Estimates of Methane Release From Gas Seeps at the Southern Hikurangi Margin, New Zealand

Abstract

The highest concentration of cold seep sites worldwide has been observed along convergent margins, where fluid migration through sedimentary sequences is enhanced by tectonic deformation and dewatering of marine sediments. In these regions, gas seeps support thriving chemosynthetic ecosystems increasing productivity and biodiversity along the margin. In this paper, we combine seismic reflection, multibeam and split-beam hydroacoustic data to identify, map and characterize five known sites of active gas seepage. The study area, on the southern Hikurangi Margin off the North Island of Aotearoa/New Zealand, is a well-established gas hydrate province and has widespread evidence for methane seepage. The combination of seismic and hydroacoustic data enable us to investigate the geological structures underlying the seep sites, the origin of the gas in the subsurface and the associated distribution of gas flares emanating from the seabed. Using multi-frequency split-beam echosounder (EK60) data we constrain the volume of gas released at the targeted seep sites that lie between 1,110 and 2,060 m deep. We estimate the total deep-water seeps in the study area emission between 8.66 and 27.21 × 106 kg of methane gas per year. Moreover, we extrpolate methane fluxes for the whole Hikurangi Margin based on an existing gas seep database, that range between 2.77 × 108 and 9.32 × 108 kg of methane released each year. These estimates can result in a potential decrease of regional pH of 0.015–0.166 relative to the background value of 7.962. This study provides the most quantitative assessment to date of total methane release on the Hikurangi Margin. The results have implications for understanding what drives variation in seafloor biological communities and ocean biogeochemistry in subduction margin cold seep sites.

Quantitatively Monitoring Bubble-Flow at a Seep Site Offshore Oregon: Field Trials and Methodological Advances for Parallel Optical and Hydroacoustical Measurements

“…Subsequently, a robust method for quantitative assessment of bubble flow rates relies on a very good understanding of the used hydroacoustic technology and the theoretical background of the acoustical backscattering response of bubbles. Particularly, over the last decade, active hydroacoustic systems have been employed during research cruises for mapping bubbling seep areas and quantitative assessments using single beam echosounder systems (SBES; Kannberg et al, 2013;Weber et al, 2014;Veloso et al, 2015;Turco et al, 2022) or multibeam echosounder systems (MBES; Römer et al, 2017;Higgs et al, 2019). Most published quantification methods used hydroacoustic inversion techniques of calibrated SBES parameters as target strength and volume backscattering strength, including the input of optically or acoustically derived bubble size distributions; fewer attempts have been made only using MBES data (e.g., Scandella et al, 2016).…”

Both Longitudinal and Transverse Extension Controlling Gas Migration Through Submarine Anticlinal Ridges, New Zealand's Southern Hikurangi Margin

“…The offshore region of the Hikurangi subduction margin hosts a large gas hydrate province, the extent of which has been determined from bottom simulating reflections (BSRs) marking the base of gas hydrate stability (BGHS) (e.g., Barnes et al., 2010; Henrys et al., 2009; Katz, 1981; Macnaughtan et al., 2022). The margin is also characterized by widespread methane seepage from the seafloor (Barnes et al., 2010; Greinert et al., 2010; Higgs et al., 2019; Turco et al., 2022; Watson et al., 2020) (Figure 2b). Focused fluid flow and methane seepage exploit a wide range of geological structures, including large upper‐plate splay faults (Crutchley et al., 2020; Hillman et al., 2020; Pecher et al., 2010, 2017; Plaza‐Faverola et al., 2016), strike‐slip fault systems (Plaza‐Faverola et al., 2014) and normal fault networks close to the seafloor (Böttner et al., 2018; Riedel et al., 2018).…”

“…. (2010),Koch et al (2015Koch et al ( , 2016,Krabbenhöft et al (2010),Krabbenhoeft et al (2013),Netzeband et al (2010), Plaza-Faverola et al (2012,Riedel et al (2018), andSchwalenberg et al (2010Schwalenberg et al ( , 2017 Urutī Ridge Seismic reflection (2D)Barnes et al (2010),Crutchley et al (2015), andKrabbenhoeft et al (2013) Maungaroa Ridge (informal name) Seismic reflection (2D)Crutchley et al (2021) andTurco et al (2022) …”

Underwater Acoustic Technology-Based Monitoring of Oil Spill: A Review

“…Active detection refers to the analysis of the movement of oil droplets and gases according to the echoes by emitting sound waves into oil droplets and gases in the deep sea [81]. Active acoustic detection is commonly used for underwater oil droplet and low solubility gas monitoring [60][61][62][63]67,73,82,83]. However, active detection requires higher working power and is suitable for short-time-period monitoring, while deep-sea oil spill monitoring requires a longer working period.…”