Characteristics And Sediment Dynamical Factors Of²¹⁰Pb Vertical Profiles Of Surficial Sediments Over The Changjiang Subaqueous Delta

The coastal waters adjacent to the Changjiang estuary is characterized by abundant supply of fine-grained sediments and active hydrodynamic effects. The growth of the Changjiang Delta is constrained by a number of factors such as sea-level rise, the original topography, sediment supply, sediment retention in the estuary, ground subsidence and large water conservancy and hydropower projects (e.g., the Three Gorges Dam). Hence, the sedimentation sequence of the Changjiang subaqueous delta in response to physical processes (i.e., changes in material source, migration of tidal channels, storm events, etc.) and anthropogenic activities (i.e., dredging of navigational channels) have become important scientific problems.Sediment cores were obtained from31stations over the Changjiang subaqueous delta, during April13to April27,2009, by utilizing a gravity corer. We took sub-samples from the cores in2cm intervals. Grain size distribution was derived for2468sub-samples.The surficial sediments over the Changjiang subaqueous delta are mainly composed of mud (mean grain size>4Φ), with only two exceptions (Stations CJ03and CJ09) mainly having very fine sand (3Φ<mean grain size<4Φ) on the surface of seabed.The grain size parameters indicate that the material of the Changjiang subaqueous delta is mainly from modern river input, with a few stations where sandy deposits that are indicative of submarine erosion were present. The vertical variations of grain size parameters show4patterns:(a) fining upwards;(b) uniform;(c) sectional or cyclical vertical variations; and (d) coarsening upwards. Core samples with different patterns of sediments grain size profiles located at different topographical region with various erosion or siltation dynamics. The variations in characteristics of sediment parameters are affected mainly by3factors:the material source; the sedimentation processes, such as the hydrodynamics at the deposition localities; and the characteristics of input suspended sediments.The sediment cores, which had the coarsening upwards pattern, were located mainly at the entrance of the South Passage, and the dredging channel associated with the North Passage. The sediment cores, which had the fining upwards pattern or had uniform grain size profiles, were obtained relatively far away from the river mouth within the study area. Besides, the sediment cores obtained at the dredging channel associated with the North Passage, and off the entrance of the North Channel are very poorly sorted (2<Sorting<4). These phenomena indicate stronger depositional hydrodynamic force at the mouth of the South Branch than that at the mouth of the North Branch of the Changjiang Estuary. The mean grain size of sediment cores obtained at Station CJ09is significantly coarser than other stations. Besides, at Station CJ10, no sediment core sample could be obtained by using the gravity corer. This indicates that the sediment at Station CJ10is consolidated sand, which is very coarse. The existence of extreme coarse sediments presented at stations near the mouth of the South Branch is caused by the human activities, such as sand-pumping and projects of deep water channel.210Pb activities of sediments were determined for511sub-samples, in4～10cm intervals, taken from the30sediment cores of the Changjiang sub-aqueous delta. According to the features shown in total210Pb activity down-core profiles and the regression analysis, the vertical distribution patterns of210Pb activities (Figure5.12) is classified as (1) normal210Pbex decay profile, and abnormal210Pbex regressing profiles such as:(2) unique total210Pb activity profile;(3) sectional210Pbex decay profile; and (4) sectional210Pbex decay and inverse210Pbex decay profile (topsy-turvy210Pb activity profile). Stations out of the South Branch usually have the210Pbex regressing profiles with vertical distribution Patterns3or4. The normal decay profile of210Pb activities only occupies a very small part of all sediment cores. This indicates that the complex source/sink processes and sedimentary process has destroyed the normal sedimentary processes of210Pb burial. And thus make the210Pb-dating very difficult over the study area. The210Pb profiles and their chronological information can only be interpreted or understood after a well picture of sedimentary processes. The variations in the distribution of210Pb activities are affected mainly by3factors:the existence of either terrestrial or marine new sediments deposition; the existence of erosion effect; and the existence of sediment transportation within the Changjiang subaqueous delta.The variations in the distribution of210Pb activities are affected mainly by3factors:the existence of either terrestrial or marine new sediments deposition; the existence of erosion effect; and the existence of sediment transportation within the Changjiang subaqueous delta. Sediment cores with relatively unique total210Pb activity profile (Pattern2) could indicate rapid sedimentation induced by thick layer of flood or storm surge deposits, or thick layer consolidation of fluid mud, or hyperpycnal flow; or no fresh deposit within100years. The Mass Transport Deposit processes (MTDs) can induce sectional decay profile (Pattern3) of210Pb activities. The unique210Pb activity profile could be a result of thick layer of fluid mud transport, or hyperpycnal flow, or no fresh deposit within100years. The sectional210Pbex decay and inverse210Pbex decay profile (topsy-turvy210Pb activity profile:Pattern4) indicate sediments come from different adjacent areas and through two processes, i.e. MTDs and continuous gravity currents in erosion area (transportation of eroded deposits).The second pattern outlined above might denote a contradictory condition:if the total210Pb activity is close to the supported210Pb activity of its adjacent stations, then the uniform total210Pb activity profile should indicate a sedimentation rate of zero; in contrast, if the total210Pb activity is much higher than the supported210Pb activity of its adjacent stations, such a profile might represent an extremely high sedimentation rate, too high to be calculated from the present short sediment core samples. In this study, the sediment samples with uniform total210Pb activities all belonged to the former condition. The third pattern outlined above might denote the sedimentary processes known as Mass Transport Deposits (MTDs). The sectional210Pbex decay and inverse210Pbex decay profile (topsy-turvy210Pb activity profile:Pattern4) includes episodes similar to Pattern3(sectional210Pbex decay profile), and episodes inverse of the normal210Pbex decay profile. In summary, the forth pattern outlined above might denote MTDs and transportation of an eroded deposit.The activities of210Pbtotai near the mouth of the South Branch, where is also the location of the estuary turbidity maximum, were higher than their adjacent stations to the west of122.2°E, i.e., the sediments near the mouth of the South Branch are younger. The activities of210Pbtotai suggest also newer deposits at the south side of the jetties than at their north side. It is also a proof of the fact that the south jetty of the deep-water channel project at the North Passage of the South Channel downstream the South Branch of the Changjiang Estuary has blocked the mass exchange between the Jiuduansha area and the North Passage, and trap the sediments at the south side of the jetties.As shown by the spatial distribution of the4types of210Pb activity profiles, at majority of the stations the sediment accumulation was not continuous. The various sedimentary records represent complicated responses of the Changjiang subaqueous delta to a number of factors such as sediment dynamic processes and dam constructions in the catchment. The distributions of the "apparent" sedimentation rates over the study area are different for different accumulation sections. This indicates that the spatio-temporal sedimentation processes of the Changjiang subaqueous delta might be affected by varying catchment’s circumstances. These catchment’s changes may include the locations of the turbidity maximum, and the depocenter; the mass flux; the material source; the anthropogenic activities such as dredging projects; and even extreme events.The spatial distributions of grain size and210Pb characteristics of the Changjiang subaqueous delta indicate the influences including changes in material source and hydrodynamics, erosion and reaccumulation of sediment over the adjacent areas, spatial variations in hydrodynamics, and event deposits.Tidal cycle measurements at4stations over the Changjiang River mouth have presented intense sediment resuspension within the bottom boundary layer over the Changjiang Estuary. The rapid increase in suspended sediment concentration might generate hyperpycnal flow over the Changjiang Estuary.With a continually reducing annual sediment load from the Changjiang River; a relatively stable annual water discharge; and enhanced sediment-trap abilities in the inter-tidal S. alterniflora saltmarshes; the future sediment retention at the Changjiang Estuary cannot remain the same magnitude as several decades ago. Under reduced sediment retention, the modern subaqueous delta of the Changjiang Estuary might suffer a switch between accumulation and erosion.