We used accelerator mass spectrometry (AMS) 14C‐dated sediments of the Holocene basal supratidal flat to upper tidal flat facies in 11 cores on the southern Yangtze delta plain to reconstruct relative sea levels of 8.5–8.0 cal ka BP. Three cores were further AMS 14C dated and used to examine the evolution of sedimentary geomorphological environments in response to the rapid sea‐level rise during the early to mid‐Holocene. Results demonstrate relative sea‐level rise of around 30 mm a−1 from 8.5 to 8.3 cal ka BP and around 10 mm a−1 from 8.3 to 8.0 cal ka BP. Retrogradation from supratidal to lower tidal flat environments occurred in response to the rapid sea‐level rise at 8.5–8.3 cal ka BP, and aggradation from middle to upper tidal flat occurred at 8.3–7.9 cal ka BP. Further retreat of the tidal flat at 7.9–7.2 cal ka BP implies a mean sea‐level rise rate exceeding 5 mm a−1 at this time. We suggest that the rapid relative sea‐level rise during 8.3–8.5 cal ka BP and subsequent slower rise caused drastic changes in the coastal zone and that these changes are key phenomena for understanding the coastal response to future sea‐level rise.

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Measurements of magnetic properties, total organic carbon (TOC) and total sulphur (TS) were performed on recent tidal flat sediments from the Feng-Xian (FX) and Bei-Bu-Gang (BBG) areas of the Yangtze coast and on Holocene tidal flat sediments from core SL67 from the southern Yangtze delta plain, China. The results indicate that greigite has likely formed in the recent upper and middle tidal flat sediments of cores FX and BBG, which are enriched in TOC and TS. Greigite is also present in association with pyrite in the early to middle Holocene saltmarsh and tidal flat sediments of SL67. The abundance of greigite in the early Holocene basal saltmarsh sediments of core SL67, along with the extremely low values of TS/TOC, suggest the presence of either a limited sulphate supply from seawater or a major terrestrial source of organic matter that decomposed slowly. The presence of greigite in mid-Holocene lower tidal flat sediments that contained low amounts of TOC indicates an upward diffusion of CH4 and H2S from the underlying upper tidal flat sediments. A comparison between the sediments of core SL67 and those of the modern tidal flat suggests that early diagenesis (including selective dissolution and pyritisation) continued after the sediments were buried during the Holocene, resulting in the depletion of soil-derived superparamagnetic (SP) particles and the enhanced magnetic properties because of authigenic greigite. However, antiferromagnetic magnetic minerals have not been altered by early diagenesis. Thus, magnetic parameter hard isothermal remanent magnetisation (HIRM) which preserved the primary magnetic signals reflects fluctuations in the detrital mineral input to the core site and records a two-stage 8.2 ka cooling event.