Study On Sedimentary Diatoms Of Late Holocene From Middle Offshore Sea Area Of Fujian Province

Micropaleontology-retrospective diatom is an important biological indicator of geologic environment change which has been widely used in the Meso-Cenozoic sediment studies to determine the evolution of ocean environment.The middle of offshore sea area of Fujian province locates in the west of Taiwan strait and can be influenced by the distribution of ocean and continent,geomorphiy,climate, tectogenesis,marine hydrology features and human activities.According to identification and analysis of diatoms from surface and core sediment samples in the research sea area,the relationships among diatom composition,distribution characters and the environment factors were illuminated.The effects of diatom distribution in respond to marine environment and marine environment change during Late Holocene were discussed in this paper.Above all,more knowledge has been got about the marine environmental evolution of studying area.The main study results are as follows:(1) 246 diatom species belonged to 59 genus(including variable species and derivative) were identified from 45 surface samples collected in the middle of offshore sea area(between Haitan Island and Chongwu peninsula).The variety of sedimentary diatom species is rich in this area.In diatom community,the quantities of euhalobic species,brackish water species and eurythermal species are very rich. Besides,it contains fresh water species,inshore species,pelagic species,coldwater species,temperate species and warm water species.(2) The diatom abundances of surface sediment samples have distinct discrepancy.On the whole,diatom abundances are very high in some sediment samples that contain small grain component such as clay and silt,as well in some samples which near to continent.The higher diatom abundances from the samples in bays mean the larger quantity of terrigenous nutrient substances import and influence of mankind's activities.However,the lower diatom abundance region appears in the east of Haitan Island,which indicates the upwelling limit to the diatom burrying. (3) In combination with the percentage compositions of dominant genus and species and the quantitative ecology analysis methods about Principal Component Analysis(PCA) and Correspondence Analysis(CA),the surface sedimentary diatoms can be categorized to 4 types and several subtypes.The property of the first type is that Melosira sulcata(Ehr.) Kutzing,Pyxidicula weyprechtii Grunow and Podosira stelliger(Bail.)Mann are the main species.In the second type,Melosira sulcata(Ehr.) Kutzing and Actinoptychus undulates(Bail.)Ralfs are the main body.In the third type, the Melosira sulcata(Ehr.) Kutzing,Thalassiosira excentrica(Ehr.) Cleve,and(or) Actinoptychus undulates(Bail.) Ralfs are the main populations.The fourth type is dominant by Melosira sulcata(Ehr.),Cyclotella stylorum Brightwell,and(or) Cyclotella striata(Kuetz.) Grunow.The principal factors that influence the sedimentary diatom in middle offshore area of Fujian province are as follows:(a) coastal low temperature water and corresponding brackish water,normal sea water environment,and(b) high temperature and salnitys' warm water environment.In other words,this area is extensively affected by coastal current and warm water. These two main factors are affected by several elements,including the distribution of ocean,and continent,geomorphy,climate,hydrologic features.(4) Some nonindigenous diatom species are found from surface sediment samples, such as Coscinodiscus africanus Janisch,Campylosira cymbelliformis(A.Schmidt) Grunow and Stephanopyxis turris(Grev.et Arnott) Ralfs,and so on.Campylosira cymbelliformis(A.Schmidt) Grunow was the first recorded species in China since 2004,and now could be found in 6 stations that reflect it properly has been adapted to the marine environment of this sea area.So we need to detect and analysis its influence for whole ecology system and whether it has potential threat to marine ecological environment.(5) 40 genus,116 diatom species(including variable species and derivative) are identified from ZK18 core in middle offshore sea area Fujian province.The dominant species contain Actinocyclus ehrenbergii Ralfs,Actinoptychus undulates(Bail.) Ralfs, Coscinodiscus argus Ehrenberg,Coscinodiscus blandus A.Schmidt,Coscinodiscus rothii(Ehr.) Grunow,Coscinodiscus radiatus Ehrenberg,Cyclotella striata(Kuetz.) Grunow,Cyclotella stylorum Brightwell,Melosira sulcata(Ehr.) Kutzing,Podosira stelliger(Bail.),Pyxidicula weyprechtii Grunow,Triceratium favus Ehrenberg,and so on.The diatom abundances change obviously and 4 diatoms assemble zones and several sub-zones could be classificated according to diatom abundance and dominant community in ZK18 core sediment samlpes.Combining grain size analysis,ZK18 core recorded the depositional environment in middle offshore area which has been through a series of environment changes.Regression occurred in the sediment layers of 250 to 100 centimeters(approximately during 600 to 1400AD),while the environment is similar to modern marine environment in the sediment layers upper of 100 centimeters.It reflects the marine environment underwent the neritic environment to near shore environment and transition to neritic environment.During the background of regression,sea level fluctuates complicated.(6) Basing on the data of earthquake,typhoon,climate changing and history records,some probable violently geological events,meteorological events occurred during last 1500 years have been recognized and the relationships between sedimentary characters and human activities have also been discussed in this paper.