Scientists from the USGS Coastal and Marine Hazards and Resources Program and Mississippi State University published a major new study that documents hundreds of previously unknown seafloor methane seeps on the U.S. Atlantic margin (USAM) and describes the sedimentary, geologic, and oceanographic processes responsible for the seeps’ formation. The published study sets the stage for the marine community’s next decade of interdisciplinary research on USAM seep evolution, seep biogeochemical processes, and benthic seep communities.  

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Gases and fluids routinely leak from the seafloor into the ocean, transferring carbon and other substances into marine waters. Where new seafloor forms at mid-ocean ridges, hot hydrothermal fluids emitted at features like black smokers can contain sulfides, dissolved metals, hydrogen, and methane. In other settings, underwater volcanoes belch large amounts of carbon dioxide into the oceans. The new study focuses on another, more common fluid flow feature—seafloor cold seeps. Cold seeps usually emit methane and can form far from plate boundaries or faults and in settings ranging from shallow-water river deltas to seafloor thousands of meters deep.  

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The recently published study uses water column imagery acquired during multibeam sonar surveys conducted on six research vessels from 2011 to 2016 to locate streams of gas bubbles leaking into the ocean at seafloor gas seeps. A previous study published in 2014 by some of the same scientists described the surprising discovery of more than 550 methane seeps between Cape Hatteras, North Carolina and Georges Bank, offshore Cape Cod. The new study adds about 1,400 new gas plume identifications on this part of the U.S. Atlantic margin. Removing possible duplicate plumes that may have been detected multiple times during different oceanographic cruises leaves about 1,100 unique seeps. When the seep locations are analyzed with spatial clustering algorithms, methane appears to be leaking from about 275 seep fields from offshore South Carolina to Georges Bank, including at 47 clusters containing 5 to 138 individual seeps. The largest clusters of seeps lie in the main channel of Hudson Canyon and in an unnamed canyon 18 kilometers south of Norfolk Canyon at a feature known as the Norfolk Seeps. 

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The new study combines the updated seeps databases with seismic data collected by the USGS in 2015 and during the 2018 MATRIX survey. The seismic data provide high-resolution images of seafloor features below the seeps. The images reveal faults, eroded sediments, gas chimneys, and submarine slide deposits that provide clues about the seeps’ formation.  

Most seeps described in the study are located on the uppermost continental slope at water depths shallower than approximately 500 meters (1,640 feet). This is too shallow for methane hydrate—a solid, icelike form of water and methane stable at cold temperatures and high pressures--to be present in the sediments. This means that methane emissions cannot originate with breakdown of methane hydrate directly beneath those seep sites. However, the paper shows that gas hydrate dynamics could explain seep formation on upper continental slope promontories overlooking major shelf-breaking canyons (e.g., Norfolk, Washington, Baltimore, and Wilmington Canyons).  

The new research also provides insight into rare seep fields mapped at water depths of about 1,000 and 1,400 meters (3,300 - 4,600 feet) offshore Virginia and Massachusetts. Methane gas leaking at these seep fields could be released through breakdown of gas hydrate in the underlying sediments.

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