Spatial And Seasonal Variability Of Channel-bed Surficial Sediments From The Mekong Delta, Vietnam: Implications For Hydro-sedimentary Dynamic Processes

High-intensity human activities are changing or have changed the evolutional process of the deltas worldwide.Particularly,the sedimentary dynamics and geomorphic processes are extremely complex in the tide-dominated river mouth.It is thus necessary to explore the changing evolution of deltas in Anthroprocene.This study takes the tide-dominated Mekong Delta as a typical example.By investigating the temporal and spatial changes of the surficial sediments,this study explores the shifting function of the delta plain's distributary channels in the sediment transporting,trapping and associated threat under the impacts of human activities and the background of sea-level rise.Author analyzed grain size,magnetic susceptibility,and organic elemental and isotopic geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong-Co Chien-Cung Hau River during the flood season.By comparing the spatial differences and seasonal changes of above proxies,the sediment sources and implications for hydro-sediment dynamic processes were discussed.The main results are as follows.1)The surficial sediments of each river channel in both flood and dry seasons were characterised by coarse sediments in the upper reaches and fine sediments in the lower reaches.Such spatial distribution reflected the fluvial domination in the upper reaches and tidal domination in the lower reaches.2)The surficial sediments in the Co Chien-Cung Hau channel have high magnetic mineral content and a terrestrial organic carbon source in the flood season,indicating the deposition of suspended sediments supplied from the drainage basin,which should be induced by the saltwater wedge intruding into the distributary channel.In the dry season,the mud content decreased in the Cung Hau reach but increased in the Co Chien reach.The magnetic mineral contents in both sections were lower than in the flood season,reflecting dissolution of the the magnetic minerals during the early diagenesis.Low C/N ratios and less negative?¹³C values occurred in the Co Chien reach,indicating early diagenesis or an additional contribution from marine source.The above characteristics reflected the intense saltwater intrusion increased in the dry season,which caused the resuspension and landward transportation of muddy sediments from downstream and offshore reaches.3)The deposition of fine-grained sediments in Bassac channel mainly occurs in the dry season.Grain size analysis indicated that there were two reaches of muddy deposits,which were charaterised by different magnetic mineral contents,C/N ratios and?¹³C values.These parameters indicated that the muddy sediments in the D-A reach near the mouth were mainly resuspended and trapped by the estuarine circulation and tidal pumping;whilst the riverine sediments dominated in the Tr section,where the fluvial domination transited to tides domination.These above results indicated suspended sediment trapping in the delta plain channels in both flood and dry seasons.Author thus speculates an ongoing landward shift of depositional center in the Mekong Delta,which was mainly induced by dam construction and sand mining in the river basin and delta plain.With the global warming and accelerated sea-level rise,strengthened marine processes can be predicted,which calls for urgently further investigation of the sediment trapping in the delta plain channels to better assess the risk of delta erosion and flood disasters.