Initiation Of Changjiang (Yangtze) Delta And Its Primary Factors During The Holocene

In recent years, the sustainable development of the coastal area has been threatened by potential hazards due to global warming, glacier melting and the sea level rising. Knowing about the evolution history and its development trend helps to predict the disasters and further to reduce the loss from the disasters. The Changjiang (Yangtze) delta in eatern China is one of the largest delta in the world which has similar challenges. To understand the initiation of Changjiang (Yangtze) delta response to climate change as well as sea-level change is beneficial for understanding the impacts on the Changjiang (Yangtze) delta from the present sea-level change and climate change. In this research, we drilled a series of cores from the upper region of Changjiang (Yangtze) delta (in the area of Nanjin-Yizhen), which recorded the information of climate, sea level and sedimentary environmental change during the period of Changing (Yangtze) delta initiation. We carried out the multi-proxy analysis including grain size, foraminifer, gastropods and palynology to reconstruct the sedmentary environment, sea level change as well asclimate change, and to further discuss the initiation of Changjiang (Yangtze) delta and its primary factors.Based on the analysis results of sediments cores PK01, HG01and XJ03from the uppermost region of the delta plain, we identify three depositional systems:a fluvial system, an estuarine system, and a deltaic system. Controlled by the high resolution AMS14C framework with the stratigraphic correlation, the maximum flooding surface was identified to format8.0cal kyr BP when brackish water intruded landward to the Nanjing area.The initiation of the Changjiang (Yangtze) delta was indicated by a progradation in the process of a transition from estuary to delta and a typical delta. After the maximum flooding time, the deposition was characterized by the appearance of progradation although aggradation had still been predominated until the Hongqiao sand bar became exposed followed by the progradational deltaic sequence a. series of river-mouth bars seaward migrating. The oldest river-mouth bar was called Hongqiao sand body dating back to6.0-5.5cal kyr BP, nearly one thousand years younger than the previous estimates.The sedimentary facies analysis from the cores have shown a highstand of relative sea level in6.0-4.0cal kyr BP, which is also supported by the prediction from Glacio-hydro-isostatic adjustment (GIA) model. This high sea level may be related to the hydro-isostatic effect.Palynological analysis for the section of core HG01spanning over a stable sedimentary environment was carried out to reflect the climate change Climate was general warming and humid associated with a strengthened East monsoon strong during8.54-7.22cal yr PB. Two centennial-scale cold and dry events were showed in the periods of8.54-8.21cal kyr PB and7.81-7.59cal kyr PB. The former might reflect the8.2ka BP event, which was characterized by a wet condition in both the north and the south regions while a dry condition in the middle region. An about20yr cycle of climate change was also shown in Changjiang (Yangtze) delta during, which may be associated with the Pacific Decadal Oscillation (PDO).The initiation of Changjiang delta was partly influenced by the sea level rising and climate warming. During9-8cal kyr BP when the sea level rose rapidly an estuary was developed. A decelerating of the sea-level rising in the period of8-6cal kyr BP resulted in progradation seaward. Moreover, strengthened East Asian Monsoon with a trend of climate warming could increase the discharge of the Changjiang.(Abundant sediments from the mid-upstream were loaded by river and deposited on the river mouth. After6.0cal kyr BP, the rate of sea level rise was gradually less than sediment accumulation rate. Then, the Changjiang (Yangtze) delta was initiated at the area between Nanjing and Yizheng.