuracy of up to 5 and 10% for the 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, such as the palaeodust content of the atmosphere, variations of the atmospheric circulation patterns and wind systems, palaeoprecipitation and palaeotemperature