بررسی اهمیت و نقش ورنی بیابان به عنوان یک شاخص کرونومتری در مطالعه نوسانات اقلیمی کواترنر اخیر (مطالعه موردی ریگ گناباد )

Introduction
Desert Varnish is a palaeogoemorphic factor in most arid regions. By studying its features, including micro-stratigraphy, morphology, thickness, and elements of it, we can find useful information about the long-term environmental conditions of the deserts. Gonabad Ergs is located south of Khorasan Razavi province and north of Gonabad city, between 34 ° 58 and 06 ° 59, east longitude, and between 34 ° 44 'and 34 ° 28' 34 'north latitude. The wind direction is dominated by the eastern region and The average temperature in the region is 17.5 degrees Celsius, and the average rainfall is 131.9 mm. The Erg,s is formed on a desert zone and clay peaks, and in some areas, Cretaceous formations include limestone, sandstone and shale, conglomerate and gypsum in low altitudes in the sandy region. Geomorphology of the sand consists of sandy hills in different forms. Most of the sandy hills of the area are of longitudinal hills, harbors and Seif.

Methodology
in this research , desert Varnish were studied as a geomorphic index for the study of late Quaternary climate fluctuations in the region. . In the field studies, we tried to collected the varnish in the area carefully, and After performing the initial processes including cutting, sample preparation (wearing the varnish level with gold palladium, covering additional parts with aluminum sheets), was transmitted to the lab for analysis by an electron scanning microscope(SEM) and Energy-X-ray diffraction (EDAX) analysis. and the required analyzes included the study of morphology, micro stratigraphy, thickness, and elements in Varnish.
Results and discussion
From the results of the experiments and analyzes carried out on the varnish of the region and their chemical analysis, it can be deduced that the varnishes in the region result from the subsidence of dust particles in the period of the abundance of dust and aerosol particles in the region, along with the extension of the period of drought It has been a source of wind activity in the region, although physicochemical processes and bacterial activity have also been effective in evolving varnishes. . An examination of morphology of varnishes in the region indicates The morphology of the varnish is in the Lamellte-shaped region indicates the frequency of wind activity with poor vegetation in the region. In the study of the elements found in Varnish, it was determined that the elements contained in Varnish are: Si, Fe, K, Mg, Au, Al, Pd, as well as a small amount of Ca and Ti, and Mn. The low amount of manganese (Mn) in the varnish and high Fe content indicate an increase in the ratio of iron to manganese in Varnish, which is indicative of the climate of dryness in the region. Also, the small amount of Ti or Titanium in Varnish indicates that the period of drought in the region and the formation of varnish is not too old and can be related to several hundred years or the last thousand years. As the age increases varnish , due to the stability of the tantalum, it is added to it and reduced to other cations such as K, Mg and Ca. with the varnish microlamination (VML) technique, Which is done using an electroscranoscope microscope, It was found that the studied vanishes had no layering in all samples, and It is a layer. This layer of Varnish has a lot of clay, aluminum, iron and other cations. That can be said, It is black varnish, And indicates the conditions for dry conditions in the area. This drought condition is most likely to occur after the last rainy season in the area and continues until now. Investigating the cationic ratio of varnishes in the region shows that This ratio between 9.8 to 11.63 using the K + Ca: Ti relationship . This ratio indicates that the beginning of the formation and sedimentation of Varnish in the region, that is, the beginning of the dry period in the region, has not been so old and is likely to be related to the beginning of the Holocene or 10,500 years ago. . According to the same study done by Dorn (1983) in the East of America, he obtained roughly the same proportions as the present study, in the work of Dorn, the cationic ratio between 9/17 and 15/55 is obtained, and the probable age for this interval is 100 It guesses up to 10,500 years. That this ageand seasonal range can range from 100 to about 10500 years.

Conclusion

What is to come from the examination of the region varnish it shows that With the onset of drought and wind activity in the area(approximately 10500 years ago), Varnish have begun to develop , and the results of the study of morphology and stratigraphy have shown The fact that the period of wind and drought activity in the region is long and that the period is not wet or wet is a long time. . Because in the event of a long wet period, the effects of this period are reflected in the morphology of the varnish, . Because the morphology of the region's Varnish is only in Lamellate form, it indicates the sovereignty of the dry period and the frequency of wind activity and dust storms in the area, if the duration of the wet period occurred in the area or was long, The morphology of the varnish was transformed from Lamellate to Botryoidal And the Varnish compounds also changed. Because in the wet period the amount and percentage of manganese(Mn(It focuses more on Varnish. Meanwhile, the amount of manganese in the varnish region is negligible And instead of iron (Fe) In Varnish more abundant, Which itself is evidence of the sovereignty of the dry period and the lack of vegetation in the region. The results also show that desert varnish can be used as a geomorphologic index to investigate the climatic fluctuations of the Ergs, especially the late Quaternary fluctuations.