Reduced methane seepage from Arctic sediments during cold bottom-water conditions

Abstract

Large amounts of methane are trapped within gas hydrate in sub-seabed sediments in the Arctic Ocean, and bottom-water warming may induce the release of methane from 2 the seafloor. Yet, the effect of seasonal temperature variations on methane seepage activity remains unknown, as surveys in Arctic seas are mainly conducted in summer. Here, we compare the activity of cold seeps along the gas hydrate stability limit offshore Svalbard during cold (May 2016) and warm (August 2012) seasons. Hydro-acoustic surveys revealed a substantially decreased seepage activity during cold bottom-water conditions, corresponding to a 43 % reduction of total cold seeps and methane release rates compared to warmer conditions. We demonstrate that cold seeps apparently hibernate during cold seasons, when more methane gas becomes trapped in the subseabed sediments. Such a greenhouse gas capacitor increases the potential for methane release during summer months. Seasonal bottom-water temperature variations are common on the Arctic continental shelves. We infer that methane-seep hibernation is a widespread phenomenon that is underappreciated in global methane budgets, leading to overestimates in current calculations. Methane (CH4) is a particularly important trace gas, as its atmospheric concentration has almost tripled since the beginning of industrialisation 1. With an equivalent warming potential that is 32 times higher than that of carbon dioxide 2 , it contributes 16 % to the global greenhouse effect 1 , and has a lifetime of ~12 years in the atmosphere 3. Natural CH4 emissions have diverse origins and vary in space and time 4 , increasing the uncertainty of the contribution of natural sources to the bulk atmospheric CH4 budget. Arctic Ocean sediments host enormous CH4 reservoirs, in the form of free gas, dissolved in pore water, or trapped in permafrost and gas hydrates 5-9. Gas hydrates are stable at low temperature and high pressure 10 , conditions typically found at ≳400 m water depth. They can dissociate if the ambient temperature rises 11 , and there is evidence for large-scale CH4 eruptions due to warming of hydrate-bearing sediments in the geological past 12,13 .

The Impact of Methane on Microbial Communities at Marine Arctic Gas Hydrate Bearing Sediment

“…However, this hypothesis would contradict previous observations where ANME-2 demonstrated higher growth rates than ANME-1 at low CH 4 flux rates (Girguis et al, 2005). Beyond these two hypotheses presented above, the hydrographic conditions above the GHPs could also induce an additional set of environmental constraints, as the bottom-water temperature seasonally varies (Ferré et al, 2020). This creates fluctuations in both CH 4 seeping activity from the sediments and subsequently CH 4 oxidation rates in the water column.…”

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

“…The seepage area with the highest density of plumes is situated at 300-400 m water depth near the shelf break. Here seepage has been associated with the dissociation of gas hydrates either by seasonal and larger term (thousands of years) temperature changes 18,27,29 or by a decrease in overburden pressure following the ice-sheet retreat since the LGM. 28,34 .…”

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).…”

“…observ.). This resilient capability could be advantageous to control methane emissions in areas with fluctuating methane fluxes, in particular seasonal fluctuations driven by up and down movements of the GHSZ as observed in the study area (Berndt, Feseker, et al, 2014; 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.…”