Ocean absorption of carbon dioxide compensates for emissions from seafloor methane seeps
USGS geochemist John Pohlman monitoring data from the USGS Gas Analysis System (GAS) that continuously measured carbon dioxide and methane concentrations in near-surface waters and in the air on the western Svalbard margin. Credit: United States Geological Survey
The ocean waters near the surface of the Arctic Ocean absorbed 2,000 times more carbon dioxide from the atmosphere than the amount of methane that escaped into the atmosphere from the same waters, according to a study by the USGS Gas Hydrates Project and collaborators in Germany and Norway. The study was conducted near Norway’s Svalbard Islands, above several seafloor methane seeps.
Methane is a more potent greenhouse gas than carbon dioxide, but the removal of carbon dioxide from the atmosphere where the study was conducted more than offset the potential warming effect of the methane emissions that were observed.
“If what we observed near Svalbard occurs more broadly at similar locations around the world, it could mean that methane seeps have a net cooling effect on climate, not a warming effect as we previously thought,” said USGS biogeochemist John Pohlman, who is the paper’s lead author. “We are looking forward to testing the hypothesis that shallow-water methane seeps are net greenhouse gas sinks in other locations.”
During the study, scientists continuously measured the concentrations of methane and carbon dioxide in near-surface waters and in the air just above the ocean surface. The measurements were taken over methane seeps fields at water depths ranging from 260 to 8530 feet (80 to 2600 meters).
Analysis of the data confirmed that methane was entering the atmosphere above the shallowest (water depth of 260-295 feet or 80-90 meters) Svalbard margin seeps. However, the data also showed that significant amounts of carbon dioxide were being absorbed by the waters near the ocean surface, and that the cooling effect resulting from carbon dioxide uptake is up to 230 times greater than the warming effect expected from the methane emitted.
Most previous studies have focused only on the sea-air flux of methane overlying seafloor seep sites and have not accounted for the drawdown of carbon dioxide that could offset some of the atmospheric warming potential of the methane.
Photosynthetic algae (marine phytoplankton) appeared to be more active in the near-surface waters overlying the seafloor methane seeps, a phenomenon that would explain why so much carbon dioxide was being absorbed. Previous research has shown that when cold, nutrient-rich waters come up from the depths, algae near the surface can use the nutrients to enhance their photosynthetic processes, resulting in more carbon dioxide being absorbed from the atmosphere. However, this study is the first to make this observation where methane-rich waters rise to the surface.