Autonomous methane seep site monitoring offshore western Svalbard: hourly to seasonal variability and associated oceanographic parameters

Abstract

Abstract. Improved quantification techniques of natural sources are needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present unique 10- and 3-month time series of bottom water measurements of physical and chemical parameters from two autonomous ocean observatories deployed at separate intense seabed methane seep sites (91 and 246 m depth) offshore western Svalbard from 2015 to 2016. Results show high short-term (100–1000 nmol L−1 within hours) and seasonal variation, as well as higher (2–7 times) methane concentrations compared to previous measurements. Rapid variability is explained by uneven distribution of seepage and changing ocean current directions. No overt influence of tidal hydrostatic pressure or water temperature variations on methane concentration was observed, but an observed negative correlation with temperature at the 246 m site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration (and variability) and wind forcing, concomitant with signs of weaker water column stratification, indicates increased potential for methane release to the atmosphere in fall and winter. We present new information about short- and long-term methane variability and provide a preliminary constraint on the uncertainties that arise in methane inventory estimates from this variability.

Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents sea

“…These results indicate persistent methane oxidation in the pore waters, but a variable proportion of methane-derived carbon incorporated into the aragonite cements during growth due to dynamic seepage conditions (Roberts and Feng, 2013). These findings are consistent with a number of studies demonstrating that seepage activity can vary both spatially and temporally (Judd and Hovland, 2007;Suess, 2014;Ferré et al, 2020;Dølven et al, 2022), influencing subsurface biogeochemical processes (e.g. AOM, carbonate precipitation) (Rooze et al, 2020), the composition and distribution of seafloor chemosynthetic communities (Levin, 2005;Fischer et al, 2012) as well as local water-column biogeochemistry (Sert et al, 2020;Zhang et al, 2020;Gründger et al, 2021).…”

Methane flux estimates from continuous atmospheric measurements and surface-water observations in the northern Labrador Sea and Baffin Bay

“…Large temporal fluctuations of dissolved CH4 levels between 9 and 609 nM within 24 hours were found close to the seafloor (~250 m) at the seep in 2018 (Cramm et al, 2021). Similarly, other studies also manifested the temporal variability of seafloor seep degassing (Boles et al, 2001;Leifer and Boles, 2005;Shakhova et al, 2014;Cramm et al, 2021;Dølven et al, 2022). However, concentrations at the water surface of the seep were in the single digits in the past (Cramm et al, 2021).…”

“…Seafloor gas seeps releasing CH4-rich bubbles into the water column are often found along continental margins. However, the contribution of seafloor gas seeps to atmospheric CH4 entails large uncertainties (Saunois et al, 2016), mostly due to significant temporal and spatial differences of emissions (Boles et al, 2001;Leifer and Boles, 2005;Shakhova et al, 2014;Cramm et al, 2021;Dølven et al, 2022). Water depth and the abundance of methanotrophic bacteria influence the oxidation of CH4, and the speed and strength of currents affects the dissolution of the gas (McGinnis et al, 2006;Reeburgh, 2007;Leonte et al, 2017;Silyakova et al, 2020).…”

Ubiquitous but unique: Water depth and oceanographic attributes shape methane seep communities

“…Methane supply at seep sites experience fluctuations at many timescales. These methane fluctuations encompass tidal (Torres et al 2002; Römer et al 2016), seasonal (Ferré et al 2020; Dølven et al 2022), and climatic (Ruppel and Kessler 2017; Skarke et al 2014; Sultan et al 2020) changes as well as methane reservoir dynamics that can operate in decade‐long (Daigle et al 2011) to geologic (Baumberger et al 2018; Himmler et al 2019) timescales. Except for the recent time‐series data from seafloor observatories (Römer et al 2016), discrete samples or observations at a seep community represent a snap‐shot view of the habitat, that is, a time‐slice of the episodic variability that undoubtedly occurs over time at the site.…”