Dynamic Change And Environmental Evolution Of Modern Coastal Dunes In Changle, Fujian

The coastal dune belt of Eastern Changle in Fujian is one of the coastal dune distribution areas with large range and typical type.Wind,wave and tide play an important role in the development of coastal dunes,therefore,the evolution of coastal environment are well recoreded in coastal dunes.Based on the analysis of dynamic factors,grain characteristics of surface sand,GPS-RTK monitoring data,and ground penetrating radar(GPR)data,the study explores the grain size and source contact between beach sand and dune sand,geomorphic changes in beach-dune system,as well as dynamics interaction.Through the analysis of optical stimulated luminescence(OSL)dating,grain size,sedimentary structure of fixed Donghai(DH)dune,the paper compares sedimentology record of activity of sand in East Asia and other regions of the northern hemisphere during the little ice age(LIA),the East Asian Winter Monsoon and Northern Hemisphere temperature sequence,the North Atlantic Oscillation Index(NAO),so as to study the growth of Changle coastal dunes and its environmental significance in century scale.The following conclusions are drawn:(1)The average wind speed is greater than critical wind speed in Changle coast,and strong shore wind promotes the morphological adjustment of beach and movement of dune.Wave and tide have an effect on beach and dune,so the beach and dune change,developing transitional coast landforms,such as tidal inlet,wave ripple,foredunes,transverse dunes and sand flat etc.(2)Grain size analysis shows that,from Meihua,Zhanggang to Xiasha,beach sand and dune sand are dominated by medium-fine sand,very fine sand and fine sand content decrease gradually,medium sand and coarse sand content increase gradually.The average particle size of beach sand and dune sand shift from coarse to fine,well sorting,nearly symmetric distribution,and flat kurtosis.The different grain size characteristics indicate wind intensity become weak after strong from Meihua to Xiasha,wave and tide determine the morphology of beach-dune system.Fixed Dune(DH)in Donghai are mainly medium sand and fine sand,well sorting,negative bias to nearly symmetrical skewness and medium kurtosis totally,basically according with characteristics of modern typical coastal aeolian sand.(3)GPS-RTK monitoring shows that,beach is dominated by erosion,coastal dune is moving towards the land gradually in recent years,which are defined as Beach stable-Dune retreat Pattern.Google earth satellite images show beach shape has experienced stable changes,dune range are decreaseing after increasing,the same changes have taken place in the area of fixed dune(DH),which are the results of sand spread inland constantly and then interference by human.(4)The images form GPR show,the coastal aeolian deposits of beach-dune system are the overlying strata on the wave-deposited beach sediments which present the horizontal bedding and hummocky cross stratification.And this's the results of interaction between wave and tide.Fixed Dune(DH)deposition is characterized by the development of gently inclined to nearly horizontal bedding,trough cross bedding and curved wedge bedding,which are the parabolic dune sedimentary structures that parallel to the wind direction,and then blocked by shrub vegetation,last formed by interaction between vegetation and sand flow.(5)In a unified time scale,through the analysis of grain size,sedimentary sequence,we comprehensively compare dunes from different regions in LIA,East Asian Winter Monsoon sequence,Northern Hemisphere temperature series,North Atlantic Oscillation Index(NAO),and Fujian coastline.We can conclude,DH dune developed in past 300 years of late little ice age.Weaker East Asia winter monsoon,dry and windy climate,and frequent human activities promoted coastal dunes.Since late LIA aeolian activity intensity represented by mean grain size,East Asian winter monsoon and Northern Hemisphere temperature series had a good relationship.Regional typhoon and storm surge may produce rapid sand accumulation and erosion,resulting in information recored by grain size loss or change.The development of local coastal dune represented by DH dune was closely related to sea level change,which recorded the secondary fluctuations of late Holocene sea level.Since Han Dynasty,Changle coast retreated gradually,and has pushed east about 5-10km,which promoted coastal dunes prograde seaward efficiently.