Shallow gas accumulation shaped by morphological evolution in the Changjiang subaqueous delta: Implications for seabed instability and geohazard management

River deltas are crucial repositories of organic carbon, particularly in their coastal regions from the river mouth to the delta front. Although these regions are influenced by a range of physical and biogeochemical processes, the impact of such processes on shallow gas, a vital form of organic carbon storage, remains poorly understood. Here, we integrate ∼2700 km of high-resolution seismic profiles with sedimentological and geomorphological datasets to investigate how the morphological evolution of the Changjiang (Yangtze River) Estuary, especially the progradation of mouth-bar shoals and their recent retreat, has shaped the accumulation, migration, and release potential of shallow gas. Our results delineate three distinct zones from the subaqueous river mouth to the delta front, each displaying contrasting gas accumulation patterns governed by morphodynamic processes. In the northern subaqueous delta, rapid sediment accretion and high organic-carbon burial enhance methane generation, causing pronounced gas-front surges over 5 m in height, accompanied by irregular or jagged front morphologies and dense gas chimneys. We attribute these features to combined deep-sourced methane migration and in-situ methanogenesis, modulated by variations in sediment thickness and sealing bed properties associated with shoal progradation and retreat. In contrast, the southern sector exhibits thinner cap beds and stronger hydrodynamic forcing, which promote gas release and near-equilibrium gas fronts. We document large gas-related pockmarks (up to 150–200 m wide) and active venting features, suggesting that methane migration contributes to localized seabed instability. These venting structures are located near major engineering corridors and ecologically sensitive areas, highlighting their relevance for infrastructure safety and environmental management. These findings have broader implications for understanding methane behavior in deltaic systems and emphasize the importance of incorporating shallow gas mapping into coastal geohazard assessments, marine spatial planning, and coastal infrastructure design in gas-bearing deltaic environment.