Abstract
Methane is the second most important greenhouse gas contributing to climate warming. The open ocean is a minor source of methane to the atmosphere. We report intense methane emissions from the near-shore southern region of the North Sea characterized by the presence of extensive areas with gassy sediments. The average flux intensities (~130 μmol m−2 d−1) are one order of magnitude higher than values characteristic of continental shelves (~30 μmol m−2 d−1) and three orders of magnitude higher than values characteristic of the open ocean (~0.4 μmol m−2 d−1). The high methane concentrations (up to 1,128 nmol L−1) that sustain these fluxes are related to the shallow and well-mixed water column that allows an efficient transfer of methane from the seafloor to surface waters. This differs from deeper and stratified seep areas where there is a large decrease of methane between bottom and surface by microbial oxidation or physical transport. Shallow well-mixed continental shelves represent about 33% of the total continental shelf area, so that marine coastal methane emissions are probably under-estimated. Near-shore and shallow seep areas are hot spots of methane emission, and our data also suggest that emissions could increase in response to warming of surface waters.
Highlights
Methane (CH4) is the second most important greenhouse gas (GHG) after CO2, accounting for 32% of the anthropogenic global radiative forcing by well-mixed GHGs in 2011 relative to 17501
The overall average CH4 concentration in the Belgian coastal zone (BCZ) near-shore area (139 nmol L−1) was ~6 times higher than in the off-shore area (24 nmol L−1), and in both areas distinctly above atmospheric equilibrium (~2 nmol L−1). These values are one to two orders of magnitude higher than the CH4 concentrations in surface waters of most of the North Sea with values typically
Values in the BCZ were high compared to estuarine plumes of the North Sea where maximal CH4 concentrations in surface waters range between 60 and 90 nmol L−1, such as for the Elbe[9] and the Rhine[6,21]
Summary
Methane (CH4) is the second most important greenhouse gas (GHG) after CO2, accounting for 32% of the anthropogenic global radiative forcing by well-mixed GHGs in 2011 relative to 17501. The CH4 concentrations in surface waters of the BCZ in spring, summer and fall 2010 and 2011 (Fig. 1) were high, with about 43% of the observed values above 50 nmol L−1, and a maximum concentration of 1,128 nmol L−1 in July 2011.
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