Acoustic and visual characterisation of methane-rich seabed seeps at Omakere Ridge on the Hikurangi Margin, New Zealand

Six active methane seeps and one cold-water reef that may represent a relict seep were mapped at Omakere Ridge on New Zealandʹs Hikurangi Margin during cruises SO191 and TAN0616. Hydroacoustic flares, interpreted to be bubbles of methane rising through the water column were identified in the area. The seep sites and the cold-water reef were characterised by regions of high backscatter intensity on sidescan sonar records, or moderate backscatter intensity where the seep was located directly below the path of the sidescan towfish. The majority of sites appear as elevated features (2–4 m) in multibeam swath data. Gas blanking and acoustic turbidity were observed in sub-bottom profiles through the sites. A seismic section across two of the sites (Bearʹs Paw and LM-9) shows a BSR suggesting the presence of gas hydrate as well as spots of high amplitudes underneath and above the BSR indicating free gas. All sites were ground truthed with underwater video observations, which showed the acoustic features to represent authigenic carbonate rock structures. Live chemosynthetic biotic assemblages, including siboglinid tube worms, vesicomyid clams, bathymodiolin mussels, and bacterial mats, were observed at the seeps. Cold-water corals were the most conspicuous biota of the cold-water reef but widespread vesicomyid clam shells indicated past seep activity at all sites. The correlation between strong backscatter features in sidescan sonar images and seep-related seabed features is a powerful tool for seep exploration, but differentiating the acoustic features as either modern or relict seeps requires judicial analysis and is most effective when supported by visual observations.