Sustained growth of the largest uninhabited alluvial island in the Changjiang Estuary under the drastic reduction of river discharged sediment

The evolution of the Changjiang Delta is obviously affected by current rapidly rising sea level and drastically declining river discharge,and it is also vital for the sustainable development of Shanghai and the Changjiang River Economic Belt,which represent China’s economic development leader and major national strategic area,respectively.In this paper,the growth pattern of Jiuduansha Island,the largest uninhabited alluvial island in the Changjiang Estuary,is studied in terms of the change in elevation,position and area over the past 50 years through using satellite-derived instantaneous shoreline positions and high/low tide exposure areas based on 497 satellite images from 1974 to 2020;and the influencing factors and future development patterns are further discussed by comparison with other alluvial islands or sandbars in the estuary.The results show that Jiuduansha Island has maintained a rapid or even accelerated area growth rate,although the sediment discharge of the Changjiang River has sharply decreased in recent decades.This sustained growth is mainly attributed to the existence of the estuarine turbidity maximum zone,coarsening fluvial sediment,onshore sediment replenishment by tide,cone-like geomorphology of Jiuduansha Island,and siltation promotion effect of vegetation.The growth rate of the low tide exposure area of Jiuduansha Island increased from 1.9 km^(2)a^(−1) in 1974–1990 to 3.0 km^(2)a^(−1) in 1990–2020,and the growth rate of the high tide exposure area reached as high as 3.7 km^(2)a^(−1) in 1994–2020.The implementation of the Deep-Water Channel Project has significantly affected the evolution of Jiuduansha Island,including shifting the heads of Shangsha and Zhongxiasha from severe retreat to rapid accretion,and promoting tidal flat progradation seaward of the Jiangyanansha and Zhongxiasha,but restricting the growth of the low tide exposure area of Jiuduansha Island.

Magnetic fabric characteristics of late Quaternary strata in Shanghai area

The result of measurements and analyses on magnetic fabrics data for the late Quaternary core at Caoyangxincun in Shanghai shows that abrupt changes of magnetic fabric curves coincide with the boundaries between the marine strata and underlying stiff clay. Littoral, neritic, subtidal and tidal flat facies in the marine strata have distinct magnetic fabric characteristics. The curves of the parameters, K, P, L, q, display clear boundaries between various facies, which could serve as a new method for the stratigraphic subdivision and correlation.