فهرست مطالب seiyed reza mehrnia

The wind erosion and the resulted landforms are the dominant landscape in flat and deep plains of Iran dry areas (Yamani, 2015). The importance of wind erosion in the deserts differs due to great variances in wind power (Goudie, 2013). This type of erosion which is controlled by the erosivity power of wind and the erodibility of impacted surfaces (sharma, 2010), is dangerous for three reasons: 1, the nutritious elements of the soil are destroyed and as a result the soil loses its power for keeping a conservative vegetation layer (Thomas, 2011). The deposition of eroded sediments can bury vegetation and river channels, pollute food and water reserves and also negatively impact the growth of vegetation, soil fertility, and the dynamics of ecosystem (Larney, 1998y; Worster, 2004; McTainsh & Strong, 2007 ). The transportation of eroded sediments through powerful winds can bring damage to buildings and products and also trouble visibility in roads and airports (Thomas, 2011).

In first step, the impacted area of fault spring in Daranjir playa are identified based on satellite images, Geology maps and field observations. In the next step, 100 milliliter of water was sampled for chemistry analysis from active spring across Bafgh – Poshte-badam fault (Kor spring). Thus, to determine the water table, chemistry analysis of the sediment and water samples across Bafgh – Poshte-badam fault, 4 points were selected for drilling and the locations of bores were determined by GPS. To this end, the sampling procedure across the fault was conducted by a hand auger with the length of 20 cm and diameter of 7.5 cm. In sum, four water samples and 22 sediment samples were collected. In the geomorphology laboratory, initially the amounts of TDS, EC, and pH in the sample waters were measured by a multi parameter device, version HI9811-5. To measure the amounts of TDS, EC and pH in sand samples, saturated paste method was utilized. Here, the samples were initially dried in a drying device, and then 50 grams of each sample was measured with an accurate scale and mixed with 50 millimeters of distilled water. In the following step, the distilled water was mixed with sediment samples and the amounts of EC and TDS was measured using the multi parameter device and other devices.

The results suggest that the active tectonic performance across Bafgh-Posht badam fault not only result in the emergence of springs across the fault but also lead to the reduction of groundwater and the creation of a wet zone across the fault in Daranjir playa due to the penetration of water to the aquifer. This wet zone across the studied fault caused a significant growth of shrubs and Tamarix mascatensis, and reduced the speed of wind in the examined area. Regarding this Pye & Tsoar and state that in desert areas the salinity degree of groundwater has an important role in the transformation and distribution of deserts’ vegetation. The analyses of the qualitative and quantitative characteristics of groundwater across Bafgh- Posht badam fault show that the minimum water table, TDS and EC across the fault are ,respectively. In fact, by injecting fresh water across the fault, the fault springs not only reduce the salinity of groundwater but also raise the the water table across the mentioned fault and bring the transportation of the sediments to a minimum level. As for this case, Silva et al., (2018) state that in areas with high water tables, the sand sources are limited and consequently the transportation of sediments reduces. Moreover, Kocurek & Nielson concluded that high water tables can reduce the transportable sediment by conserving surface moisture. In addition to the formation of vegetation, the increase of water level across the fault, the creation and change of Nebkha formation location are the main effects of spring faults which have a crucial role in controlling wind sediments. As a matter of fact, the fresh water of springs results in the formation of vegetation and this vegetation captures wind sediments and forms Nebkhas.

The results of this study show the strike-slip movement of Bafgh – Poshte-badam fault and the emergence of fault springs have a key role in controlling wind erosion and formation of eaolian landforms in Dar-Anjir playa. Indeed, the fault springs control the wind erosion in the present case study in three ways: 1: the formation of wet zone creates various types of vegetation and reduces wind speed in surface, 2: the raising of water table across Bafgh – Poshte-badam fault and the increase of the moisture surface impede the movement of sediment by wind and 3: With formation of nebkhas causes the aeoilan sediment accumulation around Shrubs.

Research on ancient and recent global environmental change has mainly been based on natural records, such as those preserved in ice cores, loess deposits, tree rings, corals, stalagmites, and deep-sea and lake sediments (Li et al, 2014). These records have been used to reconstruct the Earth’s past climate, environment, and ecological history (Zhou, 2006). However, in the arid and semi-arid desert regions that are an important part of Earth’s system, the difficulty in finding highresolution natural records has made research on environmental changes in these regions lag behind research in other areas (Xia, 2005). Nebkhas are fixed aeolian landform which are formed around Shrubs and trees in dry and semiarid areas as a result of the aggregation of aeolian sediments (Cooke et al, 1993). The unique developmental and depositional characteristics of nebkhas make them ideal carriers of proxies that can be used to study late-Holocene regional aeolian activity, the alternation between dry and wet conditions, hydrological characteristics, the ecological environment, and their evolution in arid and semi-arid desert regions (Wang et al, 2004; Hesp and Mclachlan, 2000; Wolf et al, 2000 )

In the first step, according to satellite google earth images and field observations an area with a dimension of 400 * 400 meters was specified. In the next step, the existing Nebkhas near the specified area whose vegetation was at least covering between 85 to 95 percent of the deposits were numbered. Then, through MATLAB and Random order, four Nebkhas were randomly selected. In the following stage morphometric parameters such as the height, width, Length, and the height of the Shrubs was measured. Afterwards, using field instruments, the nebkha selected for intensive analysis was excavated from its crest to its base, and sediments were sampled at 5-cm intervals throughout the profile. Overall, 79 samples from 4 Nebkhas were taken and transferred to the geomorphology laboratory for the purpose of determining organic matter and geain size of the sediments. To determine the grain size of the sediments hydrometric and granulometric methods were employed. The geain size results were analyzed in GRADISTAT4.0 and RockWorks software with Folk (1954) method. The measurement of organic matter in this study was done according to loss on ignition (LOI) method when the samples were heated at 550 °C for 2 h (Qiang et al, 2013).

The results of analyzing the organic matter of the Nebkhas’ sediments show that despite the fact the content of the organic matter during the formation of Nebkhas showed a significant change, the content of the organic matter of lower layers’ Nebkhas was higher compared to other layers, indicating a drier environmental condition after the formation of Nebkhas. The findings of the geain size of sediments show that although the threshold speed of past winds had some fluctuations in the studied area, in the upper layers of all Nebkahs a significant aggregation of higher than 250 micron sediments was observed. The findings obtained from the fluctuations of the relative speed of wind threshold based on the diameter of sediment particles in the studied area demonstrated that the size of different particles in the layers of Nebkhas of the studied area show the fluctuation of wind energy when the Nebkhas were formed.

The results of organic matter showed that in the initial stages of the formation of Nebkhas, the studied area had a suitable condition regarding the amount of water for the growth  Shrubs which cover Nebkhas but in the course of time the aridity level of the area increased. Moreover, the findings of geain size of the sediments suggested that the aggregation particles which were larger than 250 microns in the upper layers of the nebkhas was the result of human activities such as abandoning farm lands and withering of palm trees. As a result, the threshold speed decrease of wind erosion and deflation of larger particles has occurred in the studied area. In sum, the findings showed that the studied Nebkhas are relatively exact proxies which can provide us with significant insights with regard to environmental changes in the area.

Nebkhas, as one of the Eoelian Landforms in dry and semi-dry areas, have significant influence in controlling aeolian erosion and the environment stability. The main question of this study concerns different morphometry of nebkhas in two neighbor sites in Daranjir playa and the main factors which control these landforms. area special properties, the variety  of  hydrogeology properties, and the locality difference of nebkha morphomrtry introduce the  hypothesis  which hydrogeology properties have direct influence on nebkha morphomrtry. To attain this aim, remote sensing techniques, field data and geochemistry analysis have been used. Therefore, initially two sites which had  nebkha different vegetations were detached. In the next stage, nebkhas morphometry parameters in each site were measured. Thus utilizing hand auger, 8 bores were dug  with an average depth of 2.60 meters and sediment samples were collected.  The dig with hand  auger was achieved to Water table (an average 2.06 m) and was measured Water table for each bore. In addition, in each bore 300 millilitres of water was collected for chemical analysis through water sample collector device. Altogether, 46 sediment samples from the first site, 58 sediment samples from the second site and 8 water samples from eight bores were collected. After transferring the samples to geomorphology laboratory, the chemical analysis of water samples was carried out through a multi parameter device, version HI9811-5. Following that, by taking the saturated paste of sediment samples, the amounts of TDS, EC, pH were measured. The results of examinations on the qualitative nad quantitative features of the groundwater indicated that the average of ground water depth, TDS, and EC are respectively 1.69 m, 18850 ppm, 37750 µS/cm in the first site and 2.44m, 27500 ppm, and 54950 µS/cmin the second site. The chemical analysis of sample sediments indicated that the salinity of the sediments is influenced by the depth and quality of ground water, respectively. Overall, the findings show that the hydrogeology properties of the examined sites are not only the main factor in the formation  of Nebkhas, but also in control of the morphometric properties of Nebkhas.