Large ice movements activate faults in the Earth's crust and thus contribute to methane leakage on the seafloor along the continental margin west of Spitsbergen. “Our results indicate a relationship between changing amounts of continental ice and deep-sea methane emissions in the Arctic,” says first author Dr. Tobias Himmler of the Geological Survey of Norway (NGU).
Researchers from the NGU and the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) at the Arctic University of Norway (Tromsø) took several seafloor limestone samples, called seep carbonates, from active methane seeps at the Vestnesa Ridge in 1200 meters of water. Seep carbonates are formed through a microbial process in which methane generated in the sediments reacts with seawater sulfate close to the seafloor. These carbonates are therefore a very clear indication that methane has risen up through the sediments below. Detailed analyses of the seep carbonates have now shown that methane has been repeatedly emitted since the last glacial maximum 23,000 years ago.
In cooperation with scientists from MARUM, and using the sea floor drill rig MARUM-MeBo70 on an expedition with the research vessel MARIA S. MERIAN, the Norwegian researchers were able to retrieve seep carbonates from deeper layers in the sediment. Based on dates obtained from the drilled seep carbonates, using the natural radioactive isotopes uranium and thorium, the team was able to identify two additional episodes of methane leakage: one between 160,000 and 133,000 and another between 50,000 and 40,000 years ago. The drill cores that were retrieved contained records of active methane emissions at the Vestnesa Ridge at these times. These two periods are characterized by cold climate conditions, during which the ice volumes in the Barents Sea and on Spitsbergen increased significantly. The Earth's crust was depressed by the massive load of the ice cover. To compensate for this, the seafloor beyond the ice cover at Vestnesa Ridge rose. This led to movements along existing faults in the Earth's crust and allowed methane to rise from greater depths.
Over the past 23,000 years the ice has melted and the underlying land masses have been rebounding again. This, in turn, has led to subsidence at the Vestnesa Ridge due to isostatic compensation. The results obtained by the team indicate that methane was primarily emitted when the fault zones were activated by ice-sheet motions.
“Seep carbonates are geological archives for methane emissions at the seafloor. We are very pleased that we were able to drill these archives for the first time at the Vestnesa Ridge with the sea floor drill rig MARUM-MeBo,” reports Gerhard Bohrmann, chief scientist of the expedition. “Methane emissions are documented both during the build-up of ice on land as well as during its retreat.”
The MeBo drilling was carried out with financial support of the German Research Foundation (DFG – Deutsche Forschungsgemeinschaft) in cooperation with CAGE. The further investigations were financed by the Research Council of Norway under the project “Norwegian margin fluid systems and methane-derived carbonate crusts” (NORCRUST).
Prof. Dr. Gerhard Bohrmann
General Geology – Marine Geology
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Tobias Himmler, Diana Sahy, Tõnu Martma, Gerhard Bohrmann, Andreia Plaza-Faverola, Stefan Bünz, Daniel J. Condon, Jochen Knies, and Aivo Lepland: A 160,000-year-old history of tectonically controlled methane seepage in the Arctic. Science Advances 2019
Geological Survey of Norway, Trondheim (Norwegen)
MARUM - Zentrum für Marine Umweltwissenschaften, Universität Bremen
British Geological Survey, Nottingham (Großbritannien)
Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geosciences, UiT–The Arctic University of Norway, Tromsø (Norwegen)