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Discovery of a natural CO2 seep in the German North Sea: Implications for shallow dissolved gas and seep detection

Schmidt, Mark
DelSontro, Tonya
Themann, Sören
Rovelli, Lorenzo
Reitz, Anja
Linke, Peter
Published in Journal of Geophysical Research. 2011, vol. 116, no. C3
Abstract A natural carbon dioxide (CO 2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO 2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East‐Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO 2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO 2 concentration. These results are presented in the context of CO 2 seepage detection, in light of proposed subsurface CO 2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO 2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete‐bubble modeling suggest that shallow CO 2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble‐plume modeling further shows that a CO 2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO 2 ‐enriched water tends to sink toward the seabed. Results suggest that released CO 2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor.
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MCGINNIS, Daniel Frank et al. Discovery of a natural CO2 seep in the German North Sea: Implications for shallow dissolved gas and seep detection. In: Journal of Geophysical Research, 2011, vol. 116, n° C3.

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Deposited on : 2014-10-06

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