Dynamics Of The Bottom Turbid Layers And Its Deposition Effect In The Changjiang Distal Mud Region

Bottom turbid layer is a common form of sediment gravity flow.Under specific hydrodynamic conditions,large amount of sediment can be resuspended into seawater from the floor,leading to the formation of bottom turbid layer.Then,bottom turbid layer can move downward along the slope by gravity.According to previous studies,on the shelf,sediment gravity flow of this form may be widespread.Inferred from its process and mechanism,bottom turbid layer can emerge in distal mud deposit regions,and it may play an impottant role in the formation of distal mud deposits.The main objectives of this research is analyzing wether it is suitable for the formation of bottom turbid layer in Changjiang distal mud deposit region,and probing into its deposition effects.Results of field measurements show that,during the observation period in spring,the hydrodynamic conditions were relatively intensive.The winds were strong with a northeaster direction mainly,while the waves were nearly the east direction,and the mean significant wave height was 1.17 m.Tidal currents were characterized by reversing currents with a NW-SE direction;the mean tidal current velocity ranged from 0.35 to 0.42 m · s-1 during spring tide,approximately 0.21～0.22 m · s-1 during neap tide.The residual currents velocity was 3.5～6.1 m · s-1,with directions ranging from 11°to 85°.The bottom water had higher temperature and salinity than the upper water layers,and it showed the presence of thermohaline front.The vertical average suspended sediment concentration was 35～67 mg·L-1;higher concentration appeared during spring tide or under stronger wave process.The grain size of suspended sediment was larger than that of the surface sediment,and this may be the result of flocculation process.Flocs can settle much more rapidly than the suspended particles which forming flocs,and setting velocities could reach 1 ×10-3～4×10-3m·s-1。The average suspended sediment concentration through the bottom turbid layers was approximately 100～400 mg · L-1,and the thickness of bottom turbid layers was 3～8 m.Tidal currents led resuspension:occurrence frequency of resuspension was 51～74%and 33～36%for spring and neap tide conditions relatively.The strong resuspension process maintained the high suspended sediment concentration of bottom turbid layers.The gravity driven bottom turbid layers movement mainly depends on three factors:shelf slope,thickness and suspended sediment concentration of bottom turbid layers.In the Changjiang distal mud region,velocity of bottom turbid layers movement is 1.9～2.5 cm ·s-1 during neap tide period,and 2.6～3.9 cm·s-1 during spring tide period.The average transport flux of bottom turbid layers movement is 1.73～15.40 g·m-2·s-1,and the transport flux during a full tidal cycle(25 h)by bottom turbid layers movement can reach 13～34%of the tidal currents transport.Furthermore,under extreme weather conditions,stronger hydrodynamic can increase bottom shear stress greatly.Then,stronger resuspension process can enlarge the suspended sediment concentration and broaden the thickness of bottom turbid layers.Thereby,the transport flux under extreme weather conditions could increase by more than four times than normal conditions.The movement of bottom turbid layers leads to the formation of parallel clinoforms,and this effect emerge gradually from the middle-later stages of distal mud,and become more and more important with the development of distal mud.According to seismic profiles,clinoforms in Changjiang distal mud are dominated by the convergence type,only in some places the parallel ones have started to grow.While in distal mud of Yellow River,parallel clinoforms are more developed,and in distal mud of Pearl River,parallel clinoforms do not come into being yet.So,distal muds of Pearl River Changjiang River and Yellow River represent early stage,developing stage and mature stage for evolution stages of distal mud respectively.Settling suspended matter is an important carrier for the transportation of carbon in the ocean,while suspended matter is most concentrated in the bottom turbid layers.Transportation of POC by bottom turbid layers movement cause a 1.6×103 kg·m-1·yr-1 POC transportation flux.On the other hand,the relative strength of resuspension and settling processes decides whether suspended matter can settle down,and whether POC can transfer to the sea floor.Therefore,the dynamic processes of the bottom turbid layers,such as settling,resuspension and lateral movement,have great importance on the sequestration and across-shelf transportation of Organic carbon.