Response of Early Diagenesis to Methane Leakage in the inner shelf of the East China Sea Provisionally Accepted

Chenpei Wang¹ Jianfeng Su^{1, 2*} Song Lei¹ Peijun Qiao¹ Daidu Fan^{1, 3*}

• ¹State Key Laboratory of Marine Geology, Tongji University, China
• ²Zhoushan Field Scientific Observation and Research Station for Marine Geo-hazards, China Geological Survey,, China
• ³Laboratory for Marine Geology, Qingdao Marine Science and Technology Center, China

Shelf seas are experiencing a rise in shallow gas leaks, primarily methane, raising concerns due to their environmental impact. However, the effect of the leaks on early diagenesis remains poorly understood. This study analyzes sediment lithology, organic geochemistry and porewater geochemistry of two short cores collected nearby the pockmarks in the muddy inner shelf of the East China Sea. Our findings clearly demonstrate the impact of methane leakage on early diagenesis， evidenced by the shallower position of the SMTZ (sulfate-methane transition zone), higher concentrations of uranium (U), vanadium (V), and manganese (Mn) in the porewater near and above the SMTZ, and downcore decrease in Mg 2+ , Ca 2+ , and Sr 2+ concentrations versus increase in Mg 2+ /Ca 2+ and Sr 2+ /Ca 2+ ratios. Their profile variations and the difference between two cores are determined by the intensity of methane leakage. The estimated methane diffusive flux of 619 mmol m -2 yr -1 at YEC7-2 is roughly 8.5 times that at YEC7-1 (73 mmol m -2 yr -1 ), consistent with a shorter distance of YEC7-2 to the pockmark with active methane leakage. A schematic model is summarized to demonstrate the response of early diagenesis processes to the increasing methane leakages in response to changing sedimentation regimes from accretion to severe erosion. This study undoubtedly improves our understanding mutual promotion effect between seafloor erosion and gas leakage, and their impact on early diagenesis processes and resultant porewater geochemical changes and authigenic mineral records.