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

Abstract

Keywords: methane seeps ROV Hikurangi Margin sea-floor observations temperature measurements bubble-release activity During the 'New Vents' SO191 cruise in 2007, the activity and distribution of seep sites on the gas-hydratebearing 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 bubblereleasing 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 m 2 ), 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.

Physical controls of cold seep methane emission

“…At Faure Site (Fig. 6), CH 4 concentration fluctuations were documented in both the ambient bottom water and the chamber water, which coincide with tidally induced fluctuations of currents and acoustic backscatter flares , where gas seep activity was clearly evident [20]. Reference [34] describe hydro-acoustic bubble flares in the water column and interpret high-amplitude seismic reflections, extending from a shallow BSR towards the seafloor, as preferred gas migration pathways that exploit relatively high sedimentary layers through the gas hydrate stability zone (GHSZ).…”

“…Visual inspection of the landers by ROV [20] showed that the deployment sites were indeed in the immediate vicinity of vent holes discharging free gas (Fig. 5), which is clearly reflected in the backscatter data of the uplooking ADCP mounted on FLUFO (Fig.…”

“…2), as it is known by local fishermen, is the southern termination of Ritchie Ridge uplifted by the subduction of a seamount beneath the outer margin bank [30]. Several spots of active gas discharge were observed with the ROV [20], allowing accurate planning of lander deployments. …”

“…For example, large inconsistencies exist between emission rates measured in situ and fluxes derived from numerical modelling of pore water gradients for cold seep sites at Hydrate Ridge. Moreover, methane expulsion and seep distribution are extremely variable, which further impedes methane flux estimates [19,20].…”

Mechanisms of biogenic gas migration revealed by seep carbonate paragenesis, Panoche Hills, California

“…In particular, where the SMTZ is close to the seabed, it can support the establishment of a chemosynthesis-based megafauna (Gay et al, 2006;Judd and Hovland, 2007) dominated by mollusks, annelids, and crustaceans (Kiel, 2010). Gas seepage intensity varies over various time scales (Naudts et al, 2010;Ho et al, 2012) as a function of migration mechanisms and pathways, typically faults, hydrofractures, or sand injectites (Mazzini et al, 2003;Duranti and Mazzini, 2005;Jonk et al, 2005;Hurst and Cartwright, 2007;Ho et al, 2016). The Panoche giant injection complex (California) is one of the world's largest sand injection complexes.…”