Cross-front Transport Of Fine Sediment In The Western North Yellow Sea And Its Sedimentary Effects

The front is an important mesoscale ocean phenomenon,which is widely distributed along the boundary current.It is the interface between adjacent water masses with significant difference in properties(such as temperature,salinity,density),which inhibits the exchange of materials between both sides.As a result,a large number of terrigenous materials are limited in the coastal area,forming many alongshore muddy deposits.Due to the influence of tide,monsoon,ocean current and other factors,the front does not exist stably.Especially on the synoptic scale,the rapid fluctuation of the front can create an opportunity for the offshore transport of the coastal material;Furthermore,climate change may also affect the flux of offshore sediment transport.Therefore,the mud patch outside the front is an ideal place for climate change research.In this study,the "source to sink" process of fine sediment from the Shandong Peninsula to the Liaodong Peninsula and its response to climate change were studied.The research steps are as follows: 1)reveal the spatial characteristics of the two muddy deposits in the North Yellow Sea by geophysical methods(shallow seismic profiling);2)quantitatively evaluate the contents of each source through analyzing the sedimentary facies,geochemical characteristics and chronological framework of boreholes;3)reveal the transport mechanism of fine sediments are transported from the coast of Shandong Peninsula to the coast of Liaodong Peninsula,through analyzing the circulation structure and wind field characteristics.Due to the wide distribution of the front in the continental shelf area,how the fine sediment could pass through the front is the key to the diffusion process.The following aspects are involved in this study.(1)Offshore cross-front sediment transport in the eastern coast of the Shandong Peninsula.Winter is the key period for sediment diffusion in the continental shelf,since sediments are heavily resuspended by winter storm.And the resuspended sediments with high concentration are remarkably restricted in the coastal zone of the Shandong Peninsula by the front located between coastal current and the Yellow Sea Warm Current(YSWC).Through analysis of wind field,sea surface level,and suspended sediment concentration,we found that the YSWC and the Lubei coastal current were both enhanced under winter storm,along with the quick oscillation of front and the formation of frontal waves.The frontal waves tend to break when its deformation intensified under persistent strong wind,which would lead to sediment exchange between two sides of the front.Plenty of sediment could cross the front and be transported offshore due to the winter storm is strong and could last through the whole winter-half year.Taking 2017 as example,the duration of strong wind(>10 level)could reach 56 days.When the winter storm relaxes,these escaping sediment would diffuse northward under barotropic flow,which is the main sediment source of the western North Yellow Sea mud.(2)Onshore cross-front sediment transport to the eastern coast of the Liaodong Peninsula in the North Yellow Sea.Constrained by the topography,the YSWC triggered by the winter storm presents the feature of upwelling when moving onshore.Thermal analysis revealed that upwelling could cross fronts around the Liaodong Peninsula and penetrate into the coastal areas,by which the Yellow River derived sediment could be transported to the Liaodong coastal mud.This speculation was confirmed by geochemical analysis for sediment source,and these transport channel could be shown in seismic profiles.Geochemical analysis showed that the Yellow River derived sediment account for 15.3%–31.2% in the Liaodong coastal mud,namely 0.55–1.13Mt/yr sediment are from the Yellow River,which account for 1.38–2.82% of sediment going through the southern channel of the Bohai Strait.In this sense,upwelling should be widespread in the shallow East China Sea induced by the winter storm,but its upwelling signal is easily disturbed by vertical mixing,and the widespread upwelling could account for the onshore-transport of fine sediment and the multi-sources feature of coastal mud patches in the East China Shelf.(3)Effect of the evolution of continental circulation on cross-front sediment diffusion.Variation of the YSWC keep in step with the Kuroshio Current,which could regulate flux of the Yellow River derived-sediment into coastal mud of the Liaodong Peninsula.A mechanism involving how variation of the continental circulation affect cross-front sediment transport in the North Yellow Sea was carried out.In the North Yellow Sea,the range of the coastal current is constrained by the enhancement of the YSWC,which accelerates the speed of the coastal current when it flows opposite to the YSWC,leading to more cross-front sediment transport and restricts the velocity of the coastal current when they flow in the same direction,causing higher accumulation rates in the coastal area.Besides,offshore diffusion of terrigenous sediment could provide nutrition for ecosystem of the open sea and it has ecological significance.