نوع مقاله : مقاله پژوهشی
نویسندگان
1 استادیار، گروه زمینشناسی، دانشکده علوم، دانشگاه پیام نور، تهران، ایران
2 سازمان زمین شناسی و اکتشافات معدنی کشور
کلیدواژهها
موضوعات
عنوان مقاله English
نویسندگان English
1- Introduction
Lake basins have spatially and temporally index patterns and represent valuable maps of physical and chemical history. According to this point, that direct survey of climate changes has only been available as machinery in the past few decades, so paleo climatic changes indexes can give a better understanding of climate changes in the past era and its causes. Lake sediments have an extraordinary potantial to record climatic and environmental events with high sensitivity and resolution amounts. Therefore, lake sediments can be considered as regions for detecting paleo environments. Chegarman Lake formed in the northeast of Khuzestan province on the hillsides of folded Zagros and under the Izeh unit in a pit. This study has been tried to by Sedimentary geochemistry analyze, Elemental ratios, identification of sedimentary facies, and identification of sedimentary environments and changes of these environments set to the reformation the climatic and environmental conditions of Chegarman wetland in the Late Holocene.
2- Methods and Findings
The steps of this research include book studies, field works, laboratory works, statistical data processing and then interpretation and conclusion. To investigate the paleoclimate and conditions of the sedimentary environment, one intact sediment cores with a maximum depth of 7.2 m collected with using Auger corer. 21sediment samples were selected for elemental analysis with ICP-OES and XRF (X-Ray fluorescence). Statistical parameters, histogram diagrams of element frequency and cluster analysis were performed to determine the relationships between the elements in SPSS software. In this study, elemental ratios of V/Cr, Mn/Al, K/Al, Rb/Al, Si/Fe, Ti/K were used to reconstruction of the paleo climatic and environmental conditions. Which led to the study of the amount of detrital material, the severity of weathering and erosion in the catchment and fluctuations in the water level of the lake.
3- Results and discussion
In the studied core, 5 major sedimentary facies identified. Facies often contains clay, silt, gravel, and interstitial sediments along with vegetation and shell fossils which in most cases frequency and interference in the mentioned sediments are often seen. Changes were in particle size between clay to gravel due to process type changes, environment's energy and river input's amounts and sediment color variations concerning sedimentation conditions, presence of organic matter, pH amount, salinity and temperature variations, and drought occurrence, throughout the cores. Two wetland sedimentary and alluvial environments identified. According to the Chronology, the average sedimentation rate in the Chegarman wetland is 1.4 mm / year, which this amount is different for various depths (Darvishi et al., 2022). Due to the type of sediments, the high level water period of the Chegarman wetland has been established between 1250 and 150 years ago. The study of the facies sediments of the wetland bed shows that climate change occurred gradually (not suddenly) about 1250 years ago and this trend has taken at least 300 years. The relatively dry and cold period lasted about 1850 years from 3100 years to 1250 years ago and is confirmed with the reported dry periods in the Maharlou Lake about 1800 years ago, in the Mir Abad Lake about 1500 years ago and in the Urmia Lake between 2500 and 1500 years ago. From 4000 years ago to about 3100 years ago, stable and slow wetland conditions was prevailed. The conditions for creating a peatland environment have been provided in the wetland and were associated with increasing the amount of organic matter, vegetation and shell fossils and consequently increasing sedimentation rate. This period which lasted for about 900 years, has been accompanied by extremely humid conditions, high groundwater level and activity of springs in the region. The existence of light brown mud which has the age ranges from 4900 to 4000 years ago, indicates a dry period (900 years). This dry period shows suitable approximate conformity with a dry event of 4200 years ago in the northern hemisphere. This period in the studied region appeared with at least 100 years of temporal delay. After this dry period, we see again a gray sequence with plant and shell fossils that continue to a depth of 7 meters (about 5000 years ago). At depths of 7 to 7.2 m, the existence of brown-muddy sediments along with coarse-grained sediment in size of gravel, pebble and rubble, strengthen the possibility of very strong flood in the region that sent the coarse-grained sediments of around formation to bed of the wetland or maybe has hit to the area of wetland sediments bed in the region.
4. Conclusion
According to the results of surveys in the Chegarman wetland, at least 2 high water periods with humid climate and at least 2 dry periods during the Late Holocene were identified. Dry periods appear more suddenly and with more intensity and shorter duration than humid periods in the region and gradually lead to from very dry periods to semi-arid, semi-humid, and eventually humid periods.
کلیدواژهها English
