Understanding of methane seep formation and prevalence on US Atlantic margin advanced by new NOAA study

A major new study  on the distribution and origin of methane seeps on the US Atlantic margin — the seafloor between the coast and the deep ocean — expands the inventory of methane seeps (also called cold seeps) between the South Atlantic Bight and Georges Bank and explores the processes that contribute to their formation.

Until 2014, the US Atlantic margin was not generally considered an area of widespread seepage given its known geological features and lack of tectonic activity. However, using water column backscatter data collected using NOAA Ocean Exploration's multibeam sonar mapping system on NOAA Ship Okeanos Explorer between 2011 and 2013, scientists proved otherwise.

Since then, technologies and methods have evolved, more data have been collected, and more studies have been done.

Study findings

In a recent open-access research article in the journal Marine Geology, scientists report on a new and exhaustive study of US Atlantic margin seeps. The study combines water column data collected along thousands of kilometres of surveys during expeditions on Okeanos Explorer and other US research vessels with US Geological Survey (USGS) seismic data that images tens to hundreds of metres below the seafloor at seep sites.

In the article, the scientists:

• Explore how faulting, erosion, sediment mass transport processes, canyon formation, and salt diapirism contribute to the evolution of gas seeps and the migration of fluids to feed seep gas emissions at the seafloor.
• Compile all known information about US Atlantic margin seep fields that have already been assigned names by scientists to guide future exploration and research.
• Share their analysis and make available a major update to the 2014 seeps database.

The updated seeps database released simultaneously with this research article adds hundreds of new seep identifications based on water column data collected between 2011 and 2016 and removes probable duplicates. Among the additional findings are the following:

• Ninety-eight percent of the ~1,100 unique seep sites are between Cape Hatteras, North Carolina, and the southern New England margin.
• Spatial clustering analyses identify methane emissions in ~275 distinct areas, including 47 clusters with five to 138 discrete seeps.
• Most seeps are shallower than ~500 metres (1,640 feet) below sea level in the Mid-Atlantic Bight (Cape Hatteras to New York) and shallower than ~400 metres (1,312 feet) below sea level on the southern New England margin.

The new seeps database is publicly available through NOAA's National Center for Environmental Information.

Study importance

Methane seeps are important deep-ocean habitats, and methane emissions move carbon from the seafloor to the ocean, where it can be consumed by microbes. Methane is also a potent greenhouse gas, although most methane emitted at seeps is destroyed in the water column before it reaches the atmosphere.

The NOAA said the results of this study are a significant contribution to the understanding of seeps along the US Atlantic Margin. Nevertheless, the authors note that seeps have not been fully mapped on all parts of the margin and that many seeps have yet to be explored (e.g., via remotely operated vehicle).