Preliminary Study Of Clay Minerals And Its Significance As Fingerprints For Tracing The Yangtze Sediments Along Zhejiang Coast, China

On the basis of a synthesis of historical studies on fingerprinting approach in inland waters, mixing models were established to estimate the relative importance of different material sources for the sediment in its accumulation site. These models assume that tracer properties do not change during transport and temporary storage between source location and downstream sampling sites. The properties of fingerprints may be indeed stable in inland waters, since the chemical properties of water and environments of sediment dynamic processes are stable. However, the same properties can change in the open ocean system. In this case, for the application of the mixing models, additional stable fingerprints should be added, or changes in the fingerprint properties can be determined quantitatively, so that the number of fingerprints required by the model can be maintained.Composition of clay minerals is often used as tracing fingerprints and there have been some studies on modifications of clay minerals during transport and temporary storage from estuary to continental shelf. However, disagreements exist with regard to clay mineral behaviour in marine environments. In this paper, the main types of clay minerals (i.e. illite, montmorillonite, kaolinite and chlorite) are examined, to evaluate their stability as Changjiang River fingerprints, using the seabed sediment samples from an extensive region of the inner continental shelf of the East China Sea where the sediment source is relatively simple (i.e. dominated by the Changjang materials).Analysis of X-ray diffraction for five groups of samples show that the contents of illite, montmorillonite and kaolinite are stable along the inner shelf waters from Hangzhou Bay to the southern Zhejiang coast and, therefore, these contents can beused directly as the tracing fingerprints of Changjiang sediments. Statistical analysis shows that the content of chlorite does not have a sufficiently high stability and, as a result, its use as a Changjiang sediment fingerprint depends upon the determination of its changes along the transport pathways.Since the existing data are insufficient for an explanation of the varied clay mineral behaviors, further studies on the material sorting and biogeochemical alterations are required, taking into account the factors of flocculation of cohesive sediments, differential settling of clay particles and biogeochemical processes at the water-sediment interfaces.Furthermore, grain size analysis was undertaken for the sediment samples from the Cezi-Jinshan section in Hangzhou Bay by a Malvern Mastersizer 2000 laser grain size analyzer. The result indicates that mean grain size of the sediment samples has different vertical distributions. Because of the highly dynamic nature of the sedimentary environment, the thickness of the moving layer at the seabed is much larger than the deposition rate in value. Therefore, the vertical distributions of mean grain size within the short cores represent the various types of records in the same environment, rather than the information on the regional-scale evolution of the sedimentary environment. The differential settling velocity results in different distribution pattern of the mean grain size between surficial sediment and bulk sediment of the top 80 cm. In a transport environment with strong tidal action and silty sediments, if the temporal scale concerned is large, then it may be more appropriate to use 80 cm averaged mean grain size than the value for the surfical sediment. On a small temporal scale, the sorting of sediment in the sequence becomes an important research topic; this study may provide crucial clues for the interrelationships between the formation of sediment sequences, the time-scale effect of deposition rates and the preservation potential of sedimentary structures in shallow marine environments.