جستجوی مقالات مرتبط با کلیدواژه « loess » در نشریات گروه « زمین شناسی »

Loess deposits are unconsolidated sediments with mainly coarse silt, without stratification with high permeability that are mostly yellow to light brown colour. Golestan Province in northeast of Iran is one of broadest loess area in Iran that connects loesses of middle asia to urasia. In this research, texture characteristics and quantity of silt, sand and clay are compared in 2 methods (sieve- hydrometry and laser diffractometry), in 16 sampling station. Data correlation in particle measurements between sand, silt and clay in these methods are a bit different. There is weak correlation between sieve- hydrometry and laser, in loess sediments of  the study area; except sand, there are very weak correlation between silt and clay that can be realated to different measurement methods. Analysis of hydrometry is based on speed of sediment deposition particles and settling velocity is assumed to be constant, density of particles is considered equal to quartz and particles are considered spherical and smooth. Results shows that the amount of clay particles in hydrometry method are much more than laser method, which is related to different mineralogy of clays. This causes overestimation of platy particles in hydrometry method. Laser particle sizer measures medium particle diameter but sieve shows large particle diameter. This is the reason of different measurements between sand size sediments.

 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.

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.

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.

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.

The loess-paleosol sequences in Northern Iran are important archives represent several cycles of Quaternary climate change and can be used to complete the information gap on loess between Europe and central Asia. Loess geochemistry reflects paleoweathering conditions and it can be used to determine the nature and provenance of loess deposits. In the Caspian Lowlands, a pronounced precipitation gradient is reflected in mean annual precipitation rates decreasing from about 1850 mm at Bandar Anzali in the West to about 435 mm at Gonbad- e Kavoos in the East. The results of the along a loess climosequence in Northern Iran showed that with increasing precipitation, soil pH and calcium carbonate contents decreases, whereas soil organic carbon, clay content, and cation exchange capacity increases. Many studies have been carried out on the loess-paleosol sequences and modern loess soils in Northern Iran but more investigation is needed with emphasis on the forest soils, their geochemical properties and provenance. The study area is located in the northern slopes of Alborz Mountain Ranges covered with Caspian or Hyrcanian deciduous forests. Field sampling started in summer 2015. More than ten soil pedons with loess parent material were investigated based on former studies. Finally, six representatives modern pedons were selected and dug in an east-west direction on loess deposits. The climate data shows that precipitation varies from 500 mm in Qapan to up to 800 mm in Neka. Physiochemical properties of soils were studied using standard methods. Element concentrations measured on the fine-grained fractions. These extracted by sieving out material <63 µm and drying it at 105 °C for 12 hours. Eight g of the sieved material mixed with 2 g Fluxana Cereox, homogenized with a vibratory disk mill and pressed to a pellet with a pressure of 19.2 MPa for 120 s. Powdered press pills were analyzed for trace elements (V, Cr, Co, Ni, Zn, Rb, Sr, Zr, Nb, Ba, Nd, Pb) and fused discs for major (Na2O, MgO, Al2O3, P2O5, K2O, CaO, TiO2, MnO, Fe2O3) elements using energy dispersive polarized Spectro Xepos X-ray fluorescence (XRF) device. Every sample measured twice, rotating the pellet in between. Subsequently, mean values calculated from the two measurements. The results showed that the downward decalcification and the subsequent clay illuviation were the main criteria influencing assessment of soil development in this study. So, all of the soils host argillic and calcic horizons and are classified as Alfisols and Mollisols. All studied pedons had nearly uniform chemical composition in their parent materials, suggesting similar alteration history of these sediments. In general, all parent materials showed strong depletions of mobile element (Na) and weak depletion of Sr and Mg in UCC-normalized spider diagrams. Ca was distinctly enriched in loess compared to UCC. Variation trend of major and trace elements concentration in the studied regions was more similar to average of world loess composition (AWL), but a little different from PAAS (post-Archaean average Australian shale). The significant depletion in Ca and Sr in Bt horizon compared to their parent materials, evidenced in all pedons that was in line with present precipitation gradient and attributed to similar geochemical behavior of Ca and Sr suggesting the alteration of plagioclase. Also, slightly depletion trend of Na and P was in accordance with precipitation gradient. It seems that the absolute concentration of the major elements, especially Si in the studied soils, was influenced by dilution effect of carbonates and consequently amount of this element was significantly lower than AWL and PAAS chemical composition. Chemical Index of Alteration (CIA) values of the samples vary from 62 to 75, higher than those of the UCC (CIA~48) suggesting moderate degree of weathering in studied pedons. The spread of data in A–CN–K ternary diagram space parallel to A–CN line close to the plagioclase weathering trend, revealing comparatively stable K-feldspar/plagioclase ratios in loess. In general, these results support the important influence of parent material on soil geochemistry in studied areas. Additionally, calcification as prevailing pedogenic processes in these areas, can effectively affect elemental distribution within soils. So, the variation of elemental composition and distributions within soil profiles are influenced by parent materials and pedogenic processes. In addition, chemical index (CIA), elemental ratios (Ba/Sr, Rb/Sr) and the A-CN-K ternary diagram indicate a moderate degree of chemical alteration for these soils, compatible with the weathering regime prevailing in the region.

Loess deposits are the silt dominated aeolian deposits. Loess-paleosol sequences have been frequently used to determine and reconstruct paleoclimate and paleoenvironment of quaternary. The first step for this purpose, is to identify the sequence of loess and paleosols and describe their pedological, physical and chemical characteristics. Particle size distribution is the most important characteristics of loess deposits. This characteristic is a function of distance from source and pedological processes. Carbonates are the main constituents of the loess deposits which can act as a cement and affect the particle size distribution. Main loess areas in Iran are located in the north of country, especially in Golestan province. However, there are loess deposits in Sari which there is little information about their pedological and morphological properties. The objectives of this study were to 1) identify the loess - palaeosol sequences and 2) determine the effect of carbonates on particle size distribution of two loess-paleosol sequence in southern Sari.
There is a loess containing area in southern Sari. During field surveying, two loess-paleosol sections were explored and the loess layers and paleosols (horizons and pedogenic features) were described. The sections were sampled from 10 cm intervals. Calcium carbonate equivalent (CCE), soil organic carbon, pH and particle size distribution of the samples were measured. Particle size distribution was determined using pipette method. The fractions of sand, coarse silt, medium silt, fine silt and clay fractions were measured. To determine the effect of carbonate on particle size distribution, sand, silt (coarse silt, medium silt and fine silt) and clay contents of the samples were measured before and after carbonate removing. Diluted HCl was used to remove the carbonates.
The thickness of the sections were 408 and 627 cm and were morphologically similar. Both sections contained a modern soil (MS) and three palaeosol which separated by loess layers. This indicated periodic alternation of soil formation and loess accumulation. In the studied sections, the modern soils composed of cambic (Bw) and calcic (Bk) horizons. Paleosols 1 (PS1) and paleosols 2 contained argillic-calcic (Btk) horizons. Paleosols 3 (PS3) indicated vertic properties (Bsskg). In contrast to modern soil in loess in humid regions of Golestan province which contained argillic horizon, the modern soil in the studied sections were not highly developed. Slickenside and wedge shape were the two main vertic properties which observed in PS3. Seasonal saturation in PS3 due to low permeability of Bsskg horizon produced stagnic properties. Accumulation of Mn oxides and to some extent Fe oxides revealed the intermediate reduction states.
Carbonate calcium equivalent (CCE) in the sections ranged from 2 to 48.75 %. High amount of secondary carbonates were found in loess layers and was higher than overlying paleosol. This indicated that in humid periods of soil formation the carbonates translocated from the paleosols. Particle size distribution results indicated that sand fraction mostly composed of hard carbonate nodules and consequently the amount of sand fraction reached to 42 % which is unusual in loess deposits. The amount of sand, silt and clay in the sections were 2 to 48.75 %, 36.2 to 78.2 % and 16.6 to 43.8 % which after removing the carbonates changed to 0.3 to 3.8 %, 47.1 to 81.5 % and 17.6 to 50.3 % respectively. Particle size distribution firstly indicated the distance from the origin. Dominance of medium silt showed that the sediments have been originated from relatively large distance. Weathering and translocation of fine particles during humid periods increase the clay content in the paleosols. The highest amount of clay content and the lowest amount of CCE occurred in PS2 palesols which indicate suitable weathering conditions during PS2 formation. Furthermore, the thick Btk horizons in PS2 paleosols of both sections is another evidence of suitable conditions for soil formation.
Results of the present study indicated that the studied sections are suitable for future paleoclimate and paleoenvironment studies. The sequences of loess-paleosols in southern Sarin was similar to those in humid areas of Gorgan which indicated they experienced similar sequences of climate variations. On the other hand, secondary carbonates accumulated as hard nodules increase d the amount of sand content which may cause misleading interpretation. Therefore, it is suggested removing the carbonates of loess sections in humid areas of northern Iran.

Quaternary loess and loess like deposits covered more than 3200 km2 of Golestan Province. Mineralogical aspects of loess deposits were studied on collected samples with east-west trend by random method using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDXA). The results showed the presence of quartz, calcite and clay minerals like illite and chlorite in loess deposits. Based on mineralogical evidences, it could be suggested that dry and cool climate conditions were dominant at the time of loess deposition.

Loess is a particular type of silty soils with porous structure and weak cohesion which has been deposited over different periods of Quaternary. From the geological engineering view points, these soils are problematic ones whose main geological engineering feature of these soils is their erodibility and collapsibility. Loess soils are the main deposits cover north of Golestan province. These soils have spread across 4200 square kilometers. In this study, in order to examine and identify the loessic soils, undisturbed samples were collected and transferred to the lab and then Atterberg limit, specific gravity, grading, percentage of lime, specific density, rate of consolidation, and shear strength parameters were determined. Soil collapsibility was also determined using the results of the double oedometer tests. According to the unified standard, these soils are categorized in CL and ML groups. Furthermore, based on Clevenger classification, they are placed in the category of clayey loess and silty loess. The amount of calcium carbonate in these soils ranges from 12 to 22.3 percent and demonstrates low shear strength parameters in saturated condition. The double oedometer tests results demonstrate that the soils are prone to collapsibility. The results of soil dispersion tests reveal that the depression, which is due to the presence of sodium carbonate, does not have any influence on erosion of mentioned soils; therefore the high erodibility of these soils is basically a mechanical phenomenon which can be attributed to special texture and grading of loesses.

The study area is situated at about 8 km away of Neka city, on the main road of Sari Behshahr, in Mazandaran Province, north of Iran. It is bounded by Alborz fault in the south and Caspian Sea coast line in the north. We applied new environmental magnetism technique and methods including magnetic susceptibility, magnetic mineralogy, IRM and S-IRM to reconstruct the past 50 ka climate conditions and also investigate relationship between climate change and magnetic variations. We also compared the geochemistry and magnetic analysis of the loess/palaeosol deposits and the results were used to correlate these variations with climate change in the study area. The pattern of variation of geochemical data shows similarity with those of magnetic one. The increase/decrease in magnetic susceptibility is coinciding with palaeosol/ loess sequences. Plotting magnetic susceptibility variation versus lithological column of Kolet section enabled us to recognize short period of climatically cycles known as stadials/interstadials in loess/palaeosol sequences. We recognized 4 stadials for upper palaeosol, single stadial for lower palaeosol, 6 interstadials for upper loess and one interstadial for lower loess deposites.