Sea-level Change Determined Lake Formation In The Yangtze Plain

The Yangtze Plain has the highest number of freshwater lakes in China,including the three largest?Poyang Lake,Dongting Lake,and Taihu Lake?.How did these lakes form and why the Yangtze Plain became so lake-concentrated has intrigued numerous researchers.Tectonics,climate,and anthropogenic activity are three important driving forces for lake formation.According to different techniques?geomorphology,stratigraphy,or historical records?,previous researches have concluded that one or multiple factors among these three lead to lake formation.However,lack of geological records,weakness in chronological study,and limited correlation between lakes hinder us to understand formation process and mechanism of these lakes in a comprehensive way,and to explain the reason why the Yangtze Plain has so many lakes.Between 2015 and 2017,we collected 13 cores from the Jianghan Plain in the middle reaches of the Yangtze River.Apart from lithological investigations of the upper 50 m of these cores,we obtained a total of 78 age results with the optically stimulated luminesce?OSL?dating method,and utilized them to construct robust chronological frameworks for6 cores.The dating results show that,there are two distinct chronological frameworks among these 6 cores.Although sedimentary hiatuses occur in all cores,they appear at a depth of³5 m for three cores,corresponding to the period between approximately 30and 20 ka;and for another three cores,they appear at a depth of 10 m,covering the timespan from ca.30 to 10 ka.Relatively high sedimentation rates?ca.1 m/ka?were observed above the hiatus in all cores;and high sedimentation rates up to 2.42–5.16 m/ka were observed between ca.20 and 10 ka in cores with deeper hiatuses.Combining chronological and lithological characteristics from all cores,we propose a geomorphological model for the Jianghan Plain since the Last Glacial Maximum?LGM,ca.30–20 ka?:during the LGM,an incised landform prevailed in the Jianghan Plain,paleo-channel incised to a depth of 35 m,and surficial materials of paleo-interfluves evolved into stiff clay rich in Fe–Mn nodules after long-term weathering during the last ice age;during the last deglaciation?ca.20–10 ka?,rapid infilling?mainly gray sand?occurred within incised valleys,and elevation differences between incised valleys and paleo-interfluves almost disappeared by the early Holocene?ca.10 ka?;a floodplain landscape similar to the present lasted throughout the Holocene,with accumulation of ca.10-m-thick organic-rich silt and clay.Through a compilation of previous researches on the stratigraphy and chronological in the Yangtze Plain,we found that the abovementioned geomorphological evolution happened in the whole Yangtze Plain.The LGM incised valleys and stiff clay rich in Fe–Mn nodules on the top of paleo-interfluves were widespread in the Yangtze Plain,with the downcutting magnitude of incised valleys tapering from ca.80 m in the Yangtze Delta to ca.30 in the Jianghan and Dongting Plain;and limited number of ¹⁴C dating results reveal that the three largest lakes?Poyang,Dongting,and Taihu?were formed in Holocene.We presume that sea-level change was the most likely driving force for such an extensive,dramatic geomorphological change.Lowering of sea-level during the last ice age induced incision of Yangtze and its tributaries,resulting the incised landform prevailing in the Yangtze Plain in the LGM;the rapid sea-level rise during the last deglaciation raised water level of the Yangtze and triggered quick infilling of incised valleys;and continuous high elevations of sea-level throughout the Holocene maintained the floodplain landscape in the Yangtze Plain,and determined the formation of abundant lakes.Our proposed model on geomorphological evolution and lake formation reveals the mechanism responsible for a large number of lakes in the Yangtze Plain,and highlights the extensive,rapid influence of sea-level change on landform processes in drainage basins of large rivers under glacial–interglacial cycles.