بازسازي آب و هواي كواترنر پسين با استفاده از پذيرفتاري مغناطيسي و ژيوشيمي عناصر اصلي لس هاي منطقه آزادشهر

Reconstruction of Late Quaternary paleoclimate in north Iran, using magnetometery and geochemistry of Azadshahr Loess deposit

تشكيل رسوبات لسي، يكي از مهمترين پيامدهاي تغييرات اقليمي است. توالي رسوبات لس/خاك ديرين يكي از بهترين آرشيوهاي طبيعي يراي ثبت تغييرات اقليم مي باشد. از اين قابليت رسوبات لسي، براي بازسازي تغييرات اقليمي كواترنر و تغيير و تحول ژيومورفولوژيكي مي توان استفاده كرد. در اين تحقيق براي بازسازي آب و هواي كواترنر پسين منطقه آزادشهر برش لسي نودِه انتخاب شد. اين برش داراي 7/23 متر ستبرا مي باشد. در مجموع تعداد 237 نمونه از كل برش با فاصله 10 سانتيمتري جهت آناليزهاي مغناطيسي و ژيوشيمي انتخاب و برداشت شد. پذيرفتاري مغناطيسي تمامي نمونه ها اندازه گيري شده و بر اساس نتايج به دست آمده از اين اندازه گيري، 70 نمونه براي آناليز ژيوشيمي انتخاب شدند. نتايج حاصل از اين تحقيق نشان داد كه ميزان پذيرفتاري مغناطيسي با توالي لس-خاك ديرينه ارتباط مستقيم داشته و با تشكيل رسوبات لسي پذيرفتاري مغناطيسي كاهش و با تشكيل لايه هاي خاك ديرينه افزايش يافته است. 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. همچنين تغييرات نسبت اكسيد كلسيم به اكسيد آلومينيوم ((CaO/Al2O3 هم راستا با خصوصيات تشكيل خاك، دامنه زياد تغييرپذيري را نشان مي دهد. بنابراين نسبت عناصر ذكر شده مي تواند در بازسازي آب و هواي گذشته كواترنر مورد استفاده قرار گيرد.

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 changesand 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 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 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 and geochemical analysis. 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. The geochemical proxies of chemical weathering of selected 70 samples (major elements) are included to assist the paleoclimatic interpretation of the magnetic signals. 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. The changes in ratio of, CaO/Al2O3 which is coincide with changes in character of soil formation, show high variation range. Soluble element such as Ca, is very sensible to climate change and show reaction immediately, and therefore, this kind of elements ratio can be used for reconstruction of Late Quaternary climate condition in study area. Conclusion This study was conducted to investigate and evaluate the capability of magnetite susceptibility and major element geochemistry tools 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. The comparison of results of this research with the results of Antoine et al., (2013) on Loess/paleosol sediments of Central Europe, show a close relationship especially at an age of 32 Ka, which show a climate change has taken place at this age. In both sections, this change is recorded by decreasing in magnetic susceptibility approximately in 30 Ka, at the base of deposition of loess, indicating dry and cold climate in this period and increase in in magnetic susceptibility in 32 Ka, that means appearance of warm and moist climate.