ثبت تغييرات اقليم كواترنر پسين در پذيرفتاري مغناطيسي لس هاي آزادشهر

The record of late quaternary climate change on magnetic susceptibility of Azadshahr Loess

در اين پژوهش، برش رسوبي نوده واقع در شمال شرق ايران براي بررسي وضعيت آب‌وهوايي گذشته بررسي شده است. تلفيقي از مرور سامان‌مند منابع كتابخانه‌اي و ميداني و كارهاي آزمايشگاهي در اين تحقيق استفاده شده است. به‌منظور انجام كار با بررسي‌هاي ميداني متوالي، محل و روش نمونه‌برداري مشخص شد. 237 نمونه به فواصل 10 سانتي‌متري از برش نوده نمونه‌برداري شد. پذيرفتاري مغناطيسي تمامي نمونه‌ها اندازه‌گيري شد و بر اساس نتايج به‌دست‌آمده از اين آزمايش، نمونه‌هاي داراي نوسانات شديد افزايش يا كاهش ميزان پذيرفتاري مغناطيسي، براي مطالعه ساير پارامترهاي مغناطيسي انتخاب شد. نتايج به‌دست‌آمده از اين تحقيق نشان مي‌دهد كه ميزان پذيرفتاري مغناطيسي، پسماند مغناطيسي طبيعي، پسماند مغناطيسي ايزوترمال اشباع‌شده و HIRM در لايه‌هاي لس كمتر از لايه‌هاي خاك ديرينه است. در مقابل، ميزان S_0.3 در لايه‌هاي لس بيشتر از خاك‌هاي ديرينه است. نتايج اين تحقيق نشان مي‌دهد كه برش رسوبي نوده در طول 150 هزار سال گذشته، حدود هشت دوره آب‌وهوايي گرم و مرطوب (لايه‌هاي خاك ديرينه و شبه‌خاك ديرينه با ميزان پذيرفتاري مغناطيسي بالا) در بين دوره‌هاي سرد و خشك (لايه‌هاي لس با ميزان پذيرفتاري پايين) را تجربه كرده است.

Expanded Abstract Introduction In general, loess sediments are one of the most widespread forms of eolian sediments. During the past few decades, loess stratigraphy studies played key role in global climate changes. These sediments are usually yellowish in color and silt makes 70 to 90 percent of it volume. In Iran, loesses outcrop often in northeast of south Caspian Sea. The previous studies revealed that loess/paleosol sequences correspond to cold/ warm period of climate, especially during quaternary period. Therefore, loess deposits are the most important natural archive of climate changes and are ideal for reconstruction of paleoclimate and geomorphological change in Quaternary. The thick loess/paleosol sequences of northeast Iran provide unique opportunity to reconstruct the terrestrial paleoclimate changes recorded in these sediments. A number of loesse-paleosol sequences contain a magnetic record of palaeoclimate through the Quaternary period. Anisotropy of magnetic susceptibility (AMS) was mentioned as a good tool to determine paleocurrent or paleodirection. AMS measurements were mostly used in the investigation of igneous, metamorphic and sedimentary rocks with an increasing number of applications in Quaternary loess and paleosol studies since the end of the1980s. The sediment magnetic properties depend on the magnetic content and characteristics of the source material and post-depositional weathering/soil formation processes. Loess in north of Iran is part of world loess belt and such as another loess sediments are evidence of paleoclimate change in continent. Materials and Methods In this study, Azadshar (Nowdeh Loess Section) was selected to reconstruct Late Quaternary climate change. The Nowdeh loess section with about 24 m thickness were sampled in 10 cm intervals for magnetometry analysis. For this aim, sampling location and method was determined after consecutive study area. This study used of reformed review of library references and lab practices combination. Magnetic susceptibility of all samples was measured in Environmental and Paleomagnteic laboratory based at Geological Survey of Iran, Tehran, Iran. All samples were placed in an 11 cm3 plastic cylinders to be used in magnetic measurement instruments. Magnetic susceptibility was measured using AGICO company made Kappabridge model MFK1-A instrument. Magnetic susceptibility of all samples were measured. Base on this study results, samples with high frequency in magnetic susceptibility (increasing or decreasing) were selected for another studies of magnetic parameters. Results and discussion The variation of magnetic susceptibility signal in the Nowdeh section suggests variation in climate conditions and mechanisms during the Late Quaternary. The magnetic susceptibility relationship with Loess/paleosol deposits resulted in low magnetic susceptibility values in cold and dry climate periods (Loess) and high magnetic susceptibility values in warm and humid climate periods (paleosoil). Therefore one can say that Loess and paleosol sequences studied here was formed at glacial and interglacial periods and under different climate condition. Results of this study showed that Magnetic Susceptibility, Natural remanent magnetization, Saturation isothermal remanent magnetization and HIRM in loess was less than paleosol. Instead amount of S_0.3 in loess layers was more than paleosol. Results of this study show that Nowdeh section has seen 8 periods of hot and humid climate (paleosol layers and similar paleosol layers with high magnetic susceptibility) in cold and dry periods (loess layers with low magnetic susceptibility) during past 150 ka. Conclusion This study was conducted to investigate and evaluate the capability of magnetite properties in reconstruction of Late Quaternary paleoclimate condition recorded in loess/paleosol deposits of Nowdeh section in Golestan province, northeast of Iran. Nowdeh loess/paleosol sequence is an indicator for periodic dry- cool (deposition of loess) and moist-warm (formation of paleosol) conditions. Formation of the studied loess and paleosols, have probably taken place in glacial and interglacial cycles with different climatic conditions, respectively. Nowdeh section magnetic properties completely is match with Neka sediment result that has obtained by mahdipour et al (2013). In 20 to 48 ka of past years show that in two sediment sections magnetic susceptibility was similar and wherever increased, hot and humid period integrated with paleosol layer formation. Also results of this research are accordance to with bear and storm (1995) in saturation of beryllium in Xifeng section and isotope ?18 of marine sediment. This is indicate that climate change event in two section are simultaneous. Finally, should be say that magnetic properties because of depending of sensible mineral to climate change, is a very useful variable for climate change reconstruction.