Sedimentary Characteristics Of Sandy Sediment In Changxing Island

At the end of late Pleistocene,the climate of eastern and continental shelf areas in China was dry and cold,climate continents increased,ocean water decreased,sea level decreased,sea level of the East China Sea decreased by 130 meters,and all the Bohai and the Yellow Sea shelves were exposed,forming a continental environment.In addition,the lower temperature in Mongolia increased the strength of the high pressure in Asia,and the dry and cold air formed not only to blow up the vast inland area,but also to blow up the exposed offshore shelf,which caused the desertification in the continental shelf and the surrounding area.In the past,most of the researches on the desertification of the shelf were mainly based on borehole data,but the research on sandy sediments exposed to the same period at the same time is extremely rare.Through field investigation,the exposed sandy sediment profiles were found in Changxing Island,Dalian,and the grain size characteristics and geochemical characteristics of the sediments were studied.The following conclusions were drawn:1.The sand sediment in Changxing Island is mainly fine sand and medium sand,with the mean average diameter of 2.87 diameter and the mean value of the median grain size is 2.54.The average value of the standard deviation is 1.87 and the separation is poor,the mean deviation is 0.30,the mean value is 1.49,and the mean value is 1.49,which is a sharp peak,and the distribution frequency curve appears double peak state,which shows that the distribution frequency curve appears double peak state,indicating that the distribution frequency curve appears double peak state.The section is a two source sedimentary model,which is mainly composed of coarse near source sediments and a thin distant source.2.the chemical characteristics showed that the content of the oxide in the section was mainly SiO₂ and Al₂O₃,and the content of SiO₂ decreased gradually from bottom to top and then decreased,and then decreased again.It was contrary to the trend of Al₂O₃ and Fe₂O₃,showing that the leaching action gradually weakened and then changed strongly,and the overall change characteristics of trace elements were not very obvious.CIA values show that the CIA values of the underlying SHH1 to SHH4and SHH6 are between 50⁶5,indicating that the chemical weathering degree of this period is low,the climate is cold and dry,the CIA value of SHH5 and SHH7 is between 65⁸5,indicating that the chemical weathering degree of these two periods is moderate and the climate is warm and humid.Through the comparison of CIA value and silicon element and particle size,it is found that CIA is consistent with the trend of particle size and opposite to silicon element.Combined with the changes of SA,saf,Ba,cm,m value,regression coefficient,residual coefficient and Na/K,it can be deduced that the climate is dry and cold at the beginning of the formation of the whole section,and the climate gradually turns to warm and moist at the end of the formation of the section.3.The grain size characteristics in the middle and lower parts of the section are more consistent with the grain size characteristics of the Bohai shelf sediments at the end of the late Pleistocene,indicating that the two are similar in origin.The grain size and chemical element characteristics of the upper part are similar to those of Ma Lan loess.The characteristics of the grain size parameters at the bottom of the section are similar to that of Kukal's aeolian sand grain size.It shows that the bottom layer has typical characteristics of the desert sedimentary environment,and the upper part is characterized by typical loess deposition,and the whole profile is characterized by the transition from the sand layer to the loess layer.It can be deduced that in the early period of the formation of the whole section,the climate is dry and cold,the sea level falls,the continental shelf is exposed,and the desertification occurs in the area around the section and in the shelf area;in the end of the formation of the section,the exposed shelf is gradually covered by sea water as the climate gradually turns to the warmer and wetting and the sea level,and the region is converted to the loess deposit.