Methane in shallow subsurface sediments at the landward limit of the gas hydrate stability zone offshore Western Svalbard
| Type | Article | ||||||||||||
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| Date | 2017-02 | ||||||||||||
| Language | English | ||||||||||||
| Author(s) | Graves Carolyn A.1, James Rachael H.1, Sapart Celia Julia2, 3, Stott Andrew W.4, Wright Ian C.5, 6, Berndt Christian7, Westbrook Graham1, 8, Connelly Douglas P.5 | ||||||||||||
| Affiliation(s) | 1 : Univ Southampton, Natl Oceanog Ctr, Ocean & Earth Sci, Waterfront Campus, Southampton SO14 3ZH, Hants, England. 2 : Univ Utrecht, Inst Marine & Atmospher Res Utrecht, NL-3584 CC Utrecht, Netherlands. 3 : Univ Libre Bruxelles, Lab Glaciol, B-1050 Brussels, Belgium. 4 : Ctr Ecol & Hydrol, Nat Environm Res Council Life Sci Mass Spectromet, Lancaster LA1 4AP, England. 5 : Univ Southampton, Natl Oceanog Ctr, Waterfront Campus, Southampton S014 3ZH, Hants, England. 6 : Univ Canterbury, Vice Chancellors Off, Christchurch 8140, New Zealand. 7 : GEOMAR Helmholtz Ctr Ocean Res Kiel, D-24148 Kiel, Germany. 8 : IFREMER, Geosci Marines, F-29280 Plouzane, France. |
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| Source | Geochimica Et Cosmochimica Acta (0016-7037) (Pergamon-elsevier Science Ltd), 2017-02 , Vol. 198 , P. 419-438 | ||||||||||||
| DOI | 10.1016/j.gca.2016.11.015 | ||||||||||||
| WOS© Times Cited | 26 | ||||||||||||
| Keyword(s) | Methane, Seafloor sediments, Gas hydrate, Offshore Svalbard, Seabed fluxes, Anaerobic oxidation | ||||||||||||
| Abstract | 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 30550 mmol m-2 yr-1, but it is < 20 mmol m2 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. | ||||||||||||
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