Vegetation Change Of Qilihai Region During The Past 5000 Years And Responses To Climate And Sea Level Changes

Since the Mid-Late Holocene,the temperature has shown an overall downward trend,the sea level was close to the modern level,and climate and environmental changes were more closely related to human activities.This period of time provides an effective"geological similarity"reference for modern climate prediction and simulation.At the same time,in the context of global warming,rising sea levels and frequent extreme weather events,exploring the response of coastal wetland ecosystems to long-term climate change and human activities will help to further understand the background of modern climate and environment.At present,less attention has been paid to information about the response of lagoons to long-term climate change and the evolution of the ecological environment contained in wetlands in the northern region.Located in Changli County of Hebei Province,Qilihai Lagoon is the largest modern lagoon wetland in North China.As an important coastal ecological fragile area,its environmental changes are affected by the dual effects of sea level and climate change,and are very sensitive to sea-land interactions.In this study,we obtained three AMS¹⁴C dates and conducted measurements of sediment grain-sizeandsporopollen analyses of 70 sediment samples.Our aims were toexplores the effects of vegetation,climate change characteristics and pollen combinations in the study area over the past 5000 years on sea level fluctuations and the century-scale warm period in the past 2000 years.The results showed that:(1)During 4830?4080 cal yr B.P.,the forests was dominated by Pinus(51.1%)and Quercus(11.8%),its pollen contents was more than 66%and the pollen content of herbs was more than 30%,the average AP/NAP was 1.88 and the sediment grain size was relatively coarse,these characteristics all indicated that the vegetation was with relatively low coverage of Pinus and Quercus forests,and the climate was cool and dry,which corresponded to the cooling event during the 5000?4000 cal yr B.P..;during 4080?3080 cal yr B.P.,the pollen content of arbor(mostly higher than 75%)increased to the highest value in the study period,and the pollen content of Pinus(average of 57.8%)also showed the highest value,and the pollen content of Quercus(average of 13.6%)compared with the previous stage,the average pollen content of herb plants was 25.8%,which dropped to the lowest value,AP/NAP(average of 3)reached the maximum,the sand content rose to the highest value in the study section,and the decrease to the minimumof Typhaceae indicated that the coverage of trees in the forest has expanded significantly,the lagoon has expanded,the water level has risen,the climate was warmer and humid;during 3080?1880 cal yr B.P.,the content of tree pollen(mostly less than 70%)and Pinus pollen(average of 48.7%)were the lowest in this period,and the content of Quercus pollen(mostly higher than 11%)was higher than that of the previous stage.The pollen content of herbaceous plants(average of 35.3%)decreased to the highest value,AP/NAP(average of 1.82)dropped to the lowest value of the study period,the proportion of silt and clay content in the sediment increased,and the aquatic herb pollens dominated by Typhaceaereaching the peak of abundance indicated that the amount of precipitation has decreased,the lagoon has shrunk,the water level has dropped significantly from the previous stage,the forest coverage has decreased,the herbaceous plants have expanded,and the climate was cold and dry;during 1880?640 cal yr B.P.,tree pollen increased,broad-leaved tree pollen(average of 19.1%)and Quercus pollen(average of 15.7%)reached the highest value in this period,herbal pollen content decreased significantly,and AP/NAP value(average of 2.82)was higher than the previous period,with multiple obvious peaks.The sand content was the lowest.The Typhaceae pollen content has decreased significantly and also fluctuated,indicating that the water level of the lagoon has increased,the forest has expanded again.The coverage of non-arbor plants has declined,the overall climate characteristics tended to be warmer,the humidity has increased compared with the previous stage,and there were sub-level dry and wet fluctuations.The changes of grain size indicated the relative weakening of the influence of the ocean and the enhancement of the role of the river.(2)The changes in the sporo-pollen assemblage in the sediments of the Qilihai Lagoon reflected that the sea level on the west coast of the Bohai Sea has generally experienced four periods of fluctuations in the past 5000 years:During 4830?4080 cal yr B.P.,the sea level was relatively low during the regression period after the maximum transgression period of the Holocene;during 4080?3080 cal yr B.P.,the sea level has risen compared with the previous stage,corresponding to the relative sea level peak;3080?1880 cal yr B.P.was the period between two intermittent pauses in the overall regression process,and the relative sea level showed a downward trend;during the period of 1880?640 cal yr B.P.,the height has risen,which is consistent with the high sea level recorded in many places in the late Holocene.(3)The Sui-Tang Warm Period and the Medieval Warm Period have obvious manifestations in the environmental evolution sequence recorded in the Qilihai Lagoon.During the warm period of the Sui and Tang Dynasties,the climate was warm and rainy,but it was also mixed with a shorter cold period;the distinguishing feature of the medieval warm period was the decline in precipitation,the climate tended to dry obviously,and there were also warm and cold fluctuations.