High-resolution Sedimentary Records In The Subaqueous Yangtze Rive Delta And Their Responses To Climate,Environment Changes

Yangtze River is the third largest river of the world. A large-scale modern delta is developing in its mouth, which is resulted from the impact of natural and human processes. The high deposition rate and the continuous deposition in the sedimentary squence make this area be a ideal region for the research of high-resolution sedimentary record. And the research has a great significance to understanding the response to natural processes and human activities by deposition and sedimentation of the delta. And it is a universal study for this kind estuaries in the world .Based on a sediment core (18 stations, core length 225cm) collected from the Yangtze River delta front, with high– resolution subsampling by sampling intervals of 0.25cm (for grain size samples) and 0.5cm (other samples), the characteristics of grain size, elemental composition and biogenic silica content were analysed with laser particle size analyzer, X-ray fluorescence spectrometer and ultraviolet spectrophotometer. Combined with X-photograph method andγspectrometer of 210-Pb, 137-Cs radioactivity meaurement, the sequence structure and age of the core were determined. After that, the high-resolution records of grain size, biogenic silica were established. They shew periodic changs of decade, annual and seasonallity and non-periodic changes. The relationship of sedimentary records and clamite changes, sedimneatry environment changes and human activities was discussed. The following conclutions were obtained:The lithology of 18 station on subaqueous Yangtze River delta front is homogeneous, the sequence is continous. The sediments are mainly silty clay, mixed with many silt interlayers, the stratification is horizontal, and have no erosion trace. The deposition rate of the core is fast, with average deposition rate of 3.69cm /a. It is differences between different times, the deposition rate is 3.91cm /a before 1986, and after1986a, it id reduced to 3.21cm /a.The high-resolution grain size and biogenic silica records curves of latest 60 years in Yangtze River Delta front is established. The average sediment size is 11.71μm, and the biogenic silica average content is 0.831%. Both of grain size and biogenic silica records show good cyclical fluctuations, having the cycles of 11a, 3-5a, 1a, especially. At the same time, the high-resolution record was also accompanied by non-periodic fluctuations. The elements geochemical composition also shows similar fluctuation characteristics.The cyclical fluctuations of sedimentary record are related to climate changes closely. 1) The 11-year cycle of sedimentary record is consistent well with the sunspot activity, demonstrating the impact on the deposition of solar activity; but has a opposite trend with the North Pacific decadal oscillation (PDO) in January, so the impact of PDO to sedimentation needs further authentication. 2) 3 ~ 5-year cycle is corresponds with the El Nino / Southern Oscillation (ENSO). The grain size has a "U"-shaped cyclical change, in the beginning of a cycle, just at the time of El Nino, sediment grain size decreases from a larger value, and continue to reduce followed by the occurrence of La Nina, but increases in the end of the cycle; the change of biogenic silica content is against to grian size. 3) The seasonal characteristics of high-resolution sedimentary sequence due to seasonal fluctuations of flow and sediment flux in the Yangtze River and the East China Sea hydrodynamic. In summer and autumn, the sediment is of find grain size, bad sorting and high BSi contents; in winter and spring, it is of coarse grain size, worse sorting, large fluctuation and low BSi contents. The contents of P, S, Cl are basically equal between the two kinds of sediment, Si, Na, Ca, Ba, Sr, Zr have higher level in the sediment layer of winter and spring, and the other geochemical elements contents are higher in the summer and autumn deposition layer, which is mainly due to changes of particle size.Non-periodic changes related to sedimentary events and human activities. 1) The sedimentary records have a good response of the deposition events on this station. In flood years, sediment particle size has been reduced, biogenic silica content increased. In storm years, the average grain size and biogenic silica content varied regularly. When the average particle size increases, the BSi content decreases. Constrary, when the average grain size did not increase, BSi content has a peak value. These flactuations were mainly caused by whether the sediment is disturbed by storm or not. 2) The human activities have a significant impact on the Yangtze River water sediment discharge and the delivery of nutrients. There is a little change in the Yangtze River runoff in recent years, but a sharp reduction in the flux of sediment. In the latest 50a, nutrient contents of N, P have been increasing, but DSi decreasing. The changes of BSi content and grain size in the study area are mainly controlled by the water and sediment discharge in the Yangtze River before 1986, and the impact of human activity is more prominent after 1986. As results, Bsi and heavy metals contents are increasing from 1986 year by year.