پديد آورندگان :
ملكي سحر دانشگاه تهران - دانشكدة جغرافيا , عزيزي قاسم دانشگاه تهران - دانشكدة جغرافيا , خرمالي فرهاد دانشگاه علوم كشاورزي و منابع طبيعي گرگان - خاكشناسي , شهبازي رضا سازمان زمين شناسي و اكتشافات معدني كشور - مديريت مخاطرات و زيست محيطي - مهندسي منابع طبيعي , كهل مارتين دانشگاه كلن آلمان - خاكشناسي
كليدواژه :
آب و هواي ديرينه , پالئوسول , پذيرفتاري مغناطيسي , لس
چكيده فارسي :
شناخت علل تغييرات اقليمي جهاني يكي از چالش‌هاي مهم علمي است. بازسازي آب‌وهواي گذشته از بايگاني‌هاي مختلف رسوب و يخ نشان مي‌دهد كه آب‌وهوا مي‌تواند به طور چشمگيري در بازه‌هاي زماني متفاوت، از چندين دهه تا هزاره، تغيير كند. گردوغبار بادي، كه با تغييرات آب‌وهوايي جهاني در ارتباط است، يكي از مؤلفه‌هاي مهم سيستم آب‌وهوايي است كه تغييرات ناگهاني گذشته را نشان مي‌دهد. لس‌ها به طور بالقوه مي‌توانند تغييرات ديرينه محيطي را طي چندين چرخه يخبندان ثبت كنند. در اين پژوهش، برش رسوبي سعدآباد، واقع در استان گرگان، جهت بررسي آب‌وهواي ديرينه، مربوط به MIS 5e (147000 تا 57000 سال پيش) شمالشرق ايران، با توجه به نبود اطلاعات كافي در خصوص آب‌وهواي ديرينه و سيستم‌هاي تأثيرگذار بر آن در اين دوره زماني، مطالعه شد. بدين منظور ابتدا مطالعات كتابخانهاي و ميداني و سپس مطالعات آزمايشگاهي صورت گرفت و تعداد 92 نمونه به فواصل 10 سانتي‌متري از توالي مورد نظر تهيه و براي همه آن‌ها پذيرفتاري مغناطيسي اندازهگيري شد. بر اساس نتايج به‌دستآمده، نمونه ها داراي نوسانات افزايشي در بخشهاي پالئوسولي و كاهشي در بخشهاي لسي بودند. نتايج حاكي از آن بود كه برش رسوبي سعدآباد در طول حدود 147000 تا 57000 سال گذشته حدود سه دوره آب‌وهوايي گرم و مرطوب و سه دوره سرد و خشك داشته است. در دوره آب‌وهوايي گرم و مرطوب لايههاي خاك ديرينه و شبه‌خاك ديرينه با ميزان پذيرفتاري مغناطيسي بالا و در دوره آب‌وهوايي سرد و خشك لايههاي لس با ميزان پذيرفتاري پايين تشكيل شده است. آب‌وهواي ديرينه و پي بردن به جزئيات تغييرات آن در گذشته به برنامه‌ريزي‌هاي مديريتي و زيرساختي و ايجاد سناريوهاي منطقي اقليمي در كشور كمك شاياني خواهد كرد.
چكيده لاتين :
One of the processes of the Quaternary era is the glacial and interglacial periods. In the glacial period, loesses deposited and in the interglacial period, Paleosoils were formed. The northern part of Iran is geographically the same as countries such as China, a large part of which has been covered with loess sediments during the Pleistocene glacial period. Loess sediments in northern Iran reflect several cycles of climate changes and the evolution of the earthchr('39')s appearance for the middle to late Quaternary period. In this region, the Loes-Paleosoil sequences are one of the most important terrestrial archives of climate change and provide a bridge between Southeast European and Central Asian loess sediments. To reconstruct paleoclimate changes, magnetic susceptibility is used as a climate proxy. The high magnitude of the magnetic susceptibility indicates the large volume of magnetic minerals. Chemical weathering causes the formation of magnetic minerals such as magnetite and maghemitite. These minerals are abundant in Paleosoils and can be determined by analyzing the magnetic susceptibility of Loess and Paleosoils. The magnetic properties of the Loess-Paleosoil sequence are considered to be evidence of paleo precipitation and weathering.
Study area
Geographical location of Saadabad sedimentary section in the northern region is 36° 49chr('39') N and 54° 22chr('39') E, at an altitude of 140 m a.s.l. Saadabad sedimentary section is located in the construction zone of Alborz and in terms of the main sedimentary-structural zones of Iran is part of the southern Caspian coast. This zone includes areas, which are located in the north of Alborz fault and block the Caspian Sea coast on the coast of Iran and are covered to the east with thick layers of loess.
Methods
In the field work that was carried out in May 2017, after determining the exact location of the sedimentary section, sampling was performed. Before sampling, aerated sedimentary layers are dug up and removed to reveal fresh deposits. Then, using the meter, the layers were divided into 10 cm intervals. 92 samples were prepared at 10 cm intervals from the sequence. Measurement of natural residue magnetic (NRM), by the model rotating magnetometer (JR-6A), and measurement of magnetic susceptibility by magnetic intensity measuring device, in the magnetometry laboratory of the Geological Survey of Iran.
Results
Table 1 shows the measured values of the magnetic parameters. Due to the length of the table, only a few examples are given in the table.
Since high magnetic susceptibility can indicate a greater concentration of magnetic minerals in the sample, it can be concluded that the paleosils of the sequence studied have more magnetic materials than the loesses, which are inside the soils. They can be the result of podogenesis processes. Because rising temperatures and humidity cause soil erosion processes and increase oxidation and thus increase the amount of magnetic materials, it can be concluded that in paleosoils with higher magnetic susceptibility, warm and humid climates dominate in the environment. Micromorphological studies of directional clay layers, Fe-Mn hydroxide, show that the palaeosoil of section 5 is moderate to well developed. As the climate changes to higher temperatures and higher humidity, weathering and pedogenesis also increase, leading to the development of a pedogenic oxidation environment as a result this produce the formation of tiny magnetic grains such as maghemite and magnetite in this oxidizer environment and thus increases the magnetic susceptibility. In Saadabad, the high amount of xlf along with the high percentage of xfd suggests that many of the ultra-fine maghemite and magentite grains may have been formed during pedogenesis under a long, humid, warm climate. The findings show that the different behaviors of magnetic susceptibility between the loess of the drier and wetter areas are mainly caused by their different pathogenic environments, which in turn are related to local topography and climatic conditions.
Conclusions
High levels of xlf and xfd indicate high precipitation during the formation of Paleosoils. Post-sedimentation processes may increase the amount of MS by producing new ferromagnetic minerals during the oxidation of wheathered soils, and may also reduce the amount of MS by reducing the processes. The clay material in which the soil is made is the main factor influencing the change in magnetic mineralogy and magnetic susceptibility. This study also shows that magnetic susceptibility is a complex parameter and its use as a precipitation control has certain limitations and conditions, and when the magnetic properties of the loesses are used for paleo climate reconstruction, more attention should be paid to topography, environment. Sediment and weather factors.
