| Based on the grain size,detrital minerals and magnetic characteristics analyses of the surface sediments and sediment cores collected in the East China Sea continental shelf,the source to sink processes were studied.In this paper,the deposition process in the modern East China Sea continental shelf,the effects of human activities and climate disasters such as floods and typhoon on the sediment deposition and the sediment records of the climate and the East Asian Winter Monsoon changes during the past 1.5 ka have been discussed.Surficial sediment grain size distribution in the East China Sea continental shelf presents apparent spatial characteristics and seasonal variations.Mud area presented more variations in grain size than sand one.This phenomenon indicates that sediment supply is the main controlling factor of the surficial sediment grain size distribution on the East China Sea continental shelf.A total of 49 species of heavy minerals were distributed in the research region,dominated by hornblende and followed by epidote,schistose mineral,dolomite,garnet,magnetite and so on.Light minerals mainly consist of feldspar followed by quartz;schistose minerals and calcite are respectively in the third and fourth row.According to the characteristics of detrital mineral assemblages,the East China Sea continental shelf can be divided into three districts: Inner-shelf area(Unit I),Outer-shelf area(Unit II)and Tiger Reef area(Unit III).Sediments in Unit I are mainly from the Yangtze River and the Fujian and Zhejiang coastal rivers.In Unit II,sediments are mainly composed of the ancient Yangtze River material.Unit III has relatively varied provenances and complicated hydrodynamic environment.The materials from the Yellow River,the Yangtze River and submarine volcanos both influence there.Magnetic mineralogy in the East China Sea continental shelf is dominated by magnetite with a small amount of hematite and maghematite,which is consistent with the results of detrital minerals identification under the microscope.Magnetic parameters(frequency-dependent magnetic susceptibility(?fd%),?ARM,?ARM/? and ?ARM/SIRM)suggested that the magnetic mineral particles are finer in the west of the study area.In the southwest Cheju Island sediments are dominated by single-domain and pseudo-single-domain particle with relatively high concentration of super-paramagnetic particles.While in the middle-outer continental shelf the magnetic mineral particles are dominated by pseudo-single-domain and multi-domain particles.The detrital mineral and magnetic characteristics of surface sediments provide the basis for judging the provenance of sediments in the continental shelf.Compared the detrital mineral assemblages,the garnet component and the magnetic parameters of the sediments in the continental shelf with the sediments of Yangtze River?Yellow River,we found that the sediments in the East China Sea continental shelf are mainly derived from the Yangtze River and the northern part is affected by the Yellow River.The sediments of the Yellow River are widely distributed in the East China Sea shelf north of 31°N,mixing with the Yangtze river sediments.In Zhejiang Fujian coastal,parts of the sediments are affected by human activities and Oujiang material.Surficial sediment grain size distributions show significant cross-shelf transport trend in the East China Sea.Fine-grained sediment can be spread to the outer continental shelf beyond the east of 124°40'E or even farther near 29°N.This phenomenon is also reflected in the distribution of detrital minerals and the magnetic properties of sediments.In the area near 29°N,heavy mineral content appears a cross shelf low value area while schistose minerals show a high value front.This is the result of the cross transport of the schistose minerals which have relatively small density and easily to be suspended.Magnetic parameters such as ?fd%??ARM also show a high value cross shelf front in the same area.It indicates that the modern terrigenous materials on the East China Sea inner shelf not only transported along the coast but also diffused crossing the shelf.Locations and directions of the cross-shelf fine-grained sediment fronts have seasonal variations,with northeastward trend in summer and eastward one in autumn.These trends were closely related to the hydrological phenomena of Cross-shelf Penetrating Fronts.Sediments in the East China Sea inner shelf have recorded the sudden drop in sediment load of the Yangtze River since 1950 s,caused by human activities(dam constructions,especially the Three Gorges Dam closed).Sediment grain size became finer after 1969 and turned to coarsen since 1987 and then further coarsen since 2003,which accord well to the three steps of sediment load drop in the Yangtze River.Simultaneously,the East Asian Winter Monsoon has influenced the deposition process in the East China Sea inner shelf by changing the intensity of coastal current.Mean grain size variations of the fine population(divided by grain size vs.standard deviation method)are coincided with that of the East Asian Winter Monsoon strength and recorded its weakness in 1987.Abrupt changes of sediment grain size in a short time scale are generally caused by floods and typhoons.However,the effects of river floods and typhoons are limited in scope.Spectral analyses of the sediment cores show statistically periodicities centering on 10–11 and 20–22 years,corresponding to the periodicity of solar activity(Schwabe cycle and Hale cycle).Mean grain size time series also displays a 3–8 years periodicity corresponding to El Ni?o Southern Oscillation(ENSO)periodic change.The widespread weathering mica and high content of dolomite in core SS4 located in the East China Sea inner shelf indicate that the sediments are mainly from the Yangtze River.However,the relatively high content of epidote may represent the impact of the local material.The variation of the detrital mineral content and grain size of sediments in core SS4 have well recorded the climate changes in the Sui-Tang periods and in Little Ice Age(LIA)and the intensity fluctuation of the East Asian Winter Monsoon.Mean grain size variations of the sensitive components have reflected the last two cold periods in the LIA(1620–1710 a AD and 1800–1860 a AD),but showed no significant increase during the cold period in 1250–1550 a AD which has reflected both in the oxygen isotope curve and the temperature curve.However,several major detrital mineral contents have recorded this cold event.During 1250–1550 a AD,the content of amphibole,ilmenite and magnetite was high,but the content of mica was very low indicating an active hydrodynamic environment and strong East Asian Winter Monsoon.Therefore,the combination of the analysis results of detrital minerals and sensitive grain size can more accurately reflect the changes of paleoclimate and make up for the deficiency of reconstructing paleoclimate by selecting different sensitive grain components.The obvious change of the ratio of epidote / amphibole also indicates the variation of the East Asian Winter Monsoon. |
PDF Full Text Request