Diatoms And Their Palaeoenvironments From The Cores Of Xijiang Delta In China

Delta, which is interacted actively by ocean and land, respond sensitively for natural processes and human activities. So there are close connections between the environment evolvement of these areas and living space, living quality of human and social continuable development.In this paper, Basing on the data of diatom,grain size,mineral,trace element and ¹⁴C of the sediments of two core in Xijiang delta, subdividing and contrasting stratigraphic, we discussed the depositional environment evolvement of Xijiang delta since Pleistocene.Total of 168 species and variants belonging to 48 genera were identified, Among which there were 7 kinds of dominant species and 17 kinds of common species. Six bands of core ZXZ1 were classified based on diatom species property, and the primary diatom assemblages were Cyclotella stylorum Brightwell—Melasira sulcata(Ehr.) KUtzing—Actinocyclus ehrenbergii Rails, Melosira sulcata(Ehr.)KUtzing—Cyclotella stylorum Brightwell—Actinocyclus ehrenbergii Ralfs, Cyclotella stylorum Brightwell—Melosira sulcata(Ehr.)KUtzing—Surirella fluminensis Grunow, Cyclotella stylorum Brightwell—Coscinodiscus argus Ehrenberg-Coscinodiscus lineatus Ehrenberg—Melosira sulcata(Ehr.)KUtzing, Cyclotella stylorum Brightwell—Coscinodiscus lineatus Ehrenberg—Coscinodiscus rorhii(Ehr.)Grunow—Melosira sulcata(Ehr.)KUtzing. Cyclotella stylorum Brightwell—Actinocyclus ehrenbergii Ralfs; the assemblages characteristic of diatoms showed that the core ZXZ1 belonged to marine sedimentary environment.Five bands of core ZXZ2 were classified, 1he primary diatom assemblages were Cyclotella stylorum Brightwell—Melosira sulcata(Ehr.)KUtzing, Cyclotella stylorum Brightwell—Melosira sulcata(Ehr.)KUtzing, Cyclotella stylorum Brightwell—Melosira sulcata(Ehr.)KUtzing—Coscinodiscus lineatus Ehrenberg—Coscinodiscus argus Ehrenberg—Coscinodiscus rorhii(Ehr.)Grunow. The assemblages characteristic of diatoms showed that the under layer of the core ZXZ2 belonged to the land sedimentary environment, the upside belonged to marine sedimentary environment.According to grain size analysis, seven types of sediments, including sandy silt,silty sand, sandy silt, silty sand, sand, silty clay and clayey silt, were detected. The grain size of sediments in core ZXZ1 changed twice from thick to thin; the grain size in the under layer of core ZXZ2 changed by thin-thick-thin, the upside changed from thick to thin.Meanwhile 29 kinds of minerals were identified on the base of detrital mineral analysis. We found the matter of the study district mainly came from land.Trace element analysis showed that at under layer element enriched, and at upside element dispersed.Stratigraphic subdivision and contrast was studied according to the analytical results, We found that 0-11.35m(includingâ…£,â…¤,â…¥diatom band) of core ZXZ1 belonged to the Holocene sediment, 11.35-35.6m(includingâ… ,â…¡,â…¢diatom band) belonged to the late Pleistocene sediment; 0-9.9m(includingâ…¢,â…£,â…¤diatom band) belonged to the Holocene sediment, 9.9-33m(includingâ… ,â…¡diatom band) belonged to the late Pleistocene sediment. We discussed the sedimentation and environmental evolvement since Pleistocene; The study showed that the core ZXZ1 belonged to marine sedimentary environment at late Pleistocene, and the core ZXZ2 belonged to land sedimentary environment; two core were all in marine sedimentary environment at Holocene. The different sedimentary environment at late Pleistocene was mainly due to vertical motion of fault block.The grain size,mineral,trace element and ¹⁴C data were support by Guangzhou Marine Geological Survey, Ministry of Land and Resources.