Sedimentary Record Of Diatom Assemblages In The Bohai Sea And Their Palaeoceanographic Significance

Diatom is an important single-cell plant in the ocean.The cell wall is rich in silicon and sensitive to environmental changes.Siliceous algae fossils in sediments record the interdecadal paleoenvironmental information and are important methods for studying paleoenvironmental changes.At present,the research on long-term changes of diatom fossils is mostly applied in lakes and foreign seas.There are few reports on the research on the coastal areas of China's coastal shelf areas,especially the Bohai Sea.As a deep sea in the mainland of China,the Bohai Sea has a strong closed water.With the development of economy and industry,the phenomenon of eutrophication has occurred frequently in recent decades,which has seriously affected China's marine environment.On the other hand,due to the late start of marine research in China,the environmental monitoring of the Bohai Sea lacks continuous historical data,and historical data cannot be obtained to assess changes in the marine environment.Therefore,the use of sedimentary diatoms to study changes in the marine environment to make up for the lack of relevant research.In this paper,columnar sediments from three typical regions near the Bohai Sea and the Bohai Strait were selected as the research objects.The deposition rate of sediments and long-term changes of BSi were measured.Species identification and abundance statistics for siliceous algae fossils buried in sediment for more than 200years.The long-term variation of diatom fossils was further analyzed,and the possible causes of the changes in distribution were discussed.In order to provide basic data and theoretical basis for the long-term recording of diatom fossils and the study of eutrophication changes in the Bohai Sea.The main findings are as follows:A total of 35 species of siliceous algae were found in the analysis of sedimentary column samples from three survey sites in the Bohai Sea and Bohai Strait.Among them,Cyclotella stylorum?Cyclotella striata?Paralia sulcata?Dictyocha fibula?Actinocyclus ehrenbergii?Actinoptychus senarius?Coscinodiscus argus?Coscinodiscus radiatus?Coscinodiscus oculus-iridis?Coscinodiscus excentricus?Coscinodiscus oculatus?Coscinodiscus lineatus and Diploneis bombus are the main species of algae found in all three stations.The main algae species at different stations are close,but the absolute abundance of diatoms at each station is not the same.The BH6 station along the Laizhou Bay has the lowest average abundance of all species in the entire sedimentary column,only 0.86×10~4cells/g,and the abundance range is 0.08-1.91×10~4 cells/g;The average abundance of BH2 stations in central Bohai reached 1.17×10~4/g,and the abundance ranged from 0.08-5.73×10~4/g;The 5694 station in the northern part of the Yellow Sea near the Bohai Strait has the highest average abundance of 1.92×10~4/g,and the abundance range is 0.19×10~4/g-6.59×10~4/g.The dating results show that the deposition rate of BH6 station is the highest,0.35 cm/a,the age range is 1760-2015;the deposition rate of BH2 is 0.21 cm/a,the age range is 906-2015;the deposition rate of 5694 station is the lowest.It is 0.13 cm/a and the age range is 1710-2017.The results of BSi measurement are as follows:the average burial flux of BSi in BH6station is 25.15 g/m~2a,the range is 11.17-34.02 g/m~2a,which is the highest in three stations;the average burial flux of BSi in BH2 station is average The value is 17.1g/m~2a,and the variation range is 10.95-25.19 g/m~2a;the average BSi burial flux at5694 is the lowest of the three stations,5.04 g/m~2a,and the range is 1.86-9.11 g/m~2a.The diatom community structure in different areas of the three stations in Bohai Sea is similar,and Cyclotella stylorum and Coscinodiscus have advantages in absolute abundance and relative abundance,and are the main dominant species in all survey sites.However,the interdecadal trends of diatom abundance in different regions vary.The sedimentation of diatoms in the deep station of BH6 station is less likely due to the fact that the station is not conducive to diatom deposition in the sedimentary environment,and deep diatoms cannot be well preserved.The increase in diatom abundance after 1950 is due to the Yellow River in recent decades.The flow and the reduction of sediment are conducive to the breeding and survival of diatoms.The diatom abundance of the BH2 station began to rise and peaked around1850,probably due to the diversion of the Yellow Sea into the Bohai Sea,which brought about the nutrient salt,which was conducive to the growth of diatoms,and then slowly decreased due to the occurrence of red tides in the Bohai Sea.Caused a reduction in the variety of diatoms.The overall abundance of the 5694 station fluctuated less,and the overall abundance increased after the Yellow River was diverted.By comparing the burial flux of diatoms and the burial flux of bio-silicon in different stations,it is found that the burial flux of bio-silicon in BH6 station is higher and the diatom abundance is lower because BH6 station is located near the Yellow River estuary,sedimentary environment.Not conducive to siliceous shell deposition.Therefore,the diversion of the Yellow River estuary and the changes of the Yellow River runoff and the eutrophication of the Bohai Sea may be the cause of the inconsistent long-term changes in diatoms in different areas of the Bohai Sea.