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Widespread methane leakage from the sea floor on the northern US Atlantic margin

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Abstract

Methane emissions from the sea floor affect methane inputs into the atmosphere1, ocean acidification and de-oxygenation2,3, the distribution of chemosynthetic communities and energy resources. Global methane flux from seabed cold seeps has only been estimated for continental shelves4, at 8 to 65 Tg CH4 yr−1, yet other parts of marine continental margins are also emitting methane. The US Atlantic margin has not been considered an area of widespread seepage, with only three methane seeps recognized seaward of the shelf break. However, massive upper-slope seepage related to gas hydrate degradation has been predicted for the southern part of this margin5, even though this process has previously only been recognized in the Arctic2,6,7. Here we use multibeam water-column backscatter data that cover 94,000 km2 of sea floor to identify about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the northern US Atlantic passive margin. About 440 seeps originate at water depths that bracket the updip limit for methane hydrate stability. Contemporary upper-slope seepage there may be triggered by ongoing warming of intermediate waters, but authigenic carbonates observed imply that emissions have continued for more than 1,000 years at some seeps. Extrapolating the upper-slope seep density on this margin to the global passive margin system, we suggest that tens of thousands of seeps could be discoverable.

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Figure 1: Distribution of methane seeps located on the US Atlantic margin using water-column backscatter data.
Figure 2: Seep and pockmark distributions and seafloor photographs at upper-slope and deepwater seep sites offshore Virginia.
Figure 3: Schematic showing the relationship of the US Atlantic margin seeps to morphologic and geologic features.

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Acknowledgements

The National Oceanographic and Atmospheric Administration (NOAA) Office of Ocean Exploration and Research funded the 2012/2013 Atlantic canyons mapping project and managed the acquisition of the data used in this study with the vessel Okeanos Explorer and ROV Deep Discoverer. C. Van Dover analysed the length scales for Fig. 2. W. Waite, A. Demopoulos, L. Brothers and J. Chaytor provided advice and comments. C.R. had support from US Department of Energy-USGS interagency agreement DE-FE0006781. M.K. was financially supported by a NOAA Hollings Scholarship in summer 2013. Fledermaus Mid-Water software for A.S. and M.K. was provided by QPS. Mention of trade names does not imply US Government endorsement of commercial products.

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Contributions

A.S. collected some of the multibeam data, devised the approach for analysis of the water-column anomalies, conducted the second full analysis of the plumes, completed the cluster analysis and prepared the plume database. C.R. wrote the paper, prepared most of the figures, and performed the video analysis and flux calculations. M.K. completed the first full analysis of the backscatter data set. D.B. contributed the pockmark database and assisted with geologic interpretations and map preparation. E.L. collected some of the multibeam data and analysed the episodicity of some seeps using repeat multibeam data sets.

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Correspondence to A. Skarke.

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The authors declare no competing financial interests.

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Skarke, A., Ruppel, C., Kodis, M. et al. Widespread methane leakage from the sea floor on the northern US Atlantic margin. Nature Geosci 7, 657–661 (2014). https://doi.org/10.1038/ngeo2232

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