The Holocene transgression in the Bohai Sea: insights from multi-proxy sedimentary records

Understanding Holocene transgression is essential for assessing the response of low-lying coasts to future sea-level rise. The Bohai Sea, a semi-enclosed marginal sea in eastern China, preserves important records of Holocene land–sea interaction, but the timing and spatial pattern of its transgression remain debated. Based on four coastal-plain cores from Bohai Bay, Liaodong Bay, and Laizhou Bay, this study integrates sedimentary-facies and chronological frameworks with grain-size end-member analysis, organic geochemistry, and elemental geochemistry to reconstruct Holocene coastal environmental evolution along the Bohai Sea. The results show that the Holocene transgression was a stepwise and spatially heterogeneous process rather than a synchronous basin-wide event. Seawater first entered the Bohai Basin through the Bohai Strait during the Early Holocene and then progressively expanded toward the three major bays and adjacent coastal plains. Liaodong Bay records relatively early marine or brackish-water influence at approximately 8.7 cal ka BP, whereas Bohai Bay and Laizhou Bay generally show slightly later transgressive signals at approximately 8.5–8.0 cal ka BP. The strongest marine influence occurred during the Mid-Holocene, approximately 7.0–4.5 cal ka BP, broadly corresponding to the relative sea-level (RSL) highstand and the development of tidal-flat, lagoonal, shallow-marine, and delta-front environments. After approximately 4.2 cal ka BP, marine influence weakened as RSL fluctuated downward, accommodation space decreased, and deltaic and coastal-plain systems prograded. These records indicate that Holocene transgression along the Bohai coast was jointly controlled by postglacial Global Mean Sea Level (GMSL) rise, paleotopography, sediment supply, delta evolution, and local hydrodynamics, providing geological constraints for evaluating future coastal environmental change.