عنوان مقاله [English]
نویسندگان [English]چکیده [English]
During the last Quaternary period, the earth is affected by environmental changes and extreme climate fluctuations (Kaser and Osmaston, 2002). Environmental and climate changes during the last glacial maximum period from 26.5 to 19 kyr BP (Clark et al., 2009) have affected the landforms and paleoenvironments. Determining the equilibrium-line altitude is the best tool for the reconstruction of paleoclimate and paleoenvironment during former glaciations (Benn and Ballantyne, 2005, Lukas and Bradwell, 2010, Hughes et al., 2010). The Equilibrium Line Altitude (ELA) is the average elevation where, over a one-year time interval, accumulation equals ablation so the mass balance at this line is zero (Cogley et al., 2011). The purpose of this study was to reconstruction paleoclimate and paleoenvironment conditions during the last glacial maximum period using geomorphological evidence such as glacial landforms, including glacial cirques, Accumulation Area Ratio (AAR), Area-Altitude Balance Ratio (AABR) and Mean glacier elevation (MGE) or Kurowski methods at Chehel Cheshmeh mountains of Divandareh in Kurdistan province.
In this research, after the field investigation of Chehel Cheshmeh Mountains, the glacier cirques of the region were identified. Then the position of the two typical cirques, including the Shahneshin, Qholizolikha Cirques and the three cirques located in Masjedmirza Mountains, along with the location of the moraines, was determined by using GPS. The geomorphology of the glaciers was mapped using a topographic map (1: 50000) and GPS data in the Freehand 9.0.2 and ArcGIS 10.4.1 software. Then, using DEM (12.5 m) and geomorphologic maps prepared in ArcGIS 10.4.1 software, the equilibrium line altitudes in the glaciers of Chehel Cheshmeh Mountains were calculated using the methods of the Accumulation Area Ratio (AAR), Area-Altitude Balance Ratio (AABR) and Mean glacier elevation (MGE) or Kurowski (Pellitero et al., 2015). The method of Lai (2003) was used to estimate the present equilibrium-line altitude in Chehel Cheshmeh Mountains. In order to calculate the present equilibrium-line altitude, the temperature and precipitation data from the meteorological stations around the study area were used.
Based on the mean glacier elevation method, the equilibrium-line altitude during the last glacial maximum in the Shahneshin is 2812.5 (m.a.s.l), in Qolizolikha 3027.5 (m.a.s.l) and in the Masjedmirza is 2868 (m.a.s.l). Based on the area-altitude balance ratio, the equilibrium-line altitude during the last glacial maximum in the Shahneshin is 2827 (m.a.s.l), in Qolizolikha 3018 (m.a.s.l) and in the Masjedmirza is 2869 (m.a.s.l). Using the method of Lai et al. (2003) and using the meteorological data, the present equilibrium-line altitude of the Zarineh station was 4735 m, Baneh station was 4680 m. Sanandaj station was 4633 m, the average of three stations located in the region of the study area was 4683 (m.a.s.l).
The equilibrium-line altitude during the last glacial maximum was calculated about 2905 (m.a.s.l) at the Chehel Cheshmeh Mountains. The present equilibrium-line altitude was estimated about 4683 (m.a.s.l) using Lai Method and the use of meteorological data. With Consideration of the equilibrium-line altitude lowering about 1778 meters, the mean annual temperature drop was around 11.5° during the last glacial maximum. Compared to other studies by Wright et al (1962, 2004) the temperature drop in the glacial mountains of Kurdistan 12 ° C, Bobek (1937) in the glacial mountains of Kurdistan 4 ° C, Bobek (1963) in the Alborz and Zagros Mountains from 4°C to 5°C, Krinsley (1970) in Playas and deserts in central Iran from 5°C to 8°C, Frenzel et al (1992) in the Northern hemisphere from 10°C to 12°C, Seif (2015) in Oshtorankuh 9.8°C, Kuhle (2008) in the Kuh-i-Jupar 10.5°C, Yamani et al (2007) in the Karkas Mountains from 10°C to 12°C, and Jafari et al (1396) in the mountains of Ghorveh 8.8°C were observed. Also, according to previous studies, the rate of equilibrium-line altitude lowering the glacial mountains of Kurdistan by Wright et al. (1962, 2004) between 1200 to 1800 m, Bobek (1937) in the glacial mountains of Kurdistan 700 m, Bobek (1963) in the Alborz and Zagros Mountains 600-800 m, Krinsley (1970) in Playas and deserts in central Iran more than 1,800, Seif (2015) in Oshtorankuh more than 1380 m, Kuhle (2008) in the Kuh-i-Jupar 1500 m are estimated. By comparing the equilibrium-line altitude lowering and the rate of reduction in the calculated temperature with other studies in Iran and the world, the last glacial maximum at Chehel Cheshmeh Mountains corresponds to the last glacial maximum about 26.5 to 19 kyr BP. It was affected by cold and dry climate conditions. Therefore, the use of glacier geomorphologic evidence and the equilibrium-line altitude are very suitable methods for the reconstruction of the environment and paleoclimate in Chehel Cheshmeh Mountains Divandarreh, Kurdistan, during the last glacial maximum.
Keywords: The last glacial maximum (LGM), Equilibrium-line altitude (ELA), Area-Altitude Balance Ratio (AABR), Chehel Cheshmeh Mountains