Coastal Aeolian Deposition And Environmental Evolution In Northeastern Hainan Island Since The Last Glacial Maximum

Coastal aeolian dunes are developed in coastal zones of various latitudes with abundant sand source,strong wind,flat and broad terrain,and are important carriers for recording information of aeolian sand activity and environmental evolution in coastal zones.The northeast coast of Hainan Island is a typical area where coastal dunes are concentrated in tropical areas of China.In this paper,three typical coastal aeolian sediment profiles of Jinshan(JSD2),Xikeng(PQR2)and Shuigaocun(SCES)in the northeastern part of Hainan Island are selected as the research objects.In this study,field survey,systemic optically stimulated luminescence(OSL)dating,grain size measurement,magnetics,elements were applied to several coastal dune profiles.Based on the research results of many scholars,this paper establishes a chronological framework of coastal aeolian sand deposits in the study area,and analyzes the regional environmental evolution recorded by aeolian sand deposits by comparing with other climate index data on a unified chronological scale.Results show that:(1)From the perspective of coastal wind and sand dynamics in the study area,the northeast coast of Hainan Island has wind and sand transport throughout the year,mainly in winter and spring,with NE and NNE as the main sand transport winds.The results of grain size show that the JSD2,PQR2 and SCES profiles are dominated by medium and fine sand,with grain size mode of 200~300?m,accounting for 8~13%.The grain size characteristics of the sand samples in the study area are basically consistent with the typical aeolian characteristics.SEM results show that there are Dished pit,crescent-shaped pit and V-shaped pit on the surface of sand samples in the study area,which are dominated by aeolian characteristics.Generally speaking,the sand samples in the study area are mainly deposited under the action of wind power.(2)From the weathering characteristics of coastal aeolian sand,the content of fine grains< 63 ?m and EM1 decreased from brownish-red to brownish-red to grayish-yellow sand successively,and the weathering showed a decreasing trend;The results of major element analysis show that Na,Ca and other soluble components in the aeolian sand layer are leached in large quantities,with the content less than 0.2%,indicating that the coastal aeolian sand deposits in the study area have experienced strong chemical weathering;The environmental magnetic analysis shows that the coastal aeolian sand samples are mainly ferrimagnetic minerals,and the magnetic susceptibility of the old red sand layer is higher than that of the light gray sand layer,and the later reddening process of the coastal dune has an effect on the magnetic susceptibility.(3)The aeolian sand in the northeast coast of Hainan Island was mainly developed since the Last Glacial Maximum,with an age range of 2.20±1.09~22.50±1.07 ka.Based on the new OSL data of coastal dune profiles we provided and the dating results of coastal aeolian sand sections in other places of Hainan Island,we combined all the sediment characters of dune profiles,like bedding structures,unconformity,grain size parameters,magnetics and elements,to establish the OSL chronology framework of regional coastal dunes development history.Several periods of dune development and aeolian activity have taken place in the study area since the Last Glacial Maximum.They are 23.5~14ka?10~9 ka?7~5 ka and 4 ka~present,respectively.(4)Comparing with the environmental records of sea level change and East Asian winter monsoon change,the coastal aeolian sand deposition mainly occurs in the stage of high sea level and rapid rise of sea level in the long time scale of 10000 to 1000 years.Under the humid and hot climate conditions,strong land weathering and denudation transport abundant sand sources to the coastal zone,coupled with coastal wave and coastal wind The combination of sand and sand is beneficial to the development of coastal sand dunes,but these coastal sand are mostly preserved in the form of residual old red sand on higher coastal terraces.In the short time scale from millennium to centennial,for example,since the middle late Holocene,due to the weakening of solar radiation in the northern hemisphere and the southward shift of the tropical convergence zone(ITCZ),the climate has become cooler and drier,the coastline has retreated,and the winter monsoon has gradually strengthened,which is conducive to the modern coastal sand accretion and the development of coastal dunes.