Methane seeps and methane anaerobic oxidation (AOM) not only lead to the precipitation of authigenic minerals but also impact the elemental and isotopic compositions of sediment- pore water systems. This study investigated the Ba isotopes and trace element characteristics of sediments and pore waters at station HD109 in the northern South China Sea gas hydrate area. The research aimed to explore the utility of integrating redox- sensitive elements (Mo, U), Ba, and Ba isotopes to trace the sulfate- methane transition zone (SMTZ) and methane seep events in both ancient and modern settings. The elemental variations in pore water at station HD109 show a clear geochemical zonation, encompassing the Fe- Mn reduction zone, the sulfate reduction zone, and the sulfate- methane transition zone in an ascending sequence. The Ba characteristics of sediments indicate a prominent barium front above the SMTZ. The interval with Mo and U enrichment at shallow depths, coupled with the Ba/Al characteristics in adjacent intervals, suggests the presence of a past barium front and a past SMTZ, representing periods of more intense methane seeping during sedimentation. Compared to particulate and detrital Ba, pore waters generally display higher δ138/134Ba values, suggesting the dissolution of diagenetic barite in sediments. Porewater near the current barium front has even higher δ138/134Ba values, indicating a significant response to the precipitation of authigenic barite.