Magnetic Properties Of Sediments In The Yangtze Estuary And Adjacent Inner Shelf Of The East China Sea And Their Environmental Implications

The Yangtze Estuary and neighboring inner shelf of the East China Sea,a typical river-dominated continental marginal sea in the world,is a key area for land-ocean interaction studies.Due to sediment discharge decline from the catchment in recent years,the deposition/erosion processes in the estuary delta and inner shelf affect not only the geomorphic changes,but also have an impact on the bio geochemical cycle of carbon.Thus the study of sediment composition and its spatial/temporal changes in this area has drawn great attentions.Considering the sensitivity of magnetic minerals to sedimentary environment,this paper studies the spatial distribution of magnetic properties of the sediments in the Yangtze subaqueous delta and adjacent inner shelf of the East China Sea and their influencing factors.The magnetic method is combined with granulometric analysis,210Pb and 137Cs radiometric dating,geochemical and electron microscopy examinations.The environmental implications of magnetic method for geomorphologic and carbon cycling studies are discussed.The main conclusions are as follows,1.Particle size compositon indicates that sediments in the Yangtze Estuary and adjacent inner shelf of the East China Sea consist of modern Yangtze-derived sediments and late Pleistocene relict sands.Coarse sediments(sand,silty sand and slit)mostly accumulate in the estuarine channels and shoals in water depth less than 10 m,whereas fine particles(clayey silt)are mainly deposited in the subaqueous delta with water depth shallower than 30 m,and the neighboring inner continental shelf of the East China Sea.2.Sediment cores in the northern part of the study area and part of the prodelta are too old to be detected by210 Pb and 137Cs methods.The spatial distribution of sedimentation rates indicates that the region shallower than 30 m isobath outside the South Passage is the modern depocenter of subaqueous delta.There is little modern Yangtze-derived sediments transported to the areas deeper than 50 m.In addition,210Pb and 137Cs radiometric dating methods are significantly affected by sedimentary environment.The frequent re-suspension and re-deposition processes of sediments due to complicated dynamics in the study area call a cautious use of its application.3.Magnetic minerals in sediments of the study area are mainly concentrated at<16 ?m and>63 ?m fractions.Particle size variations caused by hydrodynamic sorting is an important factor affecting magnetic properties of sediments.Three zonations of surface sediments are recognized duo to magnetic and particle size distributions,which reflect influence of modern fine particles,modern coarse particles,aged sediments exposed by erosion and relict sand,respectively.Spatial distribution of ?ARM/SIRM can indicate the path way of sediment dispersal,which increases with increased distance.Most of the Yangtze-derived sediments are transported southward and southeastward,with more sediments transported farther to the sea in summer due to stronger water and sediment discharge from the Yangtze River.Due to the influence of the southeast summer monsoon in summer,some fine particles are transported northeastward when leaving the river mouth.4.Magnetic minerals could be dissolved in post-depositional diagenesis,altering original magnetic information of sediments.The depth of ferruginous to sulphidic boundary(FSB)in diageneis is influenced by total organic carbon(TOC),decreasing with the increase of TOC content and activity.In the river-dominated continental marginal sea,the degree of sulfate reduction is relatively weak due to high sedimentation rate and refractory terrestrial organic carbon.Compared to the typical marine environment,the FSB deoth in the study area is much deeper,and increases with sedimentation rate.5.Sediments modified by reductive diagenesis show decreased ?ARM values as a result of reductive dissolution,which could be recognized in the plot of ?ARM vs.<16 ?m fraction.It enables the distinguishing between modern(<100 yr)and aged(>100 yr)sediments,and therefore to determine erosion/deposition states in the subaqueous delta.Based on the magnetic mineral diagenetic zonation,it is possible to qualitatively indicate sediment erosion,as well as sediment age,which is of great significance to the morphodynamic study.We propose that the ratio of SIRM/Fe can be potentially used to trace mobilized aged sediments in this environment.6.Erosion indicated by magnetic properties can partly explain the existence of aged organic carbon in surface sediments of the study area,which provides insights into the poorly understood carbon cycling in continental margins and associated biogeochemical processes.In addition,magnetic methods provides a straightforward approach to assess organic carbon decomposition pathways,with deeper ferruginous to sulfidic reduction boundary(FSB)depth reflecting relatively limited sulfidic reduction in river-dominated margins.