Efficient preservation of old methane-derived organic carbon in deep-sea surface sediments

Methane released from the subseafloor is significantly attenuated during upward migration, yet the preservation of methane-derived organic carbon (OC) in global deep-sea sediments remains poorly understood. Here, we measured carbon isotopes coupled with temperature-ramped analyses on various methane seepage sediments. Our results reveal a correlation between the radiocarbon (14C) content in deep-sea surface sediments and sulfate-methane transition depth. Notably, our findings suggest that a substantial amount of 14C-depleted OC may originate from deep-seated methane and is efficiently preserved in surface seepage sediments. We propose that the efficient OC preservation is related to microbial-mediated aggregate formation at the seawater-sediment interface, where physical occlusion within coarse-grained matrices reduces oxygen availability and enhances OC stability. We estimate that at least 6 Tg C of methane-derived OC are preserved annually in surface sediments of the global continental slopes. This process may play a non-negligible role in OC preservation in sediments, reducing methane emission into the atmosphere. This study finds that ancient methane from deep-sea sediments can be trapped as organic carbon in the seafloor’s top layer, highlighting an overlooked ocean carbon sink, which helps to reduce greenhouse gas release from the seafloor.