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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

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard
Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard
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 ?mol 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.
methane hydrate, sea-air gas exchange, Svalbard, Arctic
2169-9275
6185-6201
Graves, Carolyn A.
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Steinle, Lea
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Rehder, Gregor
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Niemann, Helge
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Connelly, Douglas P.
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Lowry, David
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Fisher, Rebecca E.
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Stott, Andrew W.
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Sahling, Heiko
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James, Rachael H.
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Graves, Carolyn A.
9f1c821a-dc07-413b-b577-6e6b55839853
Steinle, Lea
c74c9e89-d695-41ae-b396-8413f7cb6094
Rehder, Gregor
91093308-4975-4930-8ccb-49bc7c95f5ac
Niemann, Helge
51398a14-68cb-46f0-bd02-f90068cf9848
Connelly, Douglas P.
d49131bb-af38-4768-9953-7ae0b43e33c8
Lowry, David
d708da04-c952-49d3-9dcc-c1aa21c7fada
Fisher, Rebecca E.
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Stott, Andrew W.
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Sahling, Heiko
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James, Rachael H.
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Graves, Carolyn A., Steinle, Lea, Rehder, Gregor, Niemann, Helge, Connelly, Douglas P., Lowry, David, Fisher, Rebecca E., Stott, Andrew W., Sahling, Heiko and James, Rachael H. (2015) Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard. Journal of Geophysical Research: Oceans, 120 (9), 6185-6201. (doi:10.1002/2015JC011084).

Record type: Article

Abstract

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 ?mol 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.

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More information

Accepted/In Press date: 14 August 2015
e-pub ahead of print date: 12 September 2015
Published date: 21 October 2015
Keywords: methane hydrate, sea-air gas exchange, Svalbard, Arctic
Organisations: Geochemistry, Marine Geoscience

Identifiers

Local EPrints ID: 381670
URI: http://eprints.soton.ac.uk/id/eprint/381670
ISSN: 2169-9275
PURE UUID: 7895cf3a-19bf-483a-bd5a-8c7f83aed3f4

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Date deposited: 17 Sep 2015 08:57
Last modified: 12 Mar 2020 17:33

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