The Early To Mid-Holocene Sea-level Reconstruction And Sedimentary Environmental Response On The Hang-Jia-Hu Plain

The acceleration of sea level rise is a big challenge we should not ignore in 21st century. Experience from the geological history will help predict the response of estuarine and coastal zone to the sea level rise. Holocene is the end of the latest ice age and climate warmed up globally. Sea level also rose rapidly in the early to mid-Holocene and reached the current position at-6000 cal a BP. Therefore, the rapid sea level rise and response of the coastal geomorphology in early to mid-Holocene will help us prepare for coastal change due to future sea level rise.Previous studies revealed that basal saltmarsh peat and stratigraphic approach is useful in sea level reconstruction for the unique condition of Yangtze delta plain. In this study, I used lithology, micropaleontology and sensitive indexes of grain size of 10 boreholes from Hang-Jia-Hu plain to identify Holocene stratigraphy and to seek efficient sea level indicators including basal saltmarsh peat and upper tidal flat sediments. I then reconstructed relative sea level curve of early-middle Holocene by AMS 14C dating and quantative analysis of sea level indicators. I also revealed the response of depositional environment in the Hang-Jia-Hu plain by examining changes in sedimentary facies throughout the Holocene cores.This research shows that, the sea level is 29 m below present mean sea-level at 9600 cal a BP, and is close to present position at-6800 cal a BP. Sea level rise rate is 11 mm/a during 9600-8700 cal a BP, the rate is 18 mm/a during 8700-8400 cal a BP and 13.2 mm/a during 7800-7400 cal a BP. Because of the effect of Hydro-isostasy and sediment-isostasy, relative sea level in the Hang-Jia-Hu plain is higher than that of eastern Taihu plain, and also differs from sea levels around the world.Tidal flat dominated in the study area during early to mid-Holocene. Retrogradation, aggradation and progradation occurred in association with changes in sea level rise rate. Sea level rise rate is slightly higher than the sedimentation rate of the tidal flat during 9600-7750 cal a BP, so that saltmarsh was replaced by upper tidal flat. During 7750-7450 cal a BP, deposition rate is far less than the rate of sea level rise, retrogradation then happened and low tidal flat dominated. Upper tidal flat formed again at 7450-6800 cal a BP when the sea level is relatively stable.