Evolution of multiple peat beds in postglacial transgressive deposits of western Bohai Sea and their paleoenvironmental implications

Coastal peat deposits are crucial archives for paleoenvironmental and paleoclimatic reconstructions. In transgressive depositional systems, while basal peats have been well-studied for insights into relative sea-level changes, the evolution of overlying coastal salt-marsh peats remains less understood. This study examines burial patterns and evolution of peat beds in the western Bohai Sea, utilizing seismic data and core samples from a new offshore borehole (BHB1) and nine previously published, chronologically constrained cores forming a shore-normal transect. Multiple peat horizons were identified within the postglacial transgressive succession underlying middle- and late-Holocene deltaic/prodeltaic deposits. These horizons, composed of either carbonized plant remains or peaty clayey layers, are interspersed with coastal-littoral muds, and as the shoreline migrated landward, they were progressively buried at shallower depths, with younger ages. Elemental geochemical analysis of core BHB1, along with lithological evidence from the core transect, suggests that these peat horizons formed in alternating waterlogged (anaerobic) and subaerially exposed (oxidizing) environments, leading to the observed sequence of peat layers and coastal-littoral muds. A comparison with paleoclimatic records shows that this period coincided with an intensified East Asian Summer Monsoon, increased insolation, and rising temperatures, marked by high monsoon precipitation variability. These conditions appear to supported the expansion of wetland vegetation and fluctuating water tables that drove the alternating sequence. While these conditions persisted until ∼5 ka BP, the widespread development of peat layers ceased around ∼7 ka BP, transitioning to the progradation of the Huanghe delta. This suggests that multilayered peat formation was also dependent on transgressive conditions, possibly linked to a unique backstepping, aggradational transgressive system in the western Bohai Sea, influenced by significant sediment input from the Huanghe River after the Younger Dryas. The findings from the western Bohai Sea have broader implications for understanding the paleoenvironmental significance of transgressive peat accumulations in sediment-rich coastal systems, particularly those associated with mega-river systems.