پديد آورندگان :
پاريزي اسماعيل دانشگاه تهران - ژئومورفولوژي , مقصودي مهران دانشگاه تهران - ژئومورفولوژي , يماني مجتبي دانشگاه تهران - ژئومورفولوژي , مهرنيا رضا دانشگاه پيام نور تهران - زمين شناسي
كليدواژه :
تغييرات محيطي , چاله دراَنجير , فرسايش بادي , نبكا
چكيده فارسي :
نبكاها يكي از لندفرم‌هاي بادي در مناطق خشك و نيمه‌خشك‌اند كه رسوبات آن‌ها اطلاعات باارزشي درباره تغييرات محيطي يك منطقه در طول دوره‌هاي مختلف ثبت مي‌كنند. هدف از اين پژوهش بررسي شرايط رطوبتي و نيز سرعت و مدت نسبي وزش بادهاي گذشته بر اساس ميزان مواد آلي، دانه‌بندي و ضخامت رسوبات نبكاها جهت بازسازي الگوي فرسايش بادي در چاله دراَنجير است. براي دست‌يابي به اين هدف، از تكنيك‌هاي سنجش‌ازدور، داده‌هاي ميداني، و روش‌هاي آزمايشگاهي نظير هيدرومتري، گرانولومتري و افت حرارتي استفاده شد. در اين زمينه، با استفاده از دستور Random در نرم‌افزار متلب، 4 نبكا به صورت تصادفي انتخاب گرديد. سپس، با حفر ترانشه‌هاي عمودي، نمونه‌برداري از رأس تا قاعده نبكاها با فواصل منظم 5 سانتي‌متر انجام گرفت. در مجموع 79 نمونه رسوب از 4 نبكا برداشت و براي آناليز به آزمايشگاه ژئومورفولوژي انتقال داده شد. نتايج نشان داد كه محتواي مواد آلي و دانه‌بندي رسوبات در مدت تشكيل نبكا تغييرات قابل توجهي كرده است كه حاكي از نوسانات سطح پوشش گياهي، مقدار آب و سرعت آستانه فرسايش بادي در منطقه است. تجزيه‌وتحليل مواد آلي مؤيد آن است كه در مراحل اوليه تشكيل نبكا، منطقه مطالعه‌شده براي رشد درختچه‌هاي پوشش‌دهنده نبكا شرايط آبي مناسب داشته؛ اما، به مرور زمان، بر خشكي آن افزوده شده است. نتايج بررسي دانه‌بندي رسوبات نيز بيانگر آن است كه تجمع ذرات بالاي 250 ميكرون در لايه‌هاي بالايي نبكا به دليل فعاليت‌هاي انساني، نظير رها شدن زمين‌هاي كشاورزي و خشك شدن درختان خرما اطراف نبكاها است كه نتيجه آن كاهش سرعت آستانه فرسايش بادي و رُفت‌وروب ذرات درشت‌تر در منطقه مطالعاتي است.
چكيده لاتين :
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 )
Materials and methods
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).
Results and discussion
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.
Conclusion
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.
