لس: ویژگی‌ها و کاربردها برای مطالعات اقلیم گذشته

Introduction
Loess studies have about 185 years of documented history. High agricultural suitability, sensitivity to erosion and archiving the climatic changes signals, make the loess deposits very interesting to study. Understanding the historical development and different aspects of loess sediments are necessary for further investigations on loess deposits.

Methodology
Regarding the published studies on loess, this paper has been organized into five sections including: 1) definition and characteristics of loess, 2) history of world loess studies,
3) processes of silt production, 4) importance of loess deposits for paleoclimate studies and
5) investigations carried out on Iranian loess deposits and soils. Major references for each subject were reviewed.

Results and Discussion
The word "loess" has been derived from the German word "L?s", meaning loose, which was used for the first time by Leonhard in 1824. Regarding the definitions of loess presented by many authors, aeolian origin and silt fraction dominance have been accepted as the two main characteristics of loess deposits. The particle size distribution is the most important characteristic, which is measured before any other properties. The common mineralogical composition of loess deposits is as fallows: quartz in the sand fraction and quartz, feldspars, carbonates, mica and heavy minerals in the silt fractions. Illite and smectite are dominant minerals in clay fraction. However, the mineral composition is highly related to the loess origin. For example, loess in Argentina contains a lot of glass and feldspars, while Spanish loess deposits contain anhydrite and gypsum because they have originated from volcanic ash and Tertiary formations, respectively. Furthermore, soil formation processes during interglacial periods might have caused mineral transformation.
Loess was first identified by Leonhard in 1824. Lyell in 1833 attracted a lot of attention to loess deposits worldwide through his several reports. Determining the nature (aeoloan, alluvial or in situ genesis) and clarification of silt formation processes have been two main research subjects during this period. Berg (1916) suggested the in situ formation of loess; but Richtofen in 1882, many years before Berg, had proven the aeolian origin of loess deposits. Based on the close relationship between the major loess deposits areas and ice sheet regions, Tutkovskii in 1899 explained the role of glaciers in silt production. Even until recent decades, some researchers such as Smalley believed that glacial grinding was the main process having enough energy to produce quartz silt. Obruchev in 1945 divided loess into hot and cold loess and referred to factors except glacial grinding for silt production. Currently, based on the silt origin, loess deposits are grouped into four classes including glacial or periglacial loess, desert or peridesert loess, mountain or perimountain loess and non-typic loess.
Quartz silt is the most common particle in loess deposits. In the early studies, much attention was paid to the close relation between extensive loess deposits and glacial and periglacial paleoenvironments, giving rise to the hypothesis that glacial grinding was the main silt generating mechanism. Identification of loess deposits in desert environments showed that there were mechanisms other than glacial grinding for quartz comminution. Salt weathering, frost weathering, aeolian abrasion, and fluvial comminution are responsible mechanisms for silt production. Nahon and Trompette (1982), Wright (2001) and Iriondo and Kr?hling (2007) believe that, on a global scale, the importance of glacial grinding for the production of silt is less than what was initially expected.
The other important aspect of loess studies is based on the observations of Hardcastle in 1890 who related loess formation to climate changes. Up to now, research reports have shown that loess deposition and soil formation were active during the cold glacial and hot interglacial periods, respectively. The periods of loess deposition and soil formation are well correlated to marine isotope stages. This finding is the basis for paleoclimate reconstruction and landscape evolution.
There are vast areas covered by loess deposits in northern Iran. The presence of loess deposits in other parts of the country, especially desert fringes, is expected. For example, loess deposits in southern Mashhad and in Persepolis basin have been recently identified. It is evident that the loess thickness in dry regions is much less than that in the north. In recent decade, diverse aspects of loess deposits from northern and other parts of the country have been studied. The results of these studies showed the overall correlation of loess deposition and soil formation between Iran and major loess areas of the world during last glacial-interglacial period. The most important findings are windy LGM (MIS2) and soil formation during MIS5. There are, however, many unknowns, which should be clarified in the future investigations.


Conclusion
Loess is an aeolian silt-dominant sediment. Although it is believed that quartz and mica are the main mineral constituents, however, the mineral composition is highly related to the loess origin. Loess accumulation is a climate dependent phenomenon. The common argument is that loess deposition and soil formation were active during the cold glacial and hot interglacial periods, respectively. This fact makes the loess-peleosol sequences as a reliable archive of climate changes. The loess studies in Iran are limited and it is necessary to determine the loess distribution around the country and investigate their physical, chemical and mineralogical characteristics as well as their implication for paleoclimate studies.