عنوان مقاله [English]
In general, loess sediments are one of the most widespread forms of aeolian sediments. During the past few decades, loess stratigraphy studies played key role in the investigation about global climate changes. These sediments are usually yellowish in color and silt makes 70 to 90 percent of its volume. In Iran, loesses outcrop are often in northeast part 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 evidence of paleoclimate changes 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 was sampled in 10 cm intervals for magnetometry analysis. For this aim, sampling location and method was determined after consecutive study area. This study used reformed review of library references and lab practices combination. Magnetic susceptibility of all samples was measured in Environmental and Paleomagnteic laboratory based on Geological Survey of 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 was measured. Based on results of this study, samples with high frequency in magnetic susceptibility (increasing or decreasing) were selected for other 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 of this area were formed in glacial and interglacial periods and under different climate condition. Results of this study indicated that Magnetic Susceptibility, Natural remanent magnetization, Saturation isothermal remanent magnetization and HIRM in loess was less than paleosol. Instead, the 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.
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 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 has probably taken place in glacial and interglacial cycles with different climatic conditions, respectively. Nowdeh section magnetic properties are completely matched with Neka sediment result that has obtained by Mahdipour et al. (2013). In 20 to 48 ka of past years, in two sediment sections magnetic susceptibility was similar and wherever increased, hot and humid period are integrated with paleosol layer formation. The results of this research are also in accordance with Bear and Storm (1995) in saturation of beryllium in Xifeng section and isotope δ18 of marine sediment. This indicate that climate change event in two section are simultaneous. Finally, we should say that magnetic properties depending on sensible mineral to climate change, is a very useful variable for climate change reconstruction.