The contribution of methane emitted from the world-shallow-water area is estimated to roughly 20% of the total atmospheric input. However, to accurately predict the contribution of this source to global methane sources and sinks it is necessary to understand the dynamics of shallow-water methane generation, transport and consumption in the hydrosphere, and the subsequent emission into the atmosphere.
Along the West-Spitsbergen shelf submarine methane discharges at several inter-granular and micro-seepages create an excess methane concentration in the water column. Brine-enriched shelf water accumulates near the bottom and may receive methane released from sedimentary sources. Because of density stratification primarily methane is transported and subsequently oxidized therein. Therefore, methane transported via this pathway is not a source of atmospheric methane. Indirectly however, it may influence the atmospheric methane budget because the submarine, fossil methane diluted to background concentrations and released in the West Spitsbergen Current reduces the sink capacity for atmospheric methane. Density stratification exists primary during summer while ice formation and brine release leads to convective mixing which homogenize the methane concentration in the whole water column and increase super-saturation in surface water. Hence active water mass transformation on the polar shelves may essentially influence the pathways and finally the fate of methane.
Extensive water mass formations occur especially in regions of latent heat polynyas. Theses processes may also induce resuspension of sediments and a subsequently release of more recently produced methane if high accumulation rates of organic carbon supply the environment for the formation of bacterial methane near the sediment surface. Due to the convective mixing of water masses the bottom-released methane is transported very quickly to the sea surface. The resulting difference in the partial pressure between the atmosphere and surface water and the turbulence in the surface water generated by strong winds favour a sea-air exchange during periods of open water. By this way a direct pathway for biogases like methane between surface sediments and the atmosphere is generated on polar shelves.
Thus recent change in the Arctic, which alters water mass formation and convection, may have profound effect on natural biogeochemical cycles in seawater.
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Damm, E., Mackensen, A., Budeus, G., Faber, E., Hanfland, C., (2005). Pathways of methane in seawater: Plume spreading in an Arctic shelf environment (SW-Spitsbergen), Continental Shelf Research, 25 (12-13), 1433-1452. doi: 10.1016/j.csr.2005.03.003.
Knies, J., Damm, E., Gutt, J., Mann, U., Pinturier, L. (2004). Near-surface hydrocarbon anomalies I shelf sediments off Spitsbergen: Evidences for past seepages, Geochemistry, Geophysics, Geosystems (G3) 5 (6) 1-14. doi: 10.1029/2003GC000687.