No 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 .

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.

Cruise CAGE18-5 was the first research expedition under the helm of UiT The Arctic University of Norway in Tromsø with the new ice-going research vessel R/V Kronprins Håkon. This new vessel provides new opportunities to collect cross-disciplinary data for addressing the objectives of the Norwegian Centre of Excellence for Arctic Gas Hydrate, Environment and Climate, CAGE. CAGE investigates Arctic gas hydrate and methane seepage systems in order to better understand the effects they may have on our oceans, ecosystems and global climate. The new research vessel and its facilities allows CAGE access to a state-of-art remotely-operated vehicle, the ROV Ægir 6000, opening a new domain for experimental work and acquisition of sample material from the seafloor. The overall goal of cruise CAGE18-5 therefore was to utilize the ROV in order to provide guided video imagery and to study active gas seepage systems at the gas-hydrate pingo (GHP) site located in the outer Storfjord Trough, and at Storbanken in the northeastern part of the Barents Sea. The cruise may be known as: CAGE18_5