Atlantic Methane Seeps Surprise Scientists

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Recent scientific work has confirmed the source, composition and origin of methane seeps on the Atlantic Ocean seafloor, discovered in 2012, where scientists never expected them to be.

For the first time, scientists have determined the origin of the gas, its method of formation, the longevity of the seeps, and the source of the venting fluids. The methane seeps represent a new source of global methane not previously accounted for in carbon budgets from this region.

Image: Methane Seeps along Offshore Virginia
Methane gas bubbling up out of cold seeps on the Atlantic Ocean floor offshore Virginia. Image courtesy of NOAA Okeanos Explorer Program, 2013 Northeast U.S. Canyons Expedition.Public domain

Hundreds of methane seeps along the Atlantic seafloor were remotely imaged through camera and mapping work, but uncertainty remained as to the origin and history of the seepage. In a new study published this week, U.S. Geological Survey scientists and their collaborators, including the British Geological Survey, describe the chemistry of the methane seeps by analyzing seafloor carbonate rocks, and deep-sea mussels that were collected aboard a ship by remotely operated vehicles.

When initially discovered, the scientists were able only to photograph and map the seeps as places on the seafloor where gasses were bubbling up. Now with chemical analyses, scientists have confirmed that the bubbling gas is indeed methane, and have confirmed the methane originates from the microbial decomposition of organic matter in the seafloor sediment. Hidden in the unique isotopic signature of the carbonate rocks of the seafloor is a robust chemical fingerprint revealing the origin and age of the methane seeps. These results provide the detailed insights into the chemical conditions under which the carbonate rock of the seafloor was formed.

Carbonate rocks, such as limestone or dolomite, can form at the sediment-water interface near the seafloor, where they are found today. The same chemical (isotopic) analyses confirmed that the seeping methane did not originate from a deep-seated reservoir in the sediments similar to the reservoirs found in major hydrocarbon basins such as the Gulf of Mexico.

After the methane is initially formed by decomposition of organic matter in the seafloor sediment, other microbes convert the methane to a food source. The process of metabolizing methane raises the pH (alkalinity) of the surrounding water, making the conditions just right for the precipitation of calcium carbonate, forming the carbonate or limestone rocks themselves.

“Chemical analysis of the carbonate rocks on the seafloor indicate that the methane seeps were active at least 15,000 years ago when the rock was formed, and they are still actively venting today,” said USGS geologist and lead author Nancy Prouty. “The longevity of methane releases and the age of formation of the carbonate rocks were a surprise to us given the passive northern Atlantic margin had long been considered relatively inactive.”

“Most seeps described in the new study are too deep for the methane to directly reach the atmosphere, but the methane that remains in the water column can be oxidized to carbon dioxide,” said USGS research geophysicist and co-author Carolyn Ruppel. “This methane activity plays a significant role in oceanic cycling of carbon, and forms the basis for an amazing vast ecosystem, fueled, not from the sun’s energy, as most life on Earth’s surface is, but from chemosynthetic communities of bacteria. The Atlantic margin is home to hundreds of these widely distributed seeps that support these unique communities.”

The research paper, “Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps,” by Nancy Prouty and others was published in the journal, Earth and Planetary Science Letters. The research was part of a multi-agency study with USGS, the Bureau of Ocean Energy Management and the National Oceanic and Atmospheric Administration.

 

vast field of mussels on Atlantic seafloor
A field of deep-sea mussels (Bathymodiolus sp.) on the Atlantic margin seafloor near a cold methane seep.Public domain
methane gas bubbling up from the seafloor, surrounded by mussels
Methane gas bubbles rise from the seafloor – this type of activity, originally noticed by NOAA Ship Okeanos Explorer in 2012 on a multibeam sonar survey, is what led scientists to the area.Public domain
Cusk fish under a rock ledge.
A large deep-sea fish called a Cusk (Brosme brosme) sheltering under a rocky carbonate ledge, surrounded by soft sediment and live seep mussels.Public domain
map of the U.S. Atlantic margin showing continental shelf and ocean depth
Northern U.S. Atlantic Margin map, showing major canyons that cut through the continental shelf. Boxes highlight the locations of Baltimore Canyon and Norfolk methane seep fields on the US Atlantic margin.  These sites are the focus of a just-published 2016 study highlighting methane sources and the age of methane emissions.  The yellow, red, and green circles denote methane seeps discovered earlier on the US Atlantic margin.