Quantification of seep-related methane gas emissions at Tommeliten, North Sea

• Published in: Continental shelf research 2011-05, Vol.31 (7), p.867-878
• Main Authors: Schneider von Deimling, J., Rehder, G., Greinert, J., McGinnnis, D.F., Boetius, A., Linke, P.
• Format: Article
• Language: English
• Subjects: Atmospheres; Bubbles; Flux; Gas bubbles; Gas seepage; Marine; Mathematical models; Methane; North Sea; North Sea Tommeliten; Sea beds; Water depth
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2011.02.012

Tommeliten is a prominent methane seep area in the Central North Sea. Previous surveys revealed shallow gas-bearing sediments and methane gas ebullition into the water column. In this study, the in situ methane flux at Tommeliten is re-assessed and the potential methane transport to the atmosphere is discussed, with regards to the hydrographic setting and gas bubble modeling. We have compiled previous data, acquired new video and acoustic evidence of gas bubble release, and have measured the methane concentration, and its C-isotopic composition in the water column. Parametric subbottom sonar data reveal the three-dimensional extent of shallow gas and morphologic features relevant for gas migration. Five methane ebullition areas are identified and the main seepage area appears to be 21 times larger than previously estimated. Our video, hydroacoustic, subbottom, and chemical data suggest that ∼1.5×10 6 mol CH 4/yr (∼26 tons CH 4/yr) of methane gas is being released from the seepage area of Tommeliten. Methane concentration profiles in the vicinity of the gas seeps show values of up to 268 nM (∼100 times background) close to the seafloor. A decrease in δ 13C-CH 4 values at 40 m water depth indicates an unknown additional biogenic methane source within the well oxygenated thermocline between 30 and 40 m water depth. Numerical modeling of the methane bubbles due to their migration and dissolution was performed to estimate the bubble-derived vertical methane transport, the fate of this methane in the water column, and finally the flux to the atmosphere. Modeling indicates that less than ∼4% of the gas initially released at the seafloor is transported via bubbles into the mixed layer and, ultimately, to the atmosphere. However, because of the strong seasonality of mixing in the North Sea, this flux is expected to increase as mixing increases, and almost all of the methane released at the seafloor could be transferred into the atmosphere in the stormy fall and winter time. ► 735 vents estimated. ► Methane emission ∼1.5×10 6 mol CH 4/yr; 26.3 tons CH 4/yr. ► Only 4% direct impact on atmosphere. ► Strong seasonal bias expected. ► Previous surface measurements in mid to high latitude areas might under represent the methane emissions to the atmosphere.