بازسازی نوسانات تراز آب در بازه زمانی پلیستوسن-هولوسن در سواحل کم عمق دریای خزر (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
شواهد رسوبی به جا مانده در اطراف دریای خزر در بخشها و ترازهای مختلف حاکی از نواسانات بسیار شدید ترازآب در دوره های مختلف زمین شناسی است. دو تراز مطلق دریای خزر که حدوداً در ترازها ی 22- و 24- اندازه گیری شده ا ند در اواخر و نیمه دوم هولوسن در سرتاسر دریای خزر شواهد آن وجود دارد و مورد پذیرش اکثر محققین دریای خزر می باشد. تا کنون گزارش دقیقی از تغییرات تراز آب مطلق در ابتدای هولوسن ارائه نشده است و همچنان تراز آب مطلق آن بدلیل فقدان داده های لازم مورد اتفاق نظر محققین دریای خزر قرار نگرفته است. هدف از این مقاله تمرکز بر روی تراز مطلق ابتدای هولوسن با استفاده از داده های مختلف و تعیین سن مطلق از گمانه نسبتاً عمیق 27.7 متری واقع درسواحل شرقی ایران واقع در گمیشان می باشد. بر اساس نمو نه های تعییین سن کربن 14 گمانه گمیشان نشان می دهد که تراز مطلق آب بین 10590 تا حدودا 8400 سال پیش بطور مستمر ادامه داشته و احتمالا در تراز های مطلق بیشتری نسبت به تراز های نیمه دوم هولوسن قرار داشته است . افزایش تراز آب پس از پسروی شدید تراز آب حداقل در تراز مطلق 50- در حدود 10590 سال پیش شروع شده و تا حدود 8400 سال پیش ادامه داشته است. در چنین شرایطی منطقه کم عمق ساحلی گمیشان محیط ساحلی عمیق را تجربه نموده است.Pleistocene-Holocene Sea level Shange along the shallow Caspian shore
Pleistocene-Holocene Sea level change along the shallow Caspian shoreA.A. Kakroodi Associated Prof., Dep. of Remote sensing and GIS, University of TehranIntroduction:The reconstruction of the past water level of the Caspian Sea has been carried out by many researchers throughout the Caspian Sea in its various parts, including shallow, deep marine deposits, as well as on the mainland. The study of palynology, magnetism, sedimentology and mineralogy, determination of absolute age, tectonics, and also the study of the Caspian fauna of the Caspian Sea shows now a better picture of the absolute sea level of the Caspian Sea in the past, but still there is no agreement on the sea level curve. (Rychagov, 1977, 1997; Fedrov, 1994; Boomer et al., 2005; Kroonenberg el al., 2007; Lahijani et al., 2009; Hoogendoorn et al., 2010; Leroy et al., 2013; Kakroodi et al., 2012, 2015; Naderi et al., 2015).The marine sediments recorded in different parts of the Caspian Sea indicate its drastic sea level changes of at least 50+ in the Early Pleistocene and around 20+ in the Late Pleistocene. From the Pleistocene to the Holocene, we are facing a sharp drop in the water level, therefore the expansion of the Khazar Plain. Its absolute level is not mentioned accurately, but in Russian sources it is reported up to -100 and in the Iranian part of the Gamishan area it is reported up to at least -50 (Kakroodi et al., 2015). Data and methodStudy area The studied area is located in the southeast of the Caspian Sea and the northeast coast of Iran. It corresponds to the morphological features of Gorgan Bay and Gomishan Lagoon. The studied area is one of the vast areas of Iran's coastal plain, which has a very small slope towards the land and the sea. The southeastern shores of the Caspian Sea are very sensitive to the water level due to low angle coast, a suitable place to study the water level changes. The main sources of sediment for the south-eastern Caspian coast are the Gorgan and Qare Su rivers draining the Alborz Mountains, and the Atrak River draining the Kopet Dagh Mountains. The Gorgan River provides the most sediment supply and its annual sediment load is around 1.336 million ton (Mister, 2001). The Gorgan River has formed a cuspate delta, which has constantly changed its course during the HoloceneResults:Figure 4 shows a log prepared from a part of the drilled borehole between 27.7 and 18.75 m, which includes a sequence from the pre-Holocene stage to the maximum water level rise in the Holocene (Figure 4). Each of these units is separated from the other unit by a specific fauna. Pleistocene ostracods with a homogeneous and oxidized sedimentary unit with gypsum crystals lead to the initial unit of the Caspian marine advance in the Holocene and a wetland environment, and finally to a deep environment with diatoms. In fact, in the Holocene, a wetland environment was first formed in the area and turned into a deep marine environment. The identification of these units is based on existing fauna and sedimentology (including texture, color and sedimentary structure).ConclusionThe Late Pleistocene to Holocene Caspian Sea-level was retrieved by a multi-disciplinary approach from a 27.7 m long core in the SE corner of the Iranian Caspian coast in the Gomishan Lagoon. Pleistocene and Holocene sediments are distinguished by different structures in terms of fauna and intense oxidation. After a deep sea-level fall, the wetland system was identified at the beginning of the Holocene. The continued increase in the water level led to the movement of the marine system towards the mainland and a sharp increase in depth until the middle of the Holocene. This increase in water level continued until about 8400 years ago and probably the most effective level of the Caspian Sea in the Holocene. Its absolute level is still not clear, but its sedimentary evidence shows that this level has covered major parts of the coastal plain. Most likely, the existence of old wetlands such as Almagol, Alagol and Ajigol, which are now located at levels between -5 and -8, may be the evidence of this level, although to prove it, deep boreholes are needed in the bed of these wetlands.The study of Neo-Caspian sediments in the Holocene indicates cyclic changes in the water level. The two next and very important levels, whose sedimentary evidences exist in the studied area, are around -22 and -24, which covers a major part of the low-sloping coast. Finally, the last cycle of the Caspian Sea between 1929 and 1995, with a range of changes of about 3 m, has extensive impact along the coast. Keywords: Holocene, Gomishan, Khovalyian, Sea-level, Diatom.
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