A Study On The Evolution Of Sedimentary Environment Since The Last Deglaciation In The Coastal Waters Of Zhejiang Province

Zhejiang Province is located on the southeast coast of China.Zhejiang Province is rich in marine resources.A deep understanding of the law of environmental evolution in Zhejiang coastal waters since the last deglaciation can provide a decision-making basis for the development and protection of marine resources in Zhejiang Province,which is of great significance to predict the future coastal zone environmental evolution and promote the harmonious development of human and nature.In this paper,high-resolution seismic profile and borehole sediment samples are used as materials.The seismic stratigraphic conditions are obtained by interpreting the shallow stratigraphic sections in the coastal waters of Zhejiang Province.The sediments of hole ECS-1301 were tested by AMS ¹⁴C dating,optically stimulated luminescence dating,particle size analysis and microfossil paleontology identification,and compared with hole ECS-1302 to study the evolution of sedimentary environment since the last deglaciation in the coastal waters of Zhejiang Province.The main conclusions are as follows:1.The seismic sequence revealed by the high-resolution shallow stratigraphic section in the coastal waters of Zhejiang Province shows four main seismic reflection interfaces?T1^T4 from top to bottom?,and the seismic units between these interfaces are named SU1,SU2,SU3,SU4 and SU5 from top to bottom,among which SU1,SU2?It can be further divided into SU2-1 and SU2-2?and SU3 correspond to two drill cores in this area respectively Du1,DU2?It can be further divided into DU2-1 and DU2-2?and DU3.2.The formation time and sedimentary environment of each sedimentary unit are as follows:?1?DU3 was formed in the early stage of MIS 2^MIS 1.During this period,ECS-1302experienced a transition from river to estuary environment,while ECS-1301 was dominated by river environment.?2?DU2 was formed in the early to middle Holocene of MIS 1.The ECS-1302 hole experienced the change of the tidal flat to the coastal environment affected by the tide,and then to the remote environment of the tidal sand ridge/sheet in the inland shelf of the East China Sea.The ECS-1301 borehole is interpreted as a coastal shelf environment,which gradually evolved from a tidal flat shallow water environment affected by the tide.The thin layer of fine sand at the top of the two boreholes in DU2 represents condensed section.?3?DU1 was formed in the middle Holocene?about 7.7 cal kyr BP?up to now,corresponding to the wedge-shaped muddy sedimentary body developed in the coastal area of Zhejiang Province.3.Since the last deglaciation,the change of deposition rate of Zhejiang coastal sediments is mainly controlled by location,climate,sea level change and the development of the Yangtze River Delta?the filling of the Yangtze River estuary?.The deposition of DU3 river is mainly affected by location factors.The high deposition rate of DU2 sedimentary unit is accompanied by the rise of sea level,which may reflect the influence of climate factors.In the middle Holocene,the sedimentation rate decreased obviously,which may be controlled by the material of the Yangtze River because the borehole is far away from the near source river.However,the deposition rate of² cal ka BP increased again,which may be mainly due to the fact that the estuary of the Yangtze River was basically filled,and a large number of sediments from the Yangtze River into the sea were transported to the coast of Zhejiang and Fujian by the current.4.According to the grain size standard deviation curve since Holocene high sea level in the upper part of ECS-1301 hole,three sensitive grain size intervals are identified,which are called sensitive grain size 1?<4.125??,sensitive grain size 2?4.125-6.125??and sensitive grain size 3?>6.125??.Sensitive grain size 2 fluctuates strongly in the core and controls the change of average grain size.It is a sensitive grain size component reflecting the change of environment.The change trend reflects the evolution of East Asian Winter Monsoon in the coastal waters of Zhejiang Province.