Review of Methane Seepages in the Okinawa Trough: Progress and Outlook

It has been two decades since the cold seeps were firstly found in the Okinawa Trough (OT). The scientific cruises and the geological surveys since then have unveiled the currently active submarine methane seeps and significantly improved the understanding of methane seeps in the back-arc basin of the OT. In this paper, we review the up-to-date progress of the research of methane seepages then put forward the promising, yet challenging, outlook by listing the unsolved questions of the cold seeps in the OT. Multiple approaches and techniques, including seismic and echo-sounder recording, dredging, gravity-piston and ROV coring, seafloor drilling, and isotopic and microarray-based genomic analysis, have been used to reveal the geological processes responsible for the seeping activities and the biogeochemical processes related to them. The geophysical signature associated with gas seeps mainly includes the acoustic turbidity in the subsurface, the anomaly of the backscattering intensity at the seabed, and the gas plumes observed in the water column. Pore water and methane-derived authigenic carbonate archive the intensification of methane seepage and the paleoenvironment changes at different time scales. The methane feeding of the seeps in the OT was generated mostly via the microbially mediated process and has an origin mixed by thermogenic hydrocarbon gas in the middle OT. Sulfate-driven and Fe-driven anaerobic oxidations of methane are suggested to be the key biogeochemical processes, which would shape the material cycling in the seeping environment. The future research on the cold seeps in the OT is worth looking forward to due to its geographic and potential geologic links with the nearby hydrothermal activities. Multidisciplinary studies are expected to concentrate on their link with the undiscovered gas hydrates, the amount of methane transferring into the oceans and its impact on the climatic change, and the evolution of the seeping activities accompanied by the biogeochemical processes.