Research On Environmental Changes And Human Activities In Weizhou Island Since Last 1500years

The past 2000 years have been a period of rapid human development,environmental changes have changed from nature-driven dominance to the common influence of human and nature.Weizhou Island is located in the middle of the Beibu Gulf in the northwest of the South China Sea.Due to the influence of the southern subtropical monsoon climate and frequent typhoons,the wetland deposition is well developed.It is therefore an ideal area to study the climate and environment change of subtropical islands and reveal human activities.In this study,a 160-cm wetland core,named WZD（21°03’26"N,109°07’26"E）,has been collected from Weizhou Island.Multi-proxies analysis mainly including AMS¹⁴C dating,high-resolution pollen,charcoal and particle size analysis in combination with local historical literature,provide powerful evidence for exploring paleo-environmental variability over the past 1500 years,as well as the history of vegetation evolution and climate change in Weizhou Island,and further revealing the environmental change process of Weizhou Island under the influence of human activities.The main conclusions are as follows:（1）1471～669 cal.a B.P.（479～1281cal.A.D.）,it was characterized by lowest pollen concentration,but the dominant plants were mainly drought tolerant varieties,which indicated a high temperature and drought at this period.High content of Pinus pollen suggested a large number of coniferous forests might be developed around Weizhou Island.It probably indicated a savanna landscape dominated by drought tolerant shrubs and herbs.Under the climate background of high temperature and drought,there was a high possibility of natural fire.We thus inferred that charcoal was mostly produced by local fires at this stage,and Weizhou Island basically maintained its original ecological appearance.（2）669～372 cal.a B.P.（1281～1578cal.A.D.）,the pollen concentration and the arboreal pollen type both increased,which indicated the vegetation on Weizhou Island was lush.It became a transitional type from savanna to north subtropical seasonal rain forest,and suggested a warm and humid climate during this period.The warm and humid environment made good preparations for human landing on the island in the later stage.The warm and humid climate and the increase of wood available for fire were conducive to human migration and settlement.（3）372～93 cal.a B.P.（1578～1857cal.A.D.）,this period was featured by the continuous increase of the cool-tolerant Rhododendron and the often appearance of the cold-resistant Anacardiaceae,which indicated a cold climate during this time.The common appearance of human-associated pollen such as Brassicaceae and Lactuca,and the rapid increase of the concentration of charcoal and the content of Poaceae,which inferred that human beings had carried out rice planting and other agricultural in Weizhou Island and built artificial ponds or lakes.Human beings had a certain impact on the original environment of the island,but the defensive power of the original vegetation made Weizhou Island still retain some of its original characteristics.（4）93 cal.a B.P.～now（since 1857cal.A.D.）,it roughly corresponded to the modern warm period,and the arboreal pollen decreased first and then increased sharply,and the increase of tropical plant Casuarina,which indicated that the climate became warmer during this period.During this period,Cactus has become the dominant plant on the island,and the combined vegetation of Cactus,Cinnamomum camphora,Melia azedarach,Casuarina equisetifolia and Acacia confusa became the unique vegetation landscape of Weizhou Island.In addition,the contents of charcoal and Poaceae rose sharply to the peak of the whole profile,and the median particle size was rapidly refined,suggesting strong human activities.The climate and environmental changes in Weizhou Island in recent 1500years recorded by pollen and charcoal reflected that during the warm period of the Middle Ages（900～700 cal.a B.P.）,Weizhou Island was hot and dry,and the sea level rose.During the Little Ice Age（500～350 cal.a B.P.）,the climate was warm and humid and decreased with the sea level.The climate and environment change in Weizhou Island is mainly controlled by solar radiation and affected by ENSO activities.The pollen in the wetland column samples WZD of Weizhou Island have a good corresponding relationship with the pollen in other parts of South China at the same time.In the Yuan and Ming Dynasties,it was reflected in the initial disturbance and destruction of the primitive environment of Weizhou Island by human beings,but the specific performance was not very strong.From the Qing Dynasty to modern times,due to the large increase of population,the slash and burn cultivation of primitive farming destroyed the original vegetation,which was reflected in the sharp increase of Gramineae pollen and Dicranopteris spores,the modern period was the most intense stage of human activities on Weizhou Island.