The Comparision And Perspective On Dating Methods Of Recent Sediments In The Yangtze River Estuary And Adjacent Shelf Area

Reliable dating and chronology is essential for better understanding global environmental change, including sea-level fluctuations and human impact on environmental pollution. Meanwhile, it is also important for the environmental monitoring and integrated river-basin and coastal management. There have been profound studies on chronology and associated sedimentation rate of the recent coastal sediments by the means of radiometric 210Pb and 137Cs dating in the Yangtze estuary and adjacent shelf area. However, because of limit of 210Pb and 137Cs application in the river mouth area, and also due to strong hydrodynamics, including interactive fluvial and coastal tidal currents, and storm surges in the Yangtze River estuary, there have existed many uncertainties for radiometric dating application with centennial time scale.In order to calibrate the traditional dating by radionuclide such as 210Pb and 137Cs, new dating methods using sedimentary event have been introduced by the researchers in the past decade, including the identification of Spheroida carbonaceous particles (SCP)and colonial-species pollen. The Yangtze River drainage area is significantly affected by human activities over the past century, in particular, the "Great Leap Forward" and establishment of large-scale hydraulic engineerings after the foundation of the P.R.C, and rapid economic development and urbanization since the reform and opening-up.Therefore, author proposes changes in the river mouth and offshore sediment particle composition, SCP, pollen from urban street tree, which should be certainly recorded in the estuarine sediments.In this study, various dating methods are carried out and compared each other for two shallow cores (YZE, CX38) collected in the Yangtze River estuary offshore, of which YZE is close to the North Channel, and CX38 in the adjacent shelf area. The proxies used in this study include 210Pb and 137Cs, SCP, grain size, pollen of Platanus acerifolia (Ait.) Willd. Assumption of grain size for dating is based on its close relationship with the sediment discharge fluctuations caused by reservoirs and branches migration, and the transportation of suspended sediment by the alongshore current driven by the winter monsoon.With the combination of the various dating methods abovementioned, this study discusses the availability of dating methods of the recent sediments in the Yangtze River estuary and adjacent shelf area, and improves the effectiveness of chronology series in the sediments. Based on the results from different dating methods, sedimentation rates during different periods are calculated and human activities impact on the deposition in the estuary is discussed preliminarily in this study.The result of 210Pb demonstrates a clear exponential trend in CX38, and the average sedimentation rate is computed to be 2.06 cm/a according to CIC model. Nevertheless, the result of 210Pb from CRS model is basically older than that from the CIC model, which can be attributed to the different hypothesis conditions between the two models. The content of 137Cs is very low with the maximum value of 2.21 Bq/kg merely; however, the sedimentation rate deduced from 137Cs dating is basically consistent with but generally older than that of 210Pb.In YZE, an overall exponential trend of 210Pb occurs, but the vertical distribution of 210Pb is obviously disturbed. The average sedimentation rate of 210Pb in the YZE is calculated as 1.22 cm/a from the CIC model. The content of 137Cs is low, based on which the age is apparently older.The productivity of SCP is positively related with the thermal power in drainage. Therefore, changes of SCP concentration can be used for dating purpose. The distribution of SCP in the both cores reveal several obvious phases and share some similar fluctuations, which shows a good consistence with the thermal power associated with the economic and urbanization development in China. Accordingly, four phases are determined, namely the Xinhai Revolution to the time before war against Japanese (1911-1936), the "First Five" plan period after the founding of the people's republic of China, the rapid economic growth period since the reform and opening up, and the promotion period of urbanization since the 21st century. In addition, the thermal power production levels fluctuate significantly throughout the 1960s, which corresponds well with the results of SCP record in the both cores.Some relatively low values appear during the period of the rapid increase of SCP in the both cores, and are presumed to be the results of severe flood event in the Yangtze drainage basin.The identification of Platanus acerifolia (Ait.) Willd. pollen shows a continous distribution through the core CX38, and its content increase obviously upward, especially in the top sediment, which probably correspond with the rapid urbanization since the 21st century.For the prodelta environment, the sediment composition is mainly influenced by the river-derived sediment particle composition and the winter monsoon. There exist differences on the impact from the main factors due to different locations of the two cores. The sediment of YZE is obviously coarser above 108±1cm than below, which should correspond to the branches migration of the Yangtze River to the North Channel since 1948, since this core is adjacent to the North Channel.The particle size distribution in CX38 shows a much finer sediment above the depth of 66±1cm with significant reducing content of coarse sediments(>125μm).Author suggests this change corresponds with the end of 1960s because a lot of reservoirs and dams have been constructed in the upper Yangtze River basin since the late 1960s, leading to the reduction of sediment discharge into sea and the entrapment of coarser sediment in reservoirs. Author also tried the sensitive grain size population to be corresponded with the fluctuation of winter monsoon, based on the hypotheses of winter monsoon driven mechanism for the muddy sediments of Zhejiang longshore shelf. Results indicated relatively high sedimentation rate since 1980s and low rate during 1960s, which is disagree with the sediment discharge into the sea induced by the human activities.The comparison among the dating methods indicates that the result of SCP dating is consistent with that of the grain size-the branches migration dating basically in YZE, but is largely different from 210Pb dating. We suggest that the discrepancy of Pb ages is probably due to the affects of grain size on the concentration of 210Pb, and not enough sampling both in depth and intervals. There exists a good consistency between results of the SCP dating and 210Pb CIC model above 30 cm in CX38, but apparent discrepancy occurs below 30 cm.The result of SCP dating is generally consistent with that deduce from the record of storage capacity of dams and reservoirs and the grain size variation.By the comparison of different dating methods, we believe that SCP dating is a more reliable method because its concentration is not affected by grain size and its source is simply linked with the thermal power. In contrast, the content of 137Cs in both cores is very low and is not suitable for dating, which probably can be explained by 137Cs transportion and redeposition.The sedimentation rates of different periods based on the SCP dating in both-cores show that the high values during "Great Leap Forward" in 1960s are well recorded due to large-scale deforestation and significant increase of sediment load to the sea; and the low values obviously recorded above 50 cm in the both cores since 1980s are caused by the construction of a lot of dams and reservoirs in the upper reaches of Yangtze River, which result in a significant reduction of sediment discharge into the sea.