Sedimentary Records Of The Burial Of Biogenic Elements And Environmental Changes In Nearshore Marine Areas Affected By Climate Change And Anthropogenic Activities

The nearshore marine area is a link between land and ocean,an important interface between land and water ecosystem interactions,and plays an important role in global biogeochemical cycling processes.However,anthropogenic activities and global climate change have had a profound impact on the ecological environment and the circulation of marine biogenic elements in the nearshore marine area.Due to the development of agriculture and industry and the continuous expansion of urbanization,the intense anthropogenic activities in coastal areas have impacted the ecological environment of the nearshore marine.Nutrients transported from land to coastal waters are increasing,causing eutrophication of water bodies and changes in nutrient structure.Climate change directly causes changes in meteorological factors such as temperature and precipitation,which in turn affect the physical,chemical,and biological aspects of the water environment in watersheds and coastal waters to varying degrees,thereby causing changes in the sedimentary environment,nutrient status of the water column,and primary productivity levels in offshore waters.How the burial fluxes of biogenic elements in the nearshore marine area change,and how the changing burial fluxes and environmental changes respond to anthropogenic activities and global climate change need to be further explored.In this study,sediment cores from the Arctic Krossfjorden,the Zhejiang-Fujian coastal mud area,and the nearshore area of the Pearl River Estuary,representing the Arctic region,the upwelling area,and the river delta system,were selected to reconstruct high-resolution sedimentary records and establish chronological sequences.Analyze the vertical distribution of the indicators of biogenic elements(C,N,P and Si)in each sediment core.Specific research objectives are as follows:(1)Exploring the historical changes of environmental information such as sedimentary environment,aquatic nutrient level,and primary productivity in the study area on acentury-scale.(2)To evaluate the change of burial fluxes of biogenic elements in coastal areas under the influence of climate change and anthropogenic activities,so as to understand the impact of climate change and anthropogenic activities on nutrient cycling.The following conclusions were obtained:(1)The analysis of biogenic elements and sedimentary environments in the sediment core of Krossfjord B5 indicates that this region is an area of environmental change dominated by natural climate change.Sedimentation rates in Krossfjord have increased twice as much since the 1990 s as before the 1990 s.The phenomenon of sediment grain size coarsening and the increase of TOC,TN,BSi and OP contents may be related to the increase in primary productivity level due to the temperature increase and rapid melting of sea ice caused by the recent climate change.(2)The analysis of the biogenic elements and depositional environment in the sediment core A11-1 of the coastal mud zone of the Zhejiang-Fujian shows that the area is a region where environmental changes are dominated by both climate change and anthropogenic activities.The distribution of the biogenic elements was stable before the early 1990 s.The changes of the biogenic elements were mainly dominated by climate change or natural processes at this time.Since the early 2000 s,there was a transient decrease trend of TOC,TN,BSi and TP contents,which may be related to the interception of land-sourced materials in the dam.The ecological environment changed during this period,and the influence of anthropogenic activities on biogenic elements became increasingly significant.After the early 2010 s,the contents of TOC,TN,BSi,TP,and OP showed a significant upward trend.This is the result of the combined effects of climate change and anthropogenic activities.(3)The analysis of the biogenic elements and sedimentary environment in the sediment core ZJ-3 in the nearshore area of the Pearl River Estuary indicates that the area is a region where environmental changes are dominated by anthropogenic activities.Since the early 21 st century,the increase of TOC,TN and BSi in this region is related to the intense human activities in the Pearl River Basin and the Pearl River Estuary region.With the population growth and accelerated urbanization,the Pearl River estuary region has imported excessive N to the near-shore area and the eutrophication problem has become increasingly serious.(4)The estimation of buried fluxes of biogenic elements in columnar sediments shows that the burial of biogenic elements in Krossfjord,the coastal mud zone of the Zhejiang-Fujian and the nearshore area of Pearl River Estuary all show different degrees of increase,but the leading factors affecting the increase are slightly different.Krossfjord shows the greatest growth,and the burial of biogenic elements after 1990 s is about 2-4 times of that before 1990 s.In addition,the burial fluxes of biogenic elements in the nearshore area of the Pearl River Estuary ranked first among the three cores.For example,the maximum burial flux of organic carbon in the nearshore area of the Pearl River Estuary reached 2.5 and 5.8 times that of the Zhejiang-Fujian coastal mud area and the Krossfjord,which may be related to the higher sedimentation rate and the content of biogenic elements in the area.