نوع مقاله : مقاله پژوهشی

نویسندگان

دانشگاه شهید بهشتی، تهران، ایران

چکیده

منطقه کومه بران با قرارگیری در ارتفاعات واحد شمال غربی بستری برای ثبت تمامی این تغییرات در پیشینه‌ی اقلیمی خود بوده است. تغییرات برودتی و رطوبتی در اواخر کواترنر تأثیر قابل‌توجهی بر آثار یخچالی داشته، لذا بررسی آن‌ها هدف اصلی پژوهش حاضر است. جهت مطالعه منطقه از مشاهدات میدانی، روش رایت، بازسازی اقلیمی و همچنین آنالیز دانه‌بندی و XRD نمونه‌ها استفاده شده است. آنالیز دانه‌بندی آثار به‌جامانده از عملکرد یخچال را تأیید می‌کند، که بیانگر تخریب فیزیکی سنگ‌های الترامافیک و سرپانتین در منطقه می باشد. نتایج نشان داد که خط برف دائمی در کواترنر حدود 4/2574 متری بوده است. بارش دریافتی حوضه حدود 230 میلی‌متر بیشتر از زمان حال بوده، همچنین میانگین دمای سالانه در کواترنر پایانی حدود 4/11 درجه سانتی‌گراد کمتر از زمان حال بوده است. بنابراین شرایط دمایی و بارش برای شکل‌گیری یخچال‌ها مناسب بوده است. آنچه بیش از همه در به وجود آمدن آثار یخچالی تأثیرگذار بوده؛ کاهش دمای منطقه نسبت به زمان حاضر و از طرفی پایین بودن تبخیر بوده است که امکان انباشت برف در منطقه‌ی بالاتر از خط تعادل و تبدیل آن به یخچال را فراهم کرده است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Climatic analysis of glacial evidence in the last cold period in Kumbaran valley, northwest of Piranshahr

نویسندگان [English]

  • rasoul hasani
  • Manijeh Ghahroudi Tali
  • khadijeh Alinoori

Shahid Beheshti University, Tehran, Iran

چکیده [English]

1-Introduction

The reference to the existence of glaciers in Iran goes back to the works of Jacques Demorgan (Demorgan, 1933); But the serious beginning of these studies was Bobek's fieldwork in Alborz, Sablan and Kurdistan (Bobek, 1940). Ramsht has attributed the three terraces in Gavkhoni Lake to a large-scale climate change before the Holocene (Ramsht, 2011). Yamani and his colleagues have reconstructed the snow boundary of the last glacial period in the Jajroud basin using Wright's method and Porter's model (Yemani et al. 2018). Ghahrodi and his colleagues investigated lateral moraines in mountain glacial valleys and observed at least three clear terraces on the edge of the Badin Abad River and the small Zab River (Ghahrodi et al., 2012). This research tries to reconstruct the climatic conditions of the end of the Quaternary period in the Kuma-Bran region with regard to cirques and moraines, and to analyze the terraces and their geomorphological value in re-reading the past conditions.

2-Materials and methods

In this research, the boundaries of the region were limited using 1:50000 topographic maps and Landsat 8 images.There were 3 general stages of field operations, sediment and rock sample collection. The past snow line of the study area was determined by Wright's method. According to the border of permanent snow line, both the past temperature and the current temperature were calculated . The humidity conditions of the area were checked. For the geomorphological map from topographic maps with a scale of 1:50000, geological map with a scale of 1:100000, Alos Palsar digital elevation model with a resolution of 12.5 meters from NASA and Landsat 5, 7, 8 images from 1991 to 2017 from the site of American Geology was used. Sediment samples were collected from 6 points in the cirques and Kumebaran glacier valley. 4 samples were used for grain size test and 2 samples were used for XRD test.

3-Results and discussion

Two glacial tongue has created an enlarged moraine middle mass that extends to a height of 2425 meters. On the side of the valley, the lateral moraines, especially along the end glacial moraines of cemented sediments, can be seen on both sides of the valley in the form of a destroyed terrace. The destroyed terrace in Kuma Baran Valley has a height difference of 10 meters to the river level. The new terrace is located at an altitude of 2370 meters and the upper terrace is located at an altitude of 2490 meters from the valley floor. In the last phase of the advance of the glacier, the ice sheet covered the valley up to a height of 2370 meters and the thickness of the ice in this part of the valley was more than 100 meters. The height of the valley floor in the area of the destroyed terrace is 2360 meters. The lowest terrace is 2 to 4 meters and in some cases up to 6 meters above the river level, which seems to be related to the Wurm period. The second terrace is 60-80 meters high and probably dates back to the Riss Ice Age. The third terrace is located at a height of 120 to 130 meters above the river level and is related to the Mindel Ice Age. 170 cirques are scattered in the area and the border snow line was calculated as 2574.4 meters according to Wright's method. The average annual temperature of the region is currently 5.7 degrees Celsius, and in the past it was 0.45 degrees Celsius, according to this, the temperature anomaly of Kumabaran was on average 6.39 degrees Celsius. The current average rainfall in the region is about 738.4 mm and in the past it was 969.2 mm. The granulation test of four sediment samples showed that most of the grains are made up of coarse-grained material (gravel) and the grains are very poorly sorted. The xrd test shows that the predominance of chlorite mineral in the surface sample and its absence and low amount in two old and deep sediment samples show that chlorite mineral is the result of physical destruction of ultramafic and serpentine rocks. The presence of kaolinite in the surface sediment sample can indicate the beginning of chemical weathering processes in these sediments, which are not yet complete.

4- Conclusion, Keywords

The evidence shows at least three periods of glacial expansion in the region. The terrace is exposed in a height range of 10 to 12 meters compared to the river level. The middle moraine masses of the glacial tongues are stretched up to a height of 2450 meters. Traces of limited expansion can also be seen in the Holocene period, so that inside a glacial cirque, there is another small cirque, which shows that glaciers may have expanded again in the Little Ice Age. Granulation analysis is consistent with the remaining traces of glaciers operation and confirms it. In the xrd analysis of the data, the predominance of chlorite mineral in the surface sample and its absence in the deep sample indicates the physical destruction of the ultramafic and serpentine rocks of the region. The average annual temperature anomaly of the points in the Quaternary and the present time was 11.4 degrees. In the age of ice age, the average temperature in the highest peaks of the region has reached -5.2 degrees Celsius, on the basis of which the permanence of ice is considered as the main one, which could have caused many of the morphic manifestations of ice erosion, including cirques, Create a glaciers. The average temperature of the past was 0.45 degrees Celsius and the precipitation was 937.7 mm. These temperature and precipitation conditions are suitable for the formation of glaciers. The snow line shows the past border at a height of 2574.4 meters; Above this height, due to precipitation and low evaporation, it is possible to accumulate snow in the accumulation area (above the equilibrium line) and turn it into a glacier.

کلیدواژه‌ها [English]

  • : Kumbaran
  • Granulation
  • Wright method
  • Glacier Cirques
  • XRD