Variability of Acoustically Evidenced Methane Bubble Emissions Offshore Western Svalbard

Abstract

Large reservoirs of methane present in Arctic marine sediments are susceptible to rapid warming, promoting increasing methane emissions. Gas bubbles in the water column can be detected, and flow rates can be quantified using hydroacoustic survey methods, making it possible to monitor spatiotemporal variability. We present methane (CH4) bubble flow rates derived from hydroacoustic data sets acquired during 11 research expeditions to the western Svalbard continental margin (2008–2014). Three seepage areas emit in total 725–1,125 t CH4/year, and bubble fluxes are up to 2 kg·m−2·year−1. Bubble fluxes vary between different surveys, but no clear trend can be identified. Flux variability analyses suggest that two areas are geologically interconnected, displaying alternating flow changes. Spatial migration of bubble seepage was observed to follow seasonal changes in the theoretical landward limit of the hydrate stability zone, suggesting that formation/dissociation of shallow hydrates, modulated by bottom water temperatures, influences seafloor bubble release.

High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

“…They achieved a response time of 30 min and a detection limit of 20 nmol L −1 . Wankel et al (2010) deployed a deep-sea graded in situ mass spectrometer over a brine pool in the Gulf of Mexico, where they measured high (up to 33 mM) concentrations of CH 4 . They do not specify their detection limit or the response time of the instrument but state an uncertainty of 11 %.…”

Impact of tides and sea-level on deep-sea Arctic methane emissions

“…A lthough ocean methane emissions are considered to be widespread [1][2][3][4][5][6] their dynamics and the physical processes behind their evolution are little understood. Given the impact of methane as a greenhouse gas, the dynamic of oceanic methane emissions, which could potentially reach the atmosphere, introduces a non-negligible doubt on the global budget of atmospheric methane 7 .…”

Biogeochemical Consequences of Nonvertical Methane Transport in Sediment Offshore Northwestern Svalbard

“…It is, however, possible that residual hydrates persisted in the top 5–10 m, beneath the penetration depth of our instruments, and supplied methane to the overlying chemosynthetic communities. Recent studies revealed new evidences for seasonal gas hydrate formation and dissociation in the study area, which causes temperature‐driven fluctuations of gas release along the upper continental slope depending on the location of the GHSZ (Ferré et al, 2020; Veloso‐Alarcón et al, 2019).…”

“…The calculation of the limit is based on the summer month August, which coincides with the study period (late August/early September) (Riedel et al, 2018). Gas hydrate stability in the surface sediments at this water depth likely varies seasonally (Berndt, Feseker, et al, 2014; Ferré et al, 2020; Veloso‐Alarcón et al, 2019) and moves the upper limit of the GHSZ between ~360 m during coldest (April to June) and ~410 m during warmest (November to March) conditions at the seafloor. Aside from seasonal temperature variability, rapid temperature changes associated with current‐driven hydrography occur.…”

“…That shallow stability makes Arctic hydrates more susceptible to atmospheric temperature increases and thus rises the likelihood of methane emissions reaching the atmosphere. Over the past decade, several field studies reported methane release from the shallow seafloor of the Arctic Ocean (e.g., Mau et al, 2017; Shakhova et al, 2014, 2010; Thornton et al, 2016; Veloso‐Alarcón et al, 2019; Westbrook et al, 2009). Their sources, sinks, and potential connection to gas hydrate dissociation have been the focus of several follow‐up studies and discussions (Berndt, Dumke, et al, 2014; Berndt, Feseker, et al, 2014; Graves et al, 2017, 2015; James et al, 2016; Myhre et al, 2016; Platt et al, 2018; Riedel et al, 2018; Ruppel & Kessler, 2017; H. Sahling et al, 2014; Steinle et al, 2015; M Torres & Colwell, 2018; Wallmann et al, 2018).…”