نوع مقاله : مقاله پژوهشی

نویسندگان

1 سازه های آبی، دانشکده کشاورزی، دانشگاه لرستان، لرستان، ایران

2 گروه مهندسی آب، دانشکده کشاورزی، دانشگاه لرستان، لرستان، ایران

3 گروه زمین شناسی، دانشکده علوم زمین، دانشگاه شهید چمران اهواز، خوزستان، ایران

4 گروه زمین شناسی، دانشکده علوم پایه، دانشگاه لرستان، لرستان، ایران

چکیده

دیرینه اقلیم، شاخه‌ای جدید از علم محسوب می‌شود که به بررسی تغییرات اقلیمی گذشته می‌پردازد.‎‏ در تحقیق حاضر در نظر است به کمک اطلاعات دریافتی از غار مغار واقع در استان لرستان، اقلیم گذشته بازسازی شود. یک استالاگمیت به طول 18 و قطر 8 سانتیمتر برش داده شد. بدین منظور 34 نمونه برای آنالیز ایزوتوپ پایدار، 4 نمونه برای آزمایش XRD و محاسبه درصد آراگونیت و سه نمونه نیز برای سن سنجی به آزمایشگاه کوئینزلند استرالیا ارسال شد. سن سه نمونه به ترتیب 550 ، 368 و 6/8 هزار سال برآورد شد. تحلیل نتایج ایزوتوپی O18δ در طول زمان نشان داد داده‌های ایزوتوپی با شیب نسبتاً زیادی در حال افزایش است که نشان می‌دهد اقلیم منطقه در طول 550 هزار سال گذشته به سمت خشک‌تر شدن و کاهش بارندگی می­رود. دوره­های اقلیمی همواره در حال تغییر بوده و دوره­های ترسالی و خشکسالی در مقیاس بزرگتر یعنی به صورت دوره­های یخبندان و بین یخبندان رخ داده است. در گذشته این تغییرات بسیار آهسته رخ داده اما در حدود 8 هزار سال اخیر سرعت تغییر اقلیم افزایش یافته است.  تحقیقات بیشتر مشابه تحقیق حاضر می­تواند اقلیم گذشته را با جزئیات بیشتری بازگو کند تا بتوان با کمک آن تصمیمات درستی اتخاذ نمود.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Analysis of paleoclimates and rainfall changes using stalagmites isotopic in central Zagros

نویسندگان [English]

  • Parastoo Hemeh Zadeh 1
  • Hasan Torabi Podeh 2
  • Hojjat Elah Yunsi 2
  • Seyed Yahya Mirzaei Arjanki 3
  • Hassan Zamanian 4

1 Water Structures, Faculty of Agriculture, Lorestan University, Lorestan, Iran

2 Department of Water Engineering, Faculty of Agriculture, Lorestan University, Lorestan, Iran

3 Department of Geology, Faculty of Earth Sciences, Shahid Chamran University, Ahvaz, Khuzestan, Iran

4 Department of Geology, Faculty of Basic Sciences, Lorestan University, Lorestan, Iran

چکیده [English]

Introduction
Paleoclimate is a new branch of science that investigates the climate changes of the past by using different sciences. Carbonates are a type of rocks that have been studied a lot in the geochemistry of stable isotopes and are found in all time intervals. A common type of carbonate rock is speleothem, which is capable of providing reliable records of climate change over many years. The studies conducted in the last decade show that the past climate can be understood by δ18O and δ13C. One of the common forms of speleothem is stalagmites, whose layers are known as a potential source of high-quality climatic information. By using speleothem, it is possible to reconstruct the climatic conditions up to about 600 thousand years ago. Comprehensive and complete information is not available about the climatic and environmental conditions of Iran during the Holocene period. Meanwhile, Iran's location as a transition zone between Europe, Asia and Africa is of great importance for study the past climate; However, compared to other regions, it has been less studied, which has provided a special position for this research. In the present research, it is intended to be analyzed with the help of the information received from a cave in the Zagros region of the past climate of Iran.

2-Materials and methods
For paleoclimate analysis in central Zagros, Maghar cave in Khorramabad (located in Lorestan province and in Karkheh watershed) was selected. The condition of this cave is such that it has little connection with the environment outside the cave and is not affected by the wind and other meteorological parameters outside the cave. After evaluating inside the cave, a stalagmite with a length of about 18 cm and a diameter of 8 cm was selected. After cutting it, it was transferred to the soil mechanics laboratory of Lorestan University, where its surface was polished in order to observe the layers. Since the primary core of the stalagmite is not located right in the center, it can be concluded that the primary bed of stalagmite formation is located on a sloping surface, which caused the drop of water to move on the sloping surface after hitting the surface of the cave. As the limit has increased in length, it has grown in width. For dating, three points were considered, including the stalagmite primary core (D1), middle (D2) and on top of it (D3). Age measurement was done by Multi-Collector Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-M) located in the laboratory of the University of Queensland, Australia. For stable isotope analysis, 34 samples were sent to Arak laboratory for δ13C and δ18O analysis, and the analysis was done by Isotope Ratio Mass Spectrometer (IRMS) (30 samples in the direction of stalagmite growth length and 4 other samples for Hendy test). According to the standard of this device, 50 mg of powder was prepared for each sample. This was done with the help of a dental drill and movement on the stalagmite layers. To determine the percentage of calcite and aragonite, 4 samples (50 grams each) were sent to the laboratory of Lorestan University for XRD testing.

3-Results and discussion
According to the XRD results, the percentage of aragonite was considered zero for all four samples. According to the Hendy test, stalagmites are formed in isotopic equilibrium conditions. The age of three samples was estimated to be 550, 368 and 8.6 thousand years respectively. The analysis of δ18O isotopic results over time showed that the isotopic data is increasing with a relatively large slope, which indicates that the conditions of the studied area during the last 550 thousand years are becoming drier and reducing precipitation.

4- Conclusion
Climatic periods are always changing and dry and wet periods have occurred on a larger scale in the form of glacial and interglacial periods. In the past, these changes happened very slowly, but in the last 5 thousand years, the changes are fast. These changes intensify over time. So that there have been sudden changes in the last hundred years. Today, climate change is not hidden from anyone, but unfortunately, it has not been found in a suitable solution. Many countries use different methods to prevent the damages of climate change in the future. It is clear that disturbing the order of nature disturbs the balance. Further research similar to the present research can tell the past climate in more detail.

 

کلیدواژه‌ها [English]

  • Oxygen
  • isotope
  • paleoclimate
  • Zagros
  • dating