The North Sea: high levels of methane emissions
A team of researchers from Belgium has identified important concentrations of methane in the surface waters of the North Sea, mainly near the Belgian and English coasts. In order to understand the origins of this methane concentration, it is necessary to go back 16,000 years in time when forests and peatlands connected England and Ireland to continental Europe. Trapped in marine sediment today, this organic matter produces methane which is easily released into the atmosphere from the shallower zones of the basin. This ground-breaking study includes the coastal regions in the quantification of the methane cycle. This quest was made even more difficult by the many sinks and sources of this hydrocarbon of both anthropogenic and natural origin. A better understanding of methane, the second most efficient greenhouse gas after carbon dioxide could be key to slowing down climate change.
A team of researchers has measured a significant quantity of methane dissolved in the surface waters of the North Sea. This strong concentration was observed mainly along the Belgian and English coasts. “We recorded up to 1000 nanomoles of methane (CH4) per litre, which represents 16 milligrams per cubic metre”, explains Alberto Borges, of the Chemical Oceanography Unit of the University of Liege and the leading author of the recently published work in the Scientific Reports journal(1). “This figure may seem insignificant, but the ocean surface waters contain an average of 3 nanomoles of methane per litre. This represents a concentration that is 300 times weaker but the fact that methane has such potential to cause climate change means that it contributes significantly to the phenomenon”. These measurements were taken during the course of six research campaigns that were conducted between 2010 and 1013. These campaigns enabled scientists to highlight a seasonal variation in the concentration of methane and therefore a direct influence on warmer seasons corresponding to an increase in emissions of this gas.
Peat trapped in sediments
In order to understand the singular nature of this source of methane it would be necessary to travel back in time: 16,000 years into the past, when the North Sea and the entire Celtic Sea were covered in peatlands and forests and then to travel to a point in the Atlantic Ocean beyond the westernmost part of the current island of Ireland. “This was a time of significant glaciation. Sea levels were much lower than today. Sea levels then became progressively higher but 7,000 years ago the Southern Bight of the North Sea was still dry”.
While the sea began to cover these lands again over the millennia, layers of sediment accumulated, covering and trapping these large peatlands. “There are therefore large quantities of organic matter trapped in the sediments”, continues the oceanographer. “This organic matter was degraded by bacteria. In oxygenated environments, the bacteria that destroy organic matter consume the oxygen. From a thermodynamic point of view, this is the most efficient way. Therefore the more oxygen there is, the more competitive the aerobic bacteria are”. But in an environment where oxygen is not renewed in the deepest layers of sediment, they consume all the oxygen and give way to other bacteria which develop in anaerobic conditions (absence of oxygen). New reactions are then created and these degrade the organic matter. At the end of this chain, fermentation occurs and this is the least effective way of breaking down organic matter. This fermentation produces methane. This is what occurs in the digestive systems of cattle and termites and is also what happens in humid areas saturated with organic matter such as marshes, flood plains, rivers or in these peatlands trapped under the sea floor. (Read First report on GHG emissions from African rivers)
The water column trap
This production of methane generates pockets of gas which are progressively released into the water column. Only waters close to the coasts show abnormally high levels of this gas. However, this biomass is not confined to the Belgian coast but to all lands that were above water in the past. “Other gas pockets associated with peat were identified in the northern section of the North Sea as is the case at Dogger Bank, but in deeper waters. In summer, the surface waters heat up while the lower layers remain cold. This difference in density linked to temperature creates a stratification, a cover-lid through which the waters do not mix with each other. This is known as the thermocline. The methane remains trapped in the deeper layers, is transported laterally and is not released into the atmosphere”.
(1) Alberto V. Borges, Willy Champenois, Nathalie Gypens, Bruno Delille, Jérôme Harlay, Massive marine methane emissions from near-shore shallow coastal areas, Scientific Reports, 6:27908, doi:10.1038/srep27908.