اقلیم و محیط دیرینه تالاب آبزالو در استان خوزستان با استفاده از مغزه های رسوبی در هولوسن پسین

بررسی تاریخچه تغییرات اقلیمی و محیطی تالاب آبزالو در طول دوره هولوسن پسین با استفاده از مغزه های رسوبی موضوع پژوهش حاضر است. تعداد 8 رخساره‌ رسوبی براساس خصوصیات رسوب، میزان مواد آلی، رنگ، وجود بقایای گیاهی و صدفی و سایر مولفه های ماکروسکوپی و 3 محیط مخروط‌افکنه ای و رودخانه ای و تالابی در هشت مغزه برداشت شده شناسایی و تفکیک شدند. نتایج سن‌سنجی 3 نمونه از پوسته های صدفی به روش ایزوتوپ کربن 14، میزان متوسط رسوب‌گذاری در تالاب آبزالو را 1.22 میلی متر در سال نشان داد. حداقل دو دوره پرآبی و دو دوره خشک در طول هولوسن پسین (5000 سال گذشته) شناسایی شد. دوره‌های مرطوب از 1800 تا 500 سال پیش در یک دوره 1300 ساله و از 4400 تا 2500 سال پیش در یک دوره 1900 ساله طول کشیده است. دوره‌های خشک، با مدت زمان کوتاه‌تر از دوره های مرطوب، از 2500 تا 1800 سال پیش در یک دوره 700 ساله و از 4400 تا 4700 سال پیش در یک دوره 300 ساله امتداد داشته است. دوره های خشک با شدت بیشتر و مدت زمان کمتر بصورت ناگهانی در منطقه ظاهر شده و به صورت تدریجی به پایان رسیده‌اند و دوره های مرطوب به‌تدریج شروع شده و ناگهان پایان پذیرفته‌اند.

Paleoclimate and Paleoenvironment of Abzalou wetland in Khuzestan Province with using Sedimentary Cores during the Holocene

IntroductionLake 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 past rsquo;s climate changes indexes can give a better understanding of climate changes in the past era and its causes. Of these, lake sediments have an extraordinary ability to record climatic and environmental events with high sensitivity and resolution and very high resolution amounts. Therefore, lake sediments can be considered as regions for detecting paleontological environments. Abzalou 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 examination, identification of sedimentary facies, identification of sedimentary environments and changes of these environments set to the reformation the climatic and environmental conditions of Abzalou wetland in the end Holocene. Methods and FindingsThe steps of this research include book studies, field works, laboratory works, data processing and then interpretation and conclusion. To investigate the paleontological climate and conditions of the sedimentary environment, 8 intact sediment cores with a maximum depth of 7.5 m and total of 35 m collected with using Auger corer. In this regard, tried to the position of the core be in a way that makes it possible to collect the maximum thickness of wetland bed rsquo;s sediments, give maximum coverage to wetland rsquo;s sedimentary environments and at least collect one sample in each homogenization work unit as much as possible. Based on field observations, core rsquo;s description and lab results, the cores separated into sedimentary units and their stratigraphy columns drew and determined under each sedimentary unit. Also, 3 sediment samples sent to ETH Z uuml;rich Institute of geology to age/ date measurement analyzing by the radiocarbon method of organic matters. Results and discussionIn the studied cores, 8 major sedimentary facies identified. The facies often contains clay, silt, sand, 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, environmentchr('39')s energy and river inputchr('39')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. Three wetland sedimentary, river and alluvial environments identified. According to the age measurement, the average sedimentation rate in the Abzalou wetland is 1.22 mm / year, which this amount is different for various depths. Due to the type of sediments, the Abzalou wetland has undergone human changes for about 600 500 years ago. The high water period of the Abzalou wetland has been established between 1800 and 500 years ago. The study of the facies bed sediments of the wetland shows that climate change occurred gradually (not suddenly) about 1800 years ago and this trend has taken at least 200 years. The relatively dry period lasted about 700 years from 2500 years to 1800 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 4400 years ago to about 2500 years ago, stable and slow wetland conditions 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 1900 years, has been accompanied by extremely humid conditions, high groundwater level and activity of springs feeder in the region. The existence of light brown mud which has the age ranges from 4400 to 4700 years ago, indicates a dry period (300 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 200 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 5300 years ago). At depths of 7 to 7.5 m, the existence of brownmuddy sediments along with coarsegrained sediment in size of gravel, pebble and rubble, strengthen the possibility of very strong flood in the region that sent the coarsegrained sediments of around formation to bed of the wetland or maybe has hit to the area of wetland sediments bed in the region. Conclusion According to the results of surveys in the Abzalou wetland, at least 3 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 semiarid, semihumid, and eventually humid periods.