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.
Highlights
Methane forms in marine sediments that are rich in organic matter through either microbial methanogenesis or thermogenic processes (Schoell, 1988)
When these pathways connect to the surface, gas bubbles escape the seafloor as gas seeps, which can range from diffusive sporadic and localized emanations of bubbles to widespread, vigorous gas seeps, occurring in different geological contexts, from the coastal environments to deep ocean regions (Judd, 2004; Duarte et al, 2007; Watson et al, 2020)
We identified a total of 129 individual gas flares: 46 from TAN1808, 32 from TAN1904 and 53 from TAN2012 datasets (Table 1)
Summary
Methane forms in marine sediments that are rich in organic matter through either microbial methanogenesis or thermogenic processes (Schoell, 1988). Methane formation occurs at different depths in the subsurface but, because of the buoyancy of the gas, it migrates upwards through pathways that include permeable carrier sedimentary units, faults, or densely fractured regions (Cook and Malinverno, 2013; Crutchley et al, 2015; Nole et al, 2016; Hillman et al, 2017; Hoffmann et al, 2019; Hillman et al, 2020) When these pathways connect to the surface, gas bubbles escape the seafloor as gas seeps, which can range from diffusive sporadic and localized emanations of bubbles to widespread, vigorous gas seeps, occurring in different geological contexts, from the coastal environments to deep ocean regions (Judd, 2004; Duarte et al, 2007; Watson et al, 2020). Often associated with fluid migration through geological structures such as chimneys, conduits, and faults, can be an indicator of free gas in the sediments
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