Characteristics And Mechanisms For The Formation Of Shelf Mud Deposits In The Northwestern South China Sea

Continental shelf mud deposits are characterized by high tremporal continuity and, therefore, represent appropriate sedimentary records to reveal climate and environmental changes. Thus, they provide important materials for the research of geologists, sedimentologists and biogeochemists. In China, the Huanghe and Changjiang rivers both have a delta at the river mouth and a distal shelf clinoform. Detailed studies have been carried out for these two sedimentary systems. The Pearl River also has distal mud deposit, located on the northwestern shelf of the South China Sea (SCS), but its depositional characteristics are still poorly understood. During the present study, seabed sediment samples were collected in2005～2010and tidal cycle measurements were carried at two stations; on such a basis, the distribution pattern of surficial sediments, grain size trends in relation to residual sediment transport, the environmentally sensitive grain-size component and the deposition rates are obtained, and the suspended load and bedload transport rates are calculated using the data sets of tidal currents and suspended sediment concentrations.The characteristics of sediment distribution are as follows.(1) The SCS northwestern continental shelf is covered with modern muddy and sandy sediments. Muddy sediments are extensively distributed, occupying around30%of the study area. The grain size composition of the sediment is similar to that from the Pearl and Moyang rivers. Such a similarity is also presented in the environmentally sensitive grain-size component between the depositional and the source areas; mean grain sizes associated with the relatively high standard deviations fall in a range of4.6～15.6μm (silt) and74.3～210.2μm (fine sand).(2) The strata of the study area consist of four seismic facies, i.e., U1, U2, U3and U4. The U1unit has an average inclination of less than0.01°, with the thickness variing from2.9to11.6m (mostly3～5m). The topmost core samples show that U1generally possess a fining-upward trend. The sedimentary structures of the cores are characterized by by interbeded mud and sand layers.In terms of the hydrodynamics and sediment transport, the findings of the present study are as follows.(1) The tide is characterized by standing waves, with rotary currents. The tidal current speed shows a trend of decrease towards the land. In addition, the difference between the east-west and north-south components of the current velocity decreases seaward. For example, the east-west component at C01is not more than0.4m/s, which is larger than the north-south component, whilst these two components at C02have the same order of magnitude.(2) The suspended sediment transport rate, averaged over the tidal cycle, is767.45kg/m, towards91°at CO1and704.43kg/m with towards1°at C02. Under normal conditions, the bedload transport rate at C01and C02are2.83kg/m towards the east and2.64kg/m towards the SWW, respectively. Further, the stratification of sea water temperature and salinity at both stations and the profiles of suspended sediment concentration (SSC) suggest that under normal conditions and over the area with lower SSCs, e.g., the northwestern SCS shelf, sediment transport occurs mainly in the nearbottom water column or bottom boundary layer.(3) Results of surficial sediment grain-size trend analysis show that the surficial sediments in the vicinity of the Pearl river mouth move towards the south or the southwest, those near the shoreline are uniformly transported seawards and those in offshore waters move mainly landward (i.e., towards the northwester). However, sediment transport trends between the above mentioned two areas are relatively complex. Generally, surficial sediments over the northwestern SCS continental shelf seem to converge to such areas as those adjacent to the Pearl river mouth with a bathymetry of30-50m, in the vicinity of Shangchuan and Xiachuan Islands as well as its southwestern stripe extending NE-SW, in the coastal waters to the south of Shuidong Port, and in Guangzhou Gulf with a bathymetry of20-30m.The deposition rate was0.12～0.21mm/a at a tidal cycle scale (based on sediment transport rate measurements) and1.27-5.66mm/a on a102a scale (based on210Pb dating). Assuming that the deposition rate remains the same over102to103a temporal scales, U1should be formed mainly in the later Holocene since5～3ka B.P., meanwhile the uppermost mud strata should be formed within the last200years or so. This estimate is consistent with previous studies. The strata U1has a maximum resolution of10°～101a, i.e., the Pearl river-derived distal mud deposits represent an appropriate sedimentary record for the study of the history of the Holocene environmental changes.Based on the deposition rates from210Pb dating and sediment bulk density, the contribution of the Pearl river to its distal mud deposit was estimated. The sediment discharge of the Pearl river to its newly-formed (ca.102a) distal mud deposits is0.62～4.5×107t/a, which accounts for7%-50%of the average annual sediment flux (i.e.,around8.87×107t/a), most likely11%～28%. Notably, due to the dam construction within the Pearl catchment, the future contribution of the Pearl river sediment supply will be reduced. The mechanism of suspended sediment transport is the advection via hypopycnal plume driven by the wind-forced coastal currents and the offshore diffusion of the hypopycnal plume driven by vertical shelf circulation (i.e., downwelling in winter and upwelling in summer); sediment gravity flow might play an important role in the early-stage infilling of the paleovalley in the middle-early Holocene periods.