Very high CH4 concentrations (up to 1,100 nmol L-1) were observed in surface waters of the BCZ compared to open oceanic conditions (<5 nmol L-1) due to release of CH4 from sediments (in-situ production and leakage from gassy sediments) and the well-mixed water column that allows an efficient transfer of CH4 from bottom waters to surface waters. Our data suggest that further warming of surface waters could increase CH4 emissions and provide a positive feedback on warming climate. This feedback will be expected to be acute in shallow gassy areas such as the BCZ since they are natural hotspots of CH4 emission, and the well-mixed water column will allow an efficient propagation of additional heat to the sediment that will be buffered by seasonal thermal stratification in deeper seep areas. The increase of temperature will stimulate the biogenic CH4 production, as well as, decrease Henry’s constant promoting bubbling from sediments. Poster presentation at North Sea Conference 2016, Oostende.

Abstract of an Oral Presentation or a Poster

Methane (CH4) is the second most important greenhouse gas (GHG) after CO2, accounting for 32% of the anthropogenic global radiative forcing by wellmixed GHGs in 2011 relative to 1750. Yet, there remains an important uncertainty on estimates of the sources and sinks of CH4. The open ocean is a very modest source of CH4 to the atmosphere compared to other natural and anthropogenic CH4 emissions. Coastal regions are more intense sources of CH4 to the atmosphere than open oceanic waters. The high CH4 concentrations in surface waters of continental shelves are due to direct CH4 inputs from estuaries and from sediments where methanogenesis is sustained by high organic matter sedimentation. Natural gas seeps from continental shelves contribute additionally.