Climatic And Environmental Variations In Recent 300 Years Indicated By The Records In The Sediment Of Tamarix Ramosissima Dunes In Lower Reaches Of Heihe River

Ejina oasis in the lower reaches of the Heihe River Basin is located in northern margin of the Badain Jaran Desert,where is an extreme arid area of Alashan in western Inner Mongolia Plateau.In recent decades,ecological and environmental problems in the region are increasing seriously caused by climate and human activities.As a basic geomorphic unit of the desert,dunes have been paid by more attentions for the further studies of global change,desertification and aeolian process.The shrub dunes,located the marginal area of the Badain Jaran Desert in the lower reaches of the Heihe River,well recorded climatic and environmental variations in the past.Therefore,it is valuable to reconstruct climate change in the area in the past few hundred years,not only for providing references to the ecological environment management,but also referring to climatic and environmental research.In this study,two Tamarix dunes near Ejina oasis were selected as the research medium based on the condition of high-resolution climatic substitution index in the desert area.Two sets of samples from the sediments of Tamarix dunes were collected during the fieldwork between 2014 and 2015.According to the results of AMS14 C dating,the chronological sequences of two sedimentary sections were established,and the samples were also analysized for particle size,geochemical elements and carbonate.Following conclusions are drawn according to the comprehensive analysis of each index:?1?Through the comprehensive analysis of the grain size characteristics of two sections,the sediment of the Tamarix dune is dominated by?polar?fine sand as a whole,and the longitudinal variation of the sediment particles is significant.The grain size from the bottom of the dune to the top is tapering,the specific gravity of the fine particles?<63?m?increases while the specific gravity of the coarse particles?> 63?m?decreases.The particle size distribution curve shows a single peak form,which reflects a single wind deposition.Sediment provenance and environment tended to be constant as well as the conditions of dynamic wind,which well records the process of climate warming since the end of the Little Ice Age and the two relatively cold periods in the modern warm period?the 10 th century and the 1950 s and 1960s?.It indicates that shrub dune sediments can respond to climate change in a timely and accurate manner.?2?The average particle size,SiO₂ / TiO₂ ratio,?¹⁸O,accumulation rate,and the variation of the coarse grain content in the component 3?> 178.25?m?of section S1 have been compared and analyzed,indicating that the regional winter monsoon intensity tends to weaken as well as under dry and cold climate environment,late trending to warm and humid.The changes of coarse grain content,especially 79.43 275.42?m,of component 3?> 79.43?m?in section S2 can be taken as an alternative index of regional winter monsoon intensity.According to the analysis of median particle size and SiO₂ / TiO₂ ratio,the regional winter monsoon intensity has been decreased gradually in the past 500 years,and the coarse grain components of the two sections have good consistency in the trend of the winter monsoon intensity.?3?Through the comparative analysis of particle size and geochemical elements,it is better to reconstruct the climate change sequence in the lower reaches of Heihe River in the past 300 years.Before 1825,the climate was dry and cold,and it was at the end of the small ice age.From 1825 to 1900,climate relatively turned warm.The small ice age finished and transited to modern warm period.Climate changed to the dry and cold stage,drought occured frequntly in the period from 1900 to 1930.From 1930 to 1955,the climate changed to a relatively warm and humid,and the warming rate in the northwest region was the most significant.From 1955 to 1957,the regional temperature decreased,and the climate turned dry.Since 1975,the climate began from warm to warm and wet transition.Additionally,consistency was in a well state between S1 and S2 in the process of climate change.S2 is also a good indicator of its climate change.Before 1765,climate changed to be humid partially,and then became dry and cold under the end of the small ice during 1765 1825.From 1825 to 1900,the climate is relatively humid;1900 1930,the climate turned dry and cold.Finally,climate began to change from warm to warm and wet since 1930.It is evident that the two sections have good consistency,further illustrating the feasibility and significance to reconstruct climate change in the study area.