Active venting sites on the gas-hydrate-bearing Hikurangi Margin, off New Zealand: Diffusive- versus bubble-released methane

During the ‘New Vents’ SO191 cruise in 2007, the activity and distribution of seep sites on the gas-hydrate-bearing Hikurangi Margin, off northeastern New Zealand, were subjected to a highly detailed interdisciplinary study. Here we report on the visual observations and in situ measurements of physical properties performed with a ROV (remotely operated vehicle) and other video-guided platforms at two seep sites in the Rock Garden area; Faure Site and LM-3. The ROV allowed first ever visual observations of bubble-releasing methane seeps at the Hikurangi Margin. At Faure Site, bubble release was monitored during 4 dives, up to periods of 20 min. During the first dive, this resulted in the observation of six violent outbursts, each lasting 1 min over a three minute interval. These outbursts were accompanied by the displacement and resuspension of sediment grains, and the formation of small depressions, with a maximum diameter of 50 cm and depth of 15 cm, showing what is possibly an initial stage of pockmark formation. During subsequent dives at this bubble site, bubble release rates were rather constant and the previously observed outbursts could no longer be witnessed. At LM-3, the strongest manifestation of seep activity was a large platform (100 m2), consisting of fresh authigenic carbonates, which was covered by seep fauna (live Bathymodiolus sp. mussels, Calyptogena sp. shells and live Lamellibrachia sp. tubeworms). Bubble activity near this platform was less prominent than at Faure Site. Our observations suggest that the two seep environments result from different types of methane release; mainly by bubble release at Faure Site and rather diffusive at LM-3. We propose a conceptual model where the different ways of methane release and seep environments may be explained by the depth of underlying hydrate occurrences and different tectonic histories of both seep sites.