210Pbex records the historical sediments accumulation in the water-level fluctuation zone of the Three Gorges Reservoir, China

Impoundment regulation of the Three Gorges Reservoir (TGR) has created a water-level fluctuation zone (WLFZ) with a vertical height of 30 m and a total area of 349 km2. This special zone received approximately 7% of the total sediment captured by the TGR. The present study aims to reveal the history of sediments accumulation since the WLFZ was formed in 2006. A sedimentary core was retrieved from a representative floodplain at the lower part of the WLFZ and subsequently sectioned with an interval of 5 cm. Particle composition, 137Cs and 210Pbex activities of sediment were measured after pretreatment. The result shows that the 210Pbex activity can be used as a proxy for the seasonal high-resolution dating of sediment in the WLFZ, due to the sediments deposited during the dry season have high 210Pbex activity, while the opposite is true for sediments deposited in the wet season. Then, the annual sequence of the sedimentary profile has been established based on the distribution pattern of 210Pbex activity. The inter-annual variation of the sedimentary rate is shown as 'low-high-low-high', and mutation years are 2008, 2014, and 2018, corresponding deposition rates in these four periods are 12.5, 45, 20, and 32.5 cm, respectively. In the early period of the formation of the WLFZ (2006-2007), the staged impoundment of the TGR was the key influencing factor of the sedimentary rate. Afterward, the annual sedimentary rate was consistent with the suspended sediment load supplied into the TGR. From 2014 to 2017, due to the interception of the cascade reservoirs in the lower reaches of the Jinsha River, the suspended sediment load dropped sharply, resulting in a decrease in the sedimentation rate. From 2018 to 2019, the sedimentation rate of the floodplain increased owing to the increase in the suspended sediment load caused by frequent extreme rains in the upper reaches of the Yangtze River basin.