تحولات محيطي در كواترنر پاياني و منشا دوره هاي باراني ايران

The Late Quaternary Environmental Evolution and the Origin of Rainy Periods in Iran

روند تحولات و شرايط محيطي سرزمين ايران در كواترنر، يكي از موضوعات مهمي است كه با وجود بررسي هاي چشمگير، هنوز ابهامات زيادي در مورد آن، طي دوره هاي يخچالي و بين يخچالي وجود دارد. در اين بررسي ضمن مرور شواهد و داده هاي حاصل از منابع مختلف داخلي و خارجي، با روشي تحليلي و مستدل، چارچوب كلي تحولات كواترنر در ايران مورد بحث قرار گرفته است. نتايج، بيانگر آن است كه در دوره هاي يخچالي و يخبندان، سرزمين ايران شرايط سرد و خشكي داشته و بلعكس در دوره هاي بين يخچالي و بين يخبندان، شرايط گرم و مرطوب تا نسبتاٌ گرم و مرطوبي حاكم بوده است. بررسي ها در رابطه با هولوسن نشان مي دهد كه از آغاز تا ميانه آن، شرايط بسيار گرم و مرطوبي چيره شده كه حاصل نفوذ و گسترش موسمي تابستانه هند و تقويت و تشديد شرايط مرطوب آن توسط جريان هاي غربي طي زمستان بوده است. با توجه به سردي و خشكي سرزمين ايران در دوره هاي يخچالي و يخبندان، مي توان اين فرضيه را قوت بخشيد كه شرايط بسيار مرطوبي كه به دوره هاي باراني ايران شهرت يافته و شواهد محيطي آن به گستردگي يافت شده، حاصل تكرار مكانيسم نيمه نخست هولوسن در دوره هاي بين يخچالي و احتمالاً بين يخبندان بوده است.

1.Introduction: Overall، authors apparently agree that several Quaternary climate changes occurred in Iran، also on the drier Iranian highlands. However، the timing and direction of changes have been a matter of dispute (Kehl، 2009، 13).The history of the quaternary evolution in Iran، is one of the important issues that have attracted the attention of many researchers over the past decades. Despite many studies on the nature of these changes، there are still many uncertainties about it. There are two main views about the Pleistocene climate in Iran، the First، not only does not accept an increase in precipitation during the glacial periods، but also believes in its reduction from the present values. The second is believed to increase the precipitation on the inner frontier of the Iranian plateau and the highlands. Proponents of the second theory also are two groups. The first group، such as Scharlau، gives rise to a significant increase in rainfall and the other group slight increases (Ayvazi 2013). This study، based on the hypothesis that the conditions in Iran were cold and dry during glacial periods، according to the detailed researches carried out in Iran over the past few decades، as well as in the sub-Saharan Africa، Arabia and Mesopotamia in the Quaternary، intends to find a response for the origins of rainy periods comparatively. A vast area in Iran، along with the sub-Saharan Africa and the Arabian Peninsula، are parts of the dry belt of the planet. According to this fact، this research is based on the assumption that similar mechanisms have affected these regions in the past، and Iran's rainy periods are due to a similar mechanism that affected the sub-Saharan Africa، Arabia and Mesopotamia. Although، the evidences of these changes in the glacial and partly Holocene periods have been documented in Iran، there has not yet been a clear scenario for the mechanism of these changes in a broad sense. 1.Materials and Methods: In this research on the one hand، it has been attempted in an analytical way، using the significant studies that have been carried out over the past decades in relation to the quaternary and in particular the late quaternary changes in Iran، theories and evidences are described، and the mechanisms and framework of events in glacial and interglacial periods are given. On the other hand، with a broader view of how the quaternary changes in the neighboring and distant areas، including the sub-Saharan Africa، the Arabian Peninsula، Oman and Mesopotamia، have also been examined. Thus، using the technique of comparison and reviewing existing theories and resources، including numerous books and articles، and establishing a reasonable relationship between the events in Iran and other geographic regions during the late quaternary (Pleistocene-Holocene transition)، It has been attempted to determine the general framework of changes in the Iran، during glacial and interglacial periods. In this regard، while paying special attention to the humid conditions of the first half of the Holocene in different regions، including Iran، the possibility of occurrence of periods known as rainy periods or Iran's fluvial conditions in Holocene or in general in interglacial periods is discussed. 1.Results and discussion: The set of evidences، including glacial and palynology data، show that during the last glacial period، Iran had a cold and dry conditions. This coldness and dryness were due to the expansion of the Siberian High southward. This system، even today، also governs cold and dry conditions on the territory of Iran. The aridity during the Lateglacial، probably resulted from a combination of climatic factors. The Siberian anticyclone، which today blocks all but the deepest depressions، may have been too strong or too large for low-pressure systems to penetrate into the interior of the Near East. In addition to the southward displacement of the polar front and related storm tracks، the westerlies may also have been weaker. Cyclogenesis may have been inhibited by a weak temperature gradient، and colder temperatures over the Atlantic and Mediterranean may have prevented the incorporation of substantial amounts of moisture (Stevens et al.2001). At the late Quaternary، the cold and dry conditions of the last glacial period turned into a warm and humid climate in Holocene. The mechanism of this event is related to the changes in Earth’s orbital parameters، which caused an increase in seasonal distribution of the insolation that affected much of the northern hemisphere including African Sahara، Arabia، Oman، India، Middle East، as well as Iran(Kutzbach.، 1981; Kutzbach and Guetter.، 1986; Kutzbach and Otto-Bliesner.، 1982; DeMenocal and Tierney، 2012). Humid condition in Iran was due to reactivation of Westerlies and northward penetration of Indian Monsoon from early to mid-Holocene. 1.Conclusion: In general، the geomorphic evidences، palynological data، climatic models، and archaeological data related to Iran and adjacent territories، indicate that during the last glacial period، Iran has had colder and drier conditions than today. Based on the evidences، the Indian monsoon since the beginning of Holocene، except for limited areas in the northeast، north، northwest، and parts of the west، has affected the country. In the first half of the Holocene، the monsoonal effects during summer، along with the influx of western low pressures in the cold season، led to a very humid condition، especially in the central، eastern، southern and southwestern regions of Iran throughout the year. Thus، the so called Iran’s rainy periods or fluvial periods which its different evidences have been documented، especially in the central regions، caused by these climatic systems.