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  • 11
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    Physical controls of dynamics of methane venting from a shallow seep area west of Svalbard (2020)
    Elsevier
    Details
    Publication Date: 2020-02-01
    Print ISSN: 0278-4343
    Electronic ISSN: 1873-6955
    Topics: Geosciences
    Published by Elsevier
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  • 12
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    Upwelling-induced trace gas dynamics in the Baltic Sea inferred from 8 years of autonomous measurements on a ship of opportunity (2021)
    Copernicus
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    Publication Date: 2021-04-30
    Description: Autonomous measurements aboard ships of opportunity (SOOP) provide in situ data sets with high spatial and temporal coverage. In this study, we use 8 years of carbon dioxide (CO2) and methane (CH4) observations from SOOP Finnmaid to study the influence of upwelling on trace gas dynamics in the Baltic Sea. Between spring and autumn, coastal upwelling transports water masses enriched with CO2 and CH4 to the surface of the Baltic Sea. We study the seasonality, regional distribution, relaxation, and interannual variability in this process. We use reanalysed wind and modelled sea surface temperature (SST) data in a newly established statistical upwelling detection method to identify major upwelling areas and time periods. Large upwelling-induced SST decrease and trace gas concentration increase are most frequently detected around August after a long period of thermal stratification, i.e. limited exchange between surface and underlying waters. We found that these upwelling events with large SST excursions shape local trace gas dynamics and often lead to near-linear relationships between increasing trace gas levels and decreasing temperature. Upwelling relaxation is mainly driven by mixing, modulated by air–sea gas exchange, and possibly primary production. Subsequent warming through air–sea heat exchange has the potential to enhance trace gas saturation. In 2015, quasi-continuous upwelling over several months led to weak summer stratification, which directly impacted the observed trace gas and SST dynamics in several upwelling-prone areas. Trend analysis is still prevented by the observed high variability, uncertainties from data coverage, and long water residence times of 10–30 years. We introduce an extrapolation method based on trace gas–SST relationships that allows us to estimate upwelling-induced trace gas fluxes in upwelling-affected regions. In general, the surface water reverses from CO2 sink to source, and CH4 outgassing is intensified as a consequence of upwelling. We conclude that SOOP data, especially when combined with other data sets, enable flux quantification and process studies addressing the process of upwelling on large spatial and temporal scales.
    Print ISSN: 1726-4170
    Electronic ISSN: 1726-4189
    Topics: Biology , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 13
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    Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore western Svalbard (2017)
    Elsevier
    In:  Geochimica et Cosmochimica Acta, 198 . pp. 419-438.
    Details
    Publication Date: 2020-02-06
    Description: Offshore western Svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone (LLGHSZ; ∼400 m water depth). It is hypothesized that this methane may, in part, come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters. To evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments, and the role of anaerobic oxidation in regulating methane fluxes across the sediment–seawater interface, we have characterised the chemical and isotopic compositions of the gases and sediment pore waters. The molecular and isotopic signatures of gas in the bubble plumes (C1/C2+ = 1 × 104; δ13C-CH4 = −55 to −51‰; δD-CH4 = −187 to −184‰) are similar to gas hydrate recovered from within sediments ∼30 km away from the LLGHSZ. Modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the LLGHSZ, providing no evidence for any recent change in methane supply due to gas hydrate dissociation. However, at greater water depths, within the GHSZ, there is some evidence that the supply of methane to the shallow sediments has recently increased, which is consistent with downslope retreat of the GHSZ due to bottom water warming although other explanations are possible. We estimate that the upward diffusive methane flux into shallow subsurface sediments close to the LLGHSZ is 30,550 mmol m−2 yr−1, but it is 〈20 mmol m−2 yr−1 in sediments further away from the seafloor bubble plumes. While anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10% of the total methane flux (dissolved + gas) into the shallow subsurface sediments, most of which escapes AOM as it is transported in the gas phase.
    Type: Article , PeerReviewed
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  • 14
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    Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 120 (9). pp. 6185-6201.
    Details
    Publication Date: 2017-05-02
    Description: Widespread seepage of methane from seafloor sediments offshore Svalbard close to the landward limit of the gas hydrate stability zone (GHSZ) may, in part, be driven by hydrate destabilization due to bottom water warming. To assess whether this methane reaches the atmosphere where it may contribute to further warming, we have undertaken comprehensive surveys of methane in seawater and air on the upper slope and shelf region. Near the GHSZ limit at ∼400 m water depth, methane concentrations are highest close to the seabed, reaching 825 nM. A simple box model of dissolved methane removal from bottom waters by horizontal and vertical mixing and microbially mediated oxidation indicates that ∼60% of methane released at the seafloor is oxidized at depth before it mixes with overlying surface waters. Deep waters are therefore not a significant source of methane to intermediate and surface waters; rather, relatively high methane concentrations in these waters (up to 50 nM) are attributed to isopycnal turbulent mixing with shelf waters. On the shelf, extensive seafloor seepage at 〈100 m water depth produces methane concentrations of up to 615 nM. The diffusive flux of methane from sea to air in the vicinity of the landward limit of the GHSZ is ∼4-20 μmol m-2 d-1, which is small relative to other Arctic sources. In support of this, analyses of mole fractions and the carbon isotope signature of atmospheric methane above the seeps do not indicate a significant local contribution from the seafloor source.
    Type: Article , PeerReviewed
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  • 15
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    Reduced methane seepage from Arctic sediments during cold bottom-water conditions (2020)
    Nature Research
    Details
    Publication Date: 2023-02-08
    Description: Large amounts of methane are trapped within gas hydrate in subseabed sediments in the Arctic Ocean, and bottom-water warming may induce the release of methane from the seafloor. Yet the effect of seasonal temperature variations on methane seepage activity remains unknown as surveys in Arctic seas are conducted mainly 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 with 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.
    Type: Article , PeerReviewed
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