Early Holocene tidal flat evolution in a western embayment of East China Sea, in response to sea level rise episodes

Abstract

The tidal flat in the coastal region of the East China Sea between the Yangtze estuary and Hangzhou Bay in China, interlinked with sediment transport/dispersal and sea level changes since the Last Glacial Maximum, has been a matter of rigorous research for decades. Here we focused on the evolution of the tidal flat at the westernmost extend of former marine influence in response to early Holocene sea level rise. We used sediment composition, ostracods, minerals, organic matter and carbonate contents in combination with endmember modelling analyses and principal component analysis from two sediment cores and an archaeological site close to the western boundary of former marine influence north of the Hangzhou city. The tidal flats at these sites were very sensitive to morphological changes and to sea level variation during the early Holocene. Our data show two major phases of tidal flat formation: i) an early formation stage (9.5–8.0 cal ka BP) with low sediment supply and related slow-down of the mean relative sea level rise (MRSL), followed by ii) the expansion phase with increased sediment supply, storm occurrence as well as fast depositional processes (8.0–7.5 cal ka BP) under a strong MRSL rise close to its present level. A stillstand/decline of MRSL was recorded for the last episodes (7.6–7.5 cal ka BP). The inferences match previously published local to regional sea level curves but differ from the global and modelled regional ones. We propose a connection with early Holocene unstable climatic conditions with intensified storm events and stronger variations in summer monsoon rainfall that affected river discharge and related sediment transport to the inner shelf of the East China Sea between 9.5 and 7.6 cal ka BP. Sedimentary sequences of bayhead areas in tidally-drowned coastal embayments are sensitive to sea level rise episodes and the relevant climate changes.