Sedimentary Evolution Influencing Methane Generation in Coastal Sediments of the Northern South China Sea

Methane was intermittently detected along a 52-m sedimentary core from the coastal sediments of the northern South China Sea, providing an opportunity to better understand the sedimentary factors that influence methane generation. The sediment core was categorized into seven distinct sedimentary facies (U1–U7) based on different sedimentary characteristics at varying depths and dating test results (¹⁴C dating and optically stimulated luminescence dating). Sedimentary unit U4, located at a depth of 17.03–22.04 m below the seafloor (mbsf), was the primary layer for methane generation. This unit was found to be high in total organic carbon (TOC) and trace metals associated with methanogenesis, while the freshwater algal TOC source accounted for a higher proportion, which was favorable for biological methane generation. Analyses also showed that intense historical chemical weathering in the onshore catchment provided the key metallic elements, that is, Fe, Ni, Mo, and Co, during the deposition of Unit 4. Therefore, biological methane generation was jointly controlled by supplies of organic carbon and trace elements from chemical weathering.