The transformation and burial of methane-derived organic carbon in the South China Sea

Methane seeping from the submarine has long been recognized as a driver of climate warming, owing to its oxidation that emits carbon dioxide to the atmosphere. Yet, the biogeochemical processes that transfer methane to organic carbon (OC), serving as a negative feedback on warming, remain largely under constrained. Here, we measured concentration and stable and radiocarbon isotopes of the dissolved and sedimentary OC, as well as foraminifera, across contemporary and past methane seepage settings. Our findings reveal that methane undergoes transformation into OC, promoting its long-term burial in sediments and mitigating climate change. At active methane seeps in the South China Sea, methane contributes up to 23% of dissolved OC in the contemporary bottom water. And our results suggest that methane may be emitted to the water column ~700 m above the seafloor during the Last Glacial Maximum, and subsequently undergoes transformation into OC buried in sediments. It accounts for up to 11% of methane-derived OC burial during the Last Glacial Maximum with active methane seepage events, and reduces the radiative forcing caused by methane emission over glacial cycles. Our discovery of the enhanced methane carbon burial calls for reconsideration of methane’s impact on climate warming.