Loess

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]

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

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

Loesses are 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 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 ridges have formed that had been aligned with the prevailing winds during the last 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 Central Europe by Karl Cäsar von Leonhard (1823–1824),[10] who had reported yellowish brown, silty deposits along the Rhine valley near Heidelberg.[1] Charles Lyell (1834) brought the term into widespread usage, observing similarities between "loess" and its derivatives along the loess bluffs in the Rhine and in Mississippi.[1] At the time, it was thought that the yellowish brown silt-rich sediment was of fluvial origin and had been deposited by large rivers.[1] It was not until the end of the 19th century that the aeolian origin of the loesses was recognized (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, thermoluminescence (TL), optically stimulated luminescence (OSL), and infrared stimulated luminescence (IRSL) dating have been available, providing the possibility for dating the time of loess (dust) depositions, 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 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 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 deserts, dune fields, playa lakes, and volcanic ash.

Some types of nonglacial loess are:[28]

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 weather rapidly due to their large surface area, making soils derived from loess rich. A theory says that the fertility of loess soils is due largely to 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 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 which is 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 tilled in the fall, both intensely erosive practices. 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

Central Asia

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

China

The Loess Plateau (simplified Chinese: 黄土高原; traditional Chinese: 黃土高原; pinyin: Huángtǔ Gāoyuán), also known as the Huangtu Plateau, is a plateau that covers an area of some 640,000 km2 around the upper and middle reaches of China's Yellow River. The Yellow River was so named because the loess forming its banks gave a yellowish tint to the water.[34] The soil of this region has been called the "most highly erodible soil on earth".[35] 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.

Europe

Loess deposits of varying thickness (decimeter to several tens of meters) are widely distributed over the European continent.[22] The northern European loess belt stretches from southern England and northern France to Germany, Poland and the southern Ukraine and deposits are characterized by strong influences of periglacial conditions.[36] South-eastern European loess is mainly deposited in plateau-like situations in the Danube basins, likely derived from the Danube River system.[37][38][39] In south-western Europe, relocated loess derivatives are mostly restricted to the Ebro Valley and central Spain.[40][41]

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

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 consists 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 the 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.[43]

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]

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 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.[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]

  1. Frechen, M (2011). "Loess in Europe: Guest Editorial". E&G Quaternary Science Journal. 60 (1): 3–5. doi:10.3285/eg.60.1.00.
  2. Vasiljevic, D. A.; Markovic, S. B.; Hose, T. A.; Smalley, I.; O'Hara-Dhand, K.; Basarin, B.; Lukic, T.; Vujicic, M. D. (2011). "Loess Towards (Geo) Tourism – Proposed Application on Loess in Vojvodina Region (North Serbia)". Acta Geographica Slovenica (in Slovenian). 51 (2): 390–406. doi:10.3986/AGS51305.
  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
  6. Neuendorf, K.E.K., J.P. Mehl Jr., and J.A. Jackson, 2005, Glossary of Geology. Springer-Verlag, New York City, 779 pp., ISBN 3-540-27951-2
  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.
  15. Kukla, G. J. (1977). "Pleistocene Land-Sea Correlations I. Europe". Earth-Science Reviews. 13 (4): 307–374. Bibcode:1977ESRv...13..307K. doi:10.1016/0012-8252(77)90125-8.
  16. Zöller, L.; Oches, E. A.; McCoy, W. D. (1994). "Towards a revised chronostratigraphy of loess in Austria with respect to key sections in the Czech Republic and in Hungary". Quaternary Geochronology. 13 (5–7): 465–472. Bibcode:1994QSRv...13..465Z. doi:10.1016/0277-3791(94)90059-0.
  17. Frechen, M.; Horváth, E.; Gábris, G. (1997). "Geochronology of Middle and Upper Pleistocene loess sections in Hungary". Quaternary Research. 48 (3): 291–312. Bibcode:1997QuRes..48..291F. doi:10.1006/qres.1997.1929. S2CID 128551599.
  18. Murray, A. S.; Wintle, A. G. (2000). "Luminescence dating of quartz using an improved single aliquot regenerative-dose protocol". Radiation Measurements. 32 (1): 57–73. Bibcode:2000RadM...32...57M. doi:10.1016/S1350-4487(99)00253-X.
  19. Thiel, C.; Buylaert, J. P.; Murray, A. S.; Terhorst, B.; Tsukamoto, S.; Frechen, M.; Sprafke, T. (2011). "Investigating the chronostratigraphy of prominent palaeosols in Lower Austria using post-IR IRSL dating". E&G Quaternary Science Journal. 60 (1): 137–152. doi:10.3285/eg.60.1.10.
  20. Schmidt, E. D.; Semmel, A.; Frechen, M. (2011). "Luminescence dating of the loess/palaeosol sequence at the gravel quarry Gaul/Weilbach, Southern Hesse (Germany)". E&G Quaternary Science Journal. 60 (1): 116–125. doi:10.3285/eg.60.1.08.
  21. Frechen, M. (2011). "Loess in Eurasia". Quaternary International. 234 (1–2): 1–3. Bibcode:2011QuInt.234....1F. doi:10.1016/j.quaint.2010.11.014.
  22. Schaetzl, Randall J.; Bettis, E. Arthur; Crouvi, Onn; Fitzsimmons, Kathryn E.; Grimley, David A.; Hambach, Ulrich; Lehmkuhl, Frank; Marković, Slobodan B.; Mason, Joseph A.; Owczarek, Piotr; Roberts, Helen M. (May 2018). "Approaches and challenges to the study of loess—Introduction to the LoessFest Special Issue". Quaternary Research. 89 (3): 563–618. Bibcode:2018QuRes..89..563S. doi:10.1017/qua.2018.15. ISSN 0033-5894.
  23. Újvári, Gábor; Stevens, Thomas; Molnár, Mihály; Demény, Attila; Lambert, Fabrice; Varga, György; Jull, A. J. Timothy; Páll-Gergely, Barna; Buylaert, Jan-Pieter; Kovács, János (2017-12-12). "Coupled European and Greenland last glacial dust activity driven by North Atlantic climate". Proceedings of the National Academy of Sciences. 114 (50): E10632–E10638. Bibcode:2017PNAS..11410632U. doi:10.1073/pnas.1712651114. ISSN 0027-8424. PMC 5740632. PMID 29180406.
  24. Moine, Olivier; Antoine, Pierre; Hatté, Christine; Landais, Amaëlle; Mathieu, Jérôme; Prud’homme, Charlotte; Rousseau, Denis-Didier (2017-06-13). "The impact of Last Glacial climate variability in west-European loess revealed by radiocarbon dating of fossil earthworm granules". Proceedings of the National Academy of Sciences. 114 (24): 6209–6214. Bibcode:2017PNAS..114.6209M. doi:10.1073/pnas.1614751114. ISSN 0027-8424. PMC 5474771. PMID 28559353.
  25. Pye, K (1995). "The nature, origin and accumulation of loess". Quaternary Science Reviews. 14 (7–8): 653–667. Bibcode:1995QSRv...14..653P. doi:10.1016/0277-3791(95)00047-x.
  26. Bettis, E.A.; Muhs, D.R.; Roberts, H.M.; Wintle, A.G. (2003). "Last Glacial loess in the conterminous USA". Quaternary Science Reviews. 22 (18–19): 1907–1946. Bibcode:2003QSRv...22.1907A. doi:10.1016/S0277-3791(03)00169-0. S2CID 130982847.
  27. Muhs, D.R.; Bettis, III, E.A. (2003). "Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes" (PDF). GSA Special Papers. 370: 53–74. doi:10.1130/0-8137-2370-1.53. ISBN 9780813723709.
  28. Iriondo, M.H.; Krohling, D.M. (2007). "Non-classical types of loess". Sedimentary Geology. 202 (3): 352–368. Bibcode:2007SedG..202..352I. doi:10.1016/j.sedgeo.2007.03.012. hdl:11336/114389.
  29. Whalley, W.B., Marshall, J.R., Smith, B.J. 1982, Origin of desert loess from some experimental observations, Nature, 300, 433-435.
  30. Ding, Z.; Sun, J. (1999). "Changes in Sand Content of Loess Deposits along a North–South Transect of the Chinese Loess Plateau and the Implications for Desert Variations". Quaternary Research. 52 (1): 56–62. Bibcode:1999QuRes..52...56D. doi:10.1006/qres.1999.2045. S2CID 128767602.
  31. Haberlah, D (2007). "A call for Australian loess". Area. 39 (2): 224–229. doi:10.1111/j.1475-4762.2007.00730.x.
  32. Getis, Arthur; Judith Getis and Jerome D. Fellmann (2000). Introduction to Geography, Seventh Edition. McGraw Hill. p. 99. ISBN 0-697-38506-X.
  33. Ding, Z.L. (2002). "The loess record in southern Tajikistan and correlation with Chinese loess". Earth and Planetary Science Letters. Elsevier. 200 (3–4): 387–400. Bibcode:2002E&PSL.200..387D. doi:10.1016/S0012-821X(02)00637-4., 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.
  34. "Huang He". The Columbia Encyclopedia (6th ed.). 2007. Archived from the original on June 5, 2009.
  35. John M. Laflen, Soil Erosion and Dryland Farming, 2000, CRC Press, 736 pages ISBN 0-8493-2349-5
  36. Vandenberghe, Jef; French, Hugh M.; Gorbunov, Aldar; Marchenko, Sergei; Velichko, Andrey A.; Jin, Huijun; Cui, Zhijiu; Zhang, Tingjun; Wan, Xudong (2014). "The Last Permafrost Maximum (LPM) map of the Northern Hemisphere: permafrost extent and mean annual air temperatures, 25–17 ka BP". Boreas. 43 (3): 652–666. Bibcode:2014Borea..43..652V. doi:10.1111/bor.12070. ISSN 1502-3885.
  37. Fitzsimmons, Kathryn E.; Marković, Slobodan B.; Hambach, Ulrich (2012-05-18). "Pleistocene environmental dynamics recorded in the loess of the middle and lower Danube basin". Quaternary Science Reviews. 41: 104–118. Bibcode:2012QSRv...41..104F. doi:10.1016/j.quascirev.2012.03.002. ISSN 0277-3791.
  38. Jipa, Dan C. (November 2014). "The conceptual sedimentary model of the Lower Danube loess basin: Sedimentogenetic implications". Quaternary International. 351: 14–24. Bibcode:2014QuInt.351...14J. doi:10.1016/j.quaint.2013.06.008. ISSN 1040-6182.
  39. Újvári, Gábor; Varga, Andrea; Ramos, Frank C.; Kovács, János; Németh, Tibor; Stevens, Thomas (April 2012). "Evaluating the use of clay mineralogy, Sr–Nd isotopes and zircon U–Pb ages in tracking dust provenance: An example from loess of the Carpathian Basin". Chemical Geology. 304–305: 83–96. Bibcode:2012ChGeo.304...83U. doi:10.1016/j.chemgeo.2012.02.007. ISSN 0009-2541.
  40. Boixadera, Jaume; Poch, Rosa M.; Lowick, Sally E.; Balasch, J. Carles (July 2015). "Loess and soils in the eastern Ebro Basin". Quaternary International. 376: 114–133. Bibcode:2015QuInt.376..114B. doi:10.1016/j.quaint.2014.07.046. ISSN 1040-6182. S2CID 129905410.
  41. Bertran, Pascal; Liard, Morgane; Sitzia, Luca; Tissoux, Hélène (November 2016). "A map of Pleistocene aeolian deposits in Western Europe, with special emphasis on France". Journal of Quaternary Science. 31 (8): e2909. Bibcode:2016JQS....31E2909B. doi:10.1002/jqs.2909. ISSN 0267-8179. S2CID 132258680.
  42. Muhs; et al. (2013-02-06). "Eolian History of North America: Task 2, Understand the paleoclimatic significance of loess". USGS. Archived from the original on 2013-02-18.
  43. Miller, B.J., G.C. Lewis, J.J. Alford, and W.J. Day, 1985, Loesses in Louisiana and at Vicksburg, Mississippi. Guidebook, Friends of the Pleistocene Field Trip, 12-14 April, 1985. LA Agricultural Experimental Station, Louisiana State University, Baton Rouge, Louisiana. 126 pp.
  44. John Wilson. "Canterbury Region". Encyclopedia of New Zealand. Retrieved 16 February 2020.
  45. Eileen McSaveney. "Glaciers and glaciation - Retreating ice and the glacier legacy". Te Ara - the Encyclopedia of New Zealand. Retrieved 16 February 2020.
  46. Smalley, I.J., Davin, J.E. 1980. The First Hundred Years-A Historical Bibliography of New Zealand Loess. New Zealand Soil Bureau Bibliographic Report 28, 166pp.
    • 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).
  48. Sagayo, José Manuel (1995). "The Argentine neotropical loess: An overview". Quaternary Science Reviews. Pergamon. 14 (7–8): 755–766. Bibcode:1995QSRv...14..755S. doi:10.1016/0277-3791(95)00050-X.

Further reading
  • Smalley, I. J. (editor) 1975. Loess Lithology & Genesis. Benchmark Geology 26. Dowden, Hutchinson & Ross 454pp.
  • Smalley, I. J. 1980. Loess: A Partial Bibliography. Geobooks/Elsevier. ISBN 0 86094 036 5. 103pp.
  • Rozycki, S. Z. 1991. Loess and Loess-like Deposits. Ossolineum Wroclaw ISBN 83-04-03745-9. 187pp.
Look up loess in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Loess.
Wikisource has the text of the 1920 Encyclopedia Americana article Loess.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.