دماسنجي ايزوتوپي و بازسازي تغييرات اقليمي گذشته با استفاده از شواهد پالئوپدولوژيك در بخش شرقي حوضه زاينده رود، اصفهان

Isotopic Thermometry and Past Climatic Reconstruction Using Paleo Pedologic Evidence in the Eastern Part of Zayandehrurd Watershed, Isfahan

پديده‌هاي گرمايش جهاني و تغييرات اقليمي در سال‌هاي اخير شدت گرفته و به يك چالش جهاني تبديل شده است. شناخت تغييرات اقليمي گذشته در پيش بيني نوسانات اقليمي آينده داراي اهميت است. هدف از اين پژوهش كاربرد شواهد پالئوپدولوژيك جهت بازسازي كيفي و كمي پارامترهاي اقليمي گذشته در مناطق خشك شرق اصفهان است.منطقه مورد مطالعه در فاصله حدود 50 كيلومتري شرق اصفهان قرار گرفته و داراي اقليم خشك و شديداٌ فصلي از لحاظ دما و بارش ااست. مورفولوژي خاك‌هاي مورد مطالعه پيشنهاد ميكند رژيم اقليمي گذشته منطقه شرق اصفهان بصورت فصلي با بارش زمستانه و تابستان‌هاي گرم و خشك همراه با ميانگين سالانه بارش حدود 3 تا 4 برابر مقادير فعلي (براي افق‌هاي كلسيك) و تا 6 برابر مقادير فعلي (براي افقهاي آرجيليك) بوده است. بازسازي دماهاي گذشته با استفاده از ايزوتوپ هاي اكسيژن در كربنات هاي پدوژنيك هم نشانگر تشكيل اين كربنات ها در محيطي سردتر (دوره هاي يخچالي) نسبت به شرايط فعلي است. بازسازي دماهاي گذشته پيشنهاد مي‌دهد كربنات ها در مخروطه افكنه ها در محيطي با ميانگين دماي سالانه حدود 6 درجه و در لندفرم رليكت (فلات) با ميانگين دماي سالانه حدود 2تا 3 درجه كمتر از شرايط فعلي تشكيل شده‌اند. ناهمگوني اقليمي طي دوره‌هاي يخچالي در مركز و شمال ايران مشاهده مي شود.بنظر ميرسد فعاليت‌هاي سيستم‌هاي پرفشار سيبري و جنب حاره‌اي در اين مورد موثر بوده است.

Introduction Global warming and increase in greenhouse gases concentration have been accelerated during recent years and have become a global challenge (Woodward 1992). Changes in atmospheric circulation patterns and climatic parameters considered as a consequence of greenhouse gases acceleration. Climatic change occurs on diverse scales of time and space (Hardy 2003). Today, based on past evidence, present information and the modern technology, models are developed to investigate the climate change phenomena and its consequences (Woodward, 1992). Geological and biological evidence used for reconstruction of paleo climate parameters and validate of forecasting models (Moatamed 1997, Bradley 1999, Retallack 2001). Reconstructing ancient climates require consideration of many variables including temperature, the composition of the atmosphere and precipitation intensity (Dworkin et al 2005). Soils develop in direct contact with climatic parameters and can present indicators of past climate such as temperature, precipitation, seasonality, the composition of the atmospheric gases and atmospheric circulation patterns (Amundson et al 1996, Retallack 2001, Dworkin et al 2005, Sheldon and Tobar 2009). Despite data derived from soils are usually qualitative (Catt 1991), several attempts carried out for producing quantitative data during recent years (Dworkin et al 2005, Sheldon and Tabor 2009). There are different opinions about paleoclimate of Iran according to geomorphological investigations (eg. Krinsley 1970, Ramesht 1996, Moaieri et al 2009). The aim of this study is to use paleopedologic indicators for qualitative and quantitative paleoclimate reconstruction in arid lands of eastern Isfahan. Materials and methods The study area is located in the eastern part of Zayandehrud watershed (Fig. 1). Soil leaching index (SLI) and seasonality index for study area were calculated using modern climatic data according to Bull 1991. Two soil profiles were studied on an alluvial fan and a relict landform using criteria of USDA 1979. Redness rate index for Argillic horizons was calculated by method of Torrent et al 1983. Pedogenic carbonates were calculated from pedons on landforms. Oxygen isotopic analysis was used for this carbonates. Carbonates were reacted with 100% phosphoric acid to release CO2 gas. The CO2 gas was purified and analyzed using mass spectrometry at university of California, Berkley. Isotopic values reported in the standard δ notation relative to PeeDeeBelemite (PDB). Paleotemperatures was calculated using equations suggested by Dworkin et al 2005 and Maple 9.01 software. Results and discussion Modern climatic data of eastern Isfahan synoptic station show that mean annual precipitation is lower than mean annual evapotranspiration during the year and SLI is zero for the study area. This means there is no favorable condition for pedogenic development under present climate. Seasonality indexes indicate a seasonal pattern for precipitation and temperature in eastern Isfahan. Pedogenicdevelopment of studied profiles indicates formation of calcic and argillic horizons. Calcic horizons develop under semiarid condition with seasonal aridity while argillic horizons develop under more moist condition (Retallack 2001). Mean annual precipitation about 400600 mm (according to precipitation season) with seasonal pattern suggested for carbonates translocation and calcic horizon development (Tandon and Kumar 1999). It seems that calcic horizons in eastern Isfahan were formed under past semiarid condition with seasonal pattern of precipitation. Formation and development of argillic horizons occur in more moist conditions than calcic horizons (Retallack 2001). Gvirtzman and wieder (2001) suggest annual winter precipitation about 600800 mm for remove of carbonates from calcareous parent material and argillic horizon formation in the eastern Mediterranean. Expansion of calcic and argillic horizons in the arid lands of eastern Isfahan indicate past different hydrological conditions. Redness rate index for Argillic horizons in the studied profiles suggest that soils on relict landform were developed in warmer environment than soils on alluvial fan landform. Oxygen isotopes in carbonates considered for paleotemperature determination (Faure 1986). There is a strong correlation between temperature and δ18O values in meteoric water for out tropical regions (Cerling and Quade 1993, Bradley 1999). Oxygen isotopic diffraction between water and carbonate ions is a temperature dependent process (Moatamed 1997, Sheldon and Tabor 2009) and δ18O values in secondary calcite depends on δ18O values of mother water and crystallization temperature (Sheldon and Tabor 2009). Dworkin et al (2005) suggest two equations (spatial simulation and regression) for paleotemperature determination using oxygen isotopic values in pedogenic carbonates. Application of these equations for the studied carbonates show (a) pedogenic carbonates of eastern Isfahan formed in a colder environment than today with a decrease in temperature about 6°C (for alluvial fan) to 3°C (for relict landform) (b) carbonates on relict landform formed in a warmer environment than carbonates in alluvial fan environment, This result is in corroboration with redness rate index. Finally according to geomorphologic indicators of paleoclimate in central Iran, it seems that regression equation present better estimation of paleotempoerature for eastern Isfahan. Conclusion Analysis of climatic data show there are no favorable condition for pedogenic development under modern climatic condition in the eastern Isfahan. Paleo pedological and geomorphological indicators were used for paleoclimate reconstruction in eastern Isfahan (central Iran). Results indicate that studied carbonates and argillic horizons were formed under a semiarid to subhumid and colder environment than today. Climatic asymmetry observed during glacial periods in central and north Iran. It seems that activity of Siberia high pressure and subtropical anticyclone during glacial periods had a great effect on climate of Iran and was the main control for climatic asymmetry between central and north Iran.