The YAM seep area – an active carbonate-paved gas seep field at the accretionary margin SW offshore Taiwan

The YAM seep area is an active gas seep region among the widespread seep sites offshore south-western Taiwan. The seep area covers a seafloor region of 49,000 m2 at the northern crest of Four-Way Closure Ridge in 1,347 m water depth. During several research cruises (ORI-1163, SO266), shipborne and AUV-related hydro-acoustic investigations revealed that the area of seepage is well documented by high backscatter and a changing micro-bathymetry between rough and flat in the otherwise very flat seabed in the area. 
During expedition SO266, RV Sonne, gas emission sites were observed at the center and eastern flank of the area using ship-borne multi-beam data in the water column. Seawater methane concentrations above the seafloor were collected from the middle west to east of the central transect, revealing concentrations ranging 5.5-18.2 nmol/L with general higher methane concentrations at the rim of the area. Authigenic carbonate samples were collected during gravity coring and MeBo drilling. MeBo drilling was stopped below 5 mbsf because of continuous release of free gas out of the borehole. Carbon isotope measurements of the carbonates showed δ13C values between -38.25 to -52.17 ‰, indicating a mixing of biogenic and thermogenic gas in the methane-derived carbonates. Seismic investigations of the Four-Way Closure Ridge show a well-defined fault below the ridge crest which extends from below the gas hydrate stability zone to the seafloor and acts as a pathway for the methane ascent to the seafloor. 
Seafloor observation using a TV-sled showed a seafloor paved by carbonate rocks consisting of different featured crusts, slabs, and irregular build-ups. Thin-layered crusts were mainly observed at the paved area's rim, while thick-layered slabs and free-standing build-ups were the main features at the elevated region. Chemosynthetic communities, like mussels and clams, mainly were observed within certain carbonate fractures. At the same time, other animals, bacterial mats, and tube worms are presented generally along the whole survey path among the rough seep area. Hydro-acoustic data correlated with visual observation results indicate wide variation through the region of the seep area. Based on our interpretation, the YAM seep area developed over a longer time at the crest of the accretionary ridge, seepage, uplifting of the ridge, tectonic fracturing, and seafloor erosion highly influenced the seafloor manifestation.