Sediment Record Of Lake In The Environmental Evolution Of The Southwest Of Shandong Province During The Last 1000 Years

The lower reaches of the Yellow River is the cradle of human civilization.But in the history of civilization for thousands of years,frequent flooding and avulsion resulted in great changes of the topography,landform and the formation of the numerous groups of lakes in the lower reaches of the Yellow River.The ancient Lake Liangshanpo was one of the largest lakes in Southwest Shandong,which was deeply influenced by flooding and avulsion during historical period.Based on AMS-¹⁴C dating and the comprehensive analysis of multi-proxy records such as grain size,magnetic susceptibility,TOC,TN,C/N and pollen of the core samples from the ancient Lake Liangshanpo,as well as historical documents,the paper has preliminarily reconstructed the environmental evolution of the southwest of Shandong Province during the last 1000 years and clarified the relationship between flooding and avulsion and the ancient Lake Liangshanpo during historical period.The study is of great significance for the correct understanding of flooding and avulsion and the relationship between rivers and lakes in the lower reaches of the Yellow River.The main conclusions are summarized as followings:(1)Combined the vertical distribution of grain size,magnetic susceptibility,TOC,TN and C/N indicated that the sedimentary environment of the ancient Lake Liangshanpo can be divided into several distinct phases:during 795-1168 AD,the period was marked by the deep lacustrine phase,and the input of the terrestrial plants had decreased;During1168-1215 AD,the sedimentary environment turned into a turbulent lacustrine phase,and,the lake was effected by flooding frequently during this period;And then the envrionment evoluted into a fluvial depositional environment,which lasted to 1300 AD;And then the ancient Lake Liangshanpo was disturbed by flooding again,and sedimentary environment was instable,which evolved into a turbulent lacustrine phase and turned into a deep lacustrine phase from 1332 AD,and the number of aquatic plants in Lake had increased obviously.This sedimentary environment lasted about 150 years;At last,sedimentary environment evolved into a fluvial depositional environment from 1480 AD.In the later stage,flooding still had an impact on this area.(2)The comprehensive analysis of multi-proxy records indicates that the sedimentary environment in the southwest of Shandong Province can be divided into five distinct phases:During 795-950 AD,the climate was cold and dry and the vegetation was sparse,but at the same time,the lake level was deep and the lake was vast.The Yellow River had a little effect on the ancient Lake Liangshanpo;950-1215 AD,during this period,the climate was wam and the vegetation was lush.This stage can be subdivided into two short stages:The first short stages:During 950-1058 AD,the climate was wam and wet and the vegetation was sparse.The flooding bursted into the ancient Lake Liangshanpo from time to time,which caused that the lake continued to expand;The second short stages:During1058-1215 AD,the climate was wam and dry and the vegetation cover reduced.During this period,the lake expanded and reached to the maximun depth due to the flooding of the lower Yellow River;During 1215-1300 AD,the climate was wam and wet and the vegetation cover was inferior to the previous stage.The lake began siltation and shrunk due to the southward displacement of Yellow River,the silt brought by the flooding in the previous period and the impact of human activities;During 1300-1480 AD,the climate was cold and wet and the vegetation was lush.The lake expanded temporarily due to the flooding of the lower Yellow River again;During1480 AD to present,the climate was cold and the vegetation was sparse.The lake rapidly silted and shrunk due to the southward displacement of Yellow River,and due to the impact of human activities,the ancient lake disappeared from then.In the later period,the channel of the lower Yellow River shifted back and the flooding reaffected this area,but at this time,the ancient Lake Liangshanpo can only be used as a bearing area for silt.(3)The evolution of lake in the southwest of Shandong Province is affected mutually by environmental evolution,flooding and avulsion and human activities.Under the background of climate change,the flooding and avulsion of the lower reaches Yellow River are the major reason for the evolution of the ancient Lake Liangshanpo.(4)Due to the frequent influence of the Yellow River flooding,the sedimentation rate in the southwest of Shandong Province is significantly higher than that in eastern Shandong.Environmental changes in the southwest of Shandong Province during the last1000 years revealed by the lake sediment record is well consistent with the eastern China.