تحليل اقليم ديرينه برش رسوبي نوده (استان گلستان) با استفاده از ژئوشيمي عناصر

Paleoclimate analysis of Nowdeh section (Golestan Province) using geochemical element

تشكيل رسوبات لسي، يكي از مهمترين پيامدهاي تغييرات اقليمي است. توالي رسوبات لس/خاك ديرين يكي از بهترين آرشيوهاي طبيعي براي ثبت تغييرات اقليم مي باشد. از اين قابليت رسوبات لسي، براي بازسازي تغييرات اقليمي كواترنر و تغيير و تحول ژئومورفولوژيكي مي توان استفاده كرد. در اين تحقيق براي بازسازي آب و هواي كواترنر پسين منطقه آزادشهر، برش لسي نودِه انتخاب شد. ستبراي اين برش داراي 23/7 متر مي باشد. روش تحليل داده ها مبتني بر روش هاي ژئو شيميايي و تعيين درصد فراواني عناصر ذكر شده در هر نمونه رسوبي بوده است. با استفاده از تحليل عناصر حساس به تغيير شرايط محيطي يعني عناصر اصلي اكسيدهاي آلومينيوم، آهن، كلسيم، منيزيم و سيليس و نسبت عناصر فرعي استرانسيوم به باريم، استرانسيم به منگنز و منگنز به اكسيد كلسيم نتايجي از تغييرات شرايط محيطي بدست آمد. افزايش مقادير اكسيدهاي آلومينيوم، آهن، سيليس و منيزيم با توجه به پايداري آنها در محيط هاي رسوبي، نشان از شرايط آب و هوايي گرم و مرطوب و كاهش آنها بيانگر شرايط آب و هوايي سرد و خشك است. افزايش مقدار شاخص هوازدگي شيميايي و شاخص هوازدگي شيميايي موثر، نشان از شرايط هوازدگي شديد و كاهش اين شاخص ها نشان از هوازدگي ضعيف در برش رسوبي نوده است. خاك هاي ديرين در منطقه مطالعاتي ميزان بالاي شاخص هاي ذكر شده را نشان مي دهند و بيانگر تغيير و تحول اين رسوبات و شرايط آب و هوايي گرم و مرطوب هستند. بالعكس رسوبات لسي، مقادير كمتري از شاخص ها را نشان مي دهند و بيانگر شرايط آب و هوايي سرد و خشك در برش رسوبي نوده هستند. بدين ترتيب لس ها و خاك هاي ديرين مطالعه شده به ترتيب در دوره هاي يخچالي و بين يخچالي و در شرايط آب و هوايي متفاوت تشكيل شده اند.

In general، loess sediment are one of the most widespread form of eolian sediment. Over the past three decades، studies on global change from loess stratigraphy have made remarkable achievements based on Aeolian deposits over the world. Surprisingly little direct evidence exists documenting the continuous and progressive effects of Quaternary climate changes on terrestrial landscapes across Iran and other areas around the world. These sediments are usually yellow but grizzly and approximately 70 to 90 percent of their formation is silt material. Iran loess often are located in northeast region. Last studies has showed that loess sediment and soil formation have correlation with cold and warm period especially with quaternary respectively. For this reason it is one of important natural archive in continental climate change. This loess capability can be used in quaternary climate change reconstruction and geomorphological change. Thick Northeast of Iran loess–paleosol sequences are among the best records of terrestrial paleoclimate changes. A number of loesse-paleosol sequences contain a magnetic record of palaeoclimate through the Quaternary period. Anisotropy of magnetic susceptibility (AMS) was men- tioned 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. Methodology In this research Azadshar (Nowdeh Loess section) region was selected for late quaternary climate change reconstruction. This section has 23/7 thickness. Samples were selected in 10 cm intervals for magnetometry and geochemical analysis so at the end of sampling 23/7 samples were gathered from Nodeh section. Magnetic susceptibility was measured in in the Geology and Mineral Exploration laboratory. In the laboratory all samples were packed in 11 cm3 plastic cylinders for bulk magnetic measurements. Magnetic susceptibility as well as frequency dependent susceptibility was measured using an AGICO Kappabridge instrument. In view of the geographical location of the Nowdeh Loess، this work will complement magnetic studies in northeastern of Iran and contribute to a better understanding of the relationship between loess magnetismand pedogenic intensity and hence paleoclimate. To this end، geochemical proxies of chemical weathering are included to assist the paleoclimatic interpretation of the magnetic signals. The selected samples (70 sample) were also subjected to geochemical characterization. Major elements concentrations were determined. Result Using sensitivity of some major elements such as Al2o3، Fe2O3، CaO، MgO، SiO2 and element’s ratio such as Sr/Ba، Sr/Mn، Mn/CaO to environmental condition result to change environmental condition. Increasing of CIA and MgI indexes indicate high weathering and decreasing of them indicate weak weathering in Nowdeh section. Paleosol show high content of mentioned indexes at study area that demonstrate changing and evolution of this sediments and warm and humid weather condition. In spite of loess sediment، indicate low content of indexes that demonstrate cold and dry weather condition at Nowdeh section. So، loess and paleosols of this study formed at Glacial and interglacial period and different weather condition respectively. Conclusion This study was conducted to investigate the composition and evaluation of magnetite susceptibility and major element geochemistry in the loess deposits and paleosols of Nowdeh section in Golestan province. Nowdeh loess-paleosol sequence is an indicator for periodic dry cool and moist-warm conditions. The loess layers are representative of dry-cool climate، whereas the paleosols are indicators of the moist-warm conditions. Formation of the studied loess and paleosols، have probably taken place in glacial and interglacial cycles with different climatic conditions، respectively. Results of this research and Antoine et al (2013) on Loess-paleosol sediments on central euro show a close relation at recent 32 Ka. In two section a climate change occur inn 30 Ka . This change has recorded by magnetic susceptibility decreasing that base on last studies show a dry and cold climate in this period. Also، magnetic susceptibility increase in 32 Ka at two section that this means appearance of warm and moist climate. However last sediment has a good relation with climate change in euro and Nodeh section but because of dating absence in last sediment deduction is not unpredictable.