Environmental Changes And Lagoons Evolution In Qilihai Area Of Hebei Province During The Late Holocene

Lagoons are sensitive to the interaction between marine and terrestrial environments,and their evolution are also closely related to climate and sea level changes.However,there are few high-resolution sedimentary records of the late Holocene environmental evolution of lagoons in the shallow sea areas of North China.Here,based on 11 AMS ¹⁴C dating,a high-precision chronological sequence was established.We present the results of a study of pollen,algae,geochemical elements,and grain-size of a late Holocene sediment core from Qilihai Lagoon,which is the largest lagoon in North China.In addition,the mean annual precipitation in Qilihai region also be reconstructed by using fossil pollen data,modern surface pollen data,and Weighted Average Partial Least Squares（WA-PLS）.This study aims to reveal the vegetation succession,climate change,evolution of lagoons,and the response of the Qilihai region to extreme climatic events during the 3800-1200 cal yr BP,and the results show that:（1）Vegetation and climate change during 3800-1200 cal yr BP can be divided into five stages according to the changes of pollen assemblage and quantitative reconstruction of precipitation.During 3800-2850 cal yr BP,the proportion of arboreal pollen is more than 75%,the forest coverage is relatively high,the overall proportion of coastal wetland vegetation is relatively low,and the precipitation is～610 mm,which is the second highest value in the study period,and the climate is relatively humid.During 2850-2200 cal yr BP,the proportion of arboreal pollen decreased slightly,and the proportion of terrestrial herbaceous pollen taxa dominated by Artemisia rose to～18%,indicating that the forest coverage decreased,annual precipitation decreased to 580 mm,and the climate became slightly drier than in the previous stage.During 2200-1800 cal yr BP,the proportion of tree pollen was the highest（～83%）,especially the percentage of Quercus pollen was the highest（～37%）,indicating that the forest coverage was the highest,and the annual precipitation was the highest（～620 mm）in the study period.During 1800-1450 cal yr BP,the Quercus pollen content decreased to the lowest（～14%）in the QLH-A core,and the pollen content of Chenopodiaceae,Cyperacea and other herbaceous plants increased significantly,indicating that the herbaceous vegetation expanded,the forest coverage decreased,the climate was the most cool and arid,and the precipitation was～550 mm.During 1450-1200 cal yr BP,the pollen content of trees increased slightly,indicating that the forest coverage increased and the climate changed to a humid direction.（2）QLH-A core has obvious response to both 2.8 ka（2800-2650cal yr BP）and Dark Age Cold Period（1800-1450 cal yr BP）.The main characteristics of the 2.8 ka event were the decrease of Quercus pollen content and precipitation the drying of climate.The DACP event was mainly characterized by the decrease of Quercus pollen content and the increase of herbaceous pollen content,and the climate became cool and arid.The formation of Qilihai Lagoon was closely related to this cold event.（3）The pollen,algae,geochemical elements,grain size and End-Member Modeling Analysis of QLH-A cores indicate the changes of sea level and the evolution of lagoons in Qilihai area.During 3800～1800 cal yr BP,the core site was located in a relatively open and shallow bay environment,and the sediments were mainly influenced by the sea.The sea level was relatively high.During 1800-1400 cal yr BP,the sediment sources were mainly imported from rivers,and the proportion of marine sediments decreased significantly,indicating the decline of sea level.Qilihai lagoon began to form,and the lake salinity was low.A large area of saline-alkali land and freshwater marsh appeared around the lagoon.During 1400-1200 cal yr BP,the proportion of provenance from the dune increased,indicating that the dune was pushed towards the lagoon,the lagoon became further encloused as result of incursion of sand dunes and the area of lagoon expanded,and marine influenced on the evolution of Qilihai Lagoon weakened.The lagoon basin rose,and halophytes and hygrophytes vegetation decreased.This study will provide new insights into the evolution of lagoons in northern China and the response of coastal wetland ecosystems to global change.