LATE HOLECENE ENVIRONMENTAL EVOLUTION CHARACTERISTICS OF THE NANDAGANG WETLAND IN NORTHEAST OF CANGZHOU,HEBEI PROVINCE
Coastal wetland is an ecological fragile zone sensitive to environmental change under the background of global change.Revealing the evolution law of coastal wetlands under the influence of long-term climate change and human activities is helpful to determine the threshold of ecosystem imbalance and realize the protection and sustainable utilization of coastal wetlands.Based on the proxy changes of pollen,algae,grain size,geochemical elements and oxygen isotopes,this study employed 8 AMS 14C dating data and revealed the environmental evolution of the Nandagang wetland in northeast of Cangzhou,Hebei Province during the past 4000 years and its driving factors.The samples were taken from the 400～64 cm depth(3850～0 cal.a B.P.)of the NDG core(38°32′18.6″N,117°27′10.9″E)with a total length of 39 m,which was located in the northeastern part of Cangzhou City on the western side of Bohai Bay.This composite coastal wetland is a land-sea junction zone formed by marine encroachment and fluvial siltation.The lithology consists mainly of clayey silt with minor amounts of clay and sand.The results showed that:(1)During the period of 3850～3570 cal.a B.P.in Nandagang wetland,EM3(Marine deposition)was the main end member,dinoflagellates concentration was high,the contents of Cl and Sr/Ca were the highest in the study section,and the contents of Ti and K were low,indicating that the sedimentary environment was mainly influenced by the ocean and was an open shallow sea environment.During the period from 3570 cal.a B.P.to 3450 cal.a B.P.,the end member became EM1(river deposition),dinoflagellates concentration was decreased,the content of Cl and Sr/Ca was decreased,the content of Ti and K was increased,and the terrigenous sediment obviously ascended,and the Marine influence was weakened.This stage should be a lagoon environment.Since 3450 cal.a B.P.,the end member of EM1(fluvial sediment)decreased,EM2(aeolian or proximate sediment)increased,and the concentration of dinoflagellates decreased significantly,indicating the demise of the lagoon and the formation of coastal wetlands.(2)The characteristics of pollen assemblages in core drilling in Nandagang Wetland show that regional vegetation showed significant differences before and after 3500 cal.a B.P.Before 3500 cal.a B.P.,the vegetation in the region was dominated by woody plants such as Pinus and Quercus,but after 3500 cal.a B.P.,the woody plants greatly decreased,and the herb of Chenopodiaceae and Poaceae became dominant.(3)The decrease of arboreal pollen after 3500 cal.a B.P.was mainly related to the regression and weak monsoon events.The root cause may be the decrease of global solar radiation during this period,the weakening of East Asian summer monsoon,and the development of cold and dry climate,which led to the decrease of vegetation coverage in the western area of Bohai Bay,the intensification of soil erosion,the enhancement of river sediment transport capacity,and the accumulation of large amounts of sediment in the coast.The relative sea level decreased,and the Nandagang wetland was formed.
Late HoloceneNandagang wetlandsedimentary environmentpollen assemblages3.5 cal.a B.P.weak monsoon event
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