Cold Seeps off the U.S. Atlantic Coast: Methane-Fuelled Oases on the Deep Seafloor

Methane bubbling from the seafloor off Virginia, north of Washington Canyon, ringed by quill worms, anemones and microbial mats. Image from NOAA Ocean Exploration.

Cold seeps are places where salty fluids rich in hydrocarbons, such as methane and sulfides, leak out of the seafloor at about the temperature of the surrounding water. In 2019 NOAA's Windows to the Deep expedition off the southeastern United States planned to explore at least one near the North Carolina coast. Cheryl Morrison of the USGS, a co-science lead, explained why seeps matter.

Oases in the deep

Seeps feed whole communities through chemosynthesis: bacteria turn chemicals like hydrocarbons into food. Some bacteria live free and form mats visible on the seafloor. Others live inside tube worms or mussels, which shelter them from predators while the bacteria feed their hosts. These partnerships let mussels and tube worms dominate seeps, building complex habitat for squat lobsters, crabs, shrimps, sea cucumbers, worms and snails. With food plentiful where the deep sea usually has little, seeps are oases that draw animals from all around.

A recent discovery

Such communities were discovered only recently. The first cold seep was found in 1983 by Charles Paull at the Florida Escarpment in the Gulf of Mexico, 3,200 meters down. Since then, imaging of the water column and better tools for extreme environments have revealed many more.

Bubble plumes from seeps detected acoustically by NOAA Ship Okeanos Explorer, with seafloor depth and backscatter. Image from NOAA Ocean Exploration.

Until recently, the only seep confirmed by sight off the southeastern U.S. Atlantic coast was at the Blake Ridge Diapir, found in 1995. Then mapping by NOAA Ship Okeanos Explorer picked up bubble plumes, signs of leaking hydrocarbon gas, suggesting there could be up to 550 seeps along the U.S. Atlantic seaboard, though not every plume means a strong seep or a thriving community.

Methane seeps (red) from NOAA surveys and the 2014 database of Skarke and others, updated by USGS surveys in 2016 and 2017; yellow marks seeps targeted by Okeanos Explorer in 2018. Image by Carolyn Ruppel, USGS.

Seeing is believing

Proving a seep community takes eyes on the bottom: remotely operated vehicles, crewed submersibles like Alvin, or autonomous vehicles. Dives have confirmed seeps near canyons off the Northeast and in the Mid-Atlantic. Because hydrocarbons leak in patches, seep communities are patchy too, often far from their nearest neighbours, so each new site may hold familiar species or ones never seen before.

The ROV Deep Discoverer. Image from NOAA Ocean Exploration.

Mussels by depth

At known seeps off North Carolina, bathymodiolin mussels dominate. Their larvae can drift high in the water and ride fast currents a long way. Which species lives at a seep depends on depth, with a change below about 1,000 meters: in the Gulf of Mexico, Bathymodiolus childressi dominates shallower seeps, while B. heckerae, known from 2,200 to 3,300 meters, lives at deeper Gulf seeps and at the Blake Ridge Diapir, 2,500 meters down. Because the species are hard to tell apart by eye, scientists sequence DNA from tissue samples.

Bathymodiolus mussels, living and dead, under a carbonate overhang with methane hydrate. Image from NOAA Ocean Exploration.

Many seeps remain to be found. Knowing where these communities are and what lives there helps manage offshore resources and shows how these islands of life may be connected around the world.

Sources

• Cheryl Morrison, USGS, "An Update on Cold Seeps in the Northwestern Atlantic Ocean," NOAA Ocean Exploration, Windows to the Deep 2019; rewritten in hubnx's own words. Images from NOAA Ocean Exploration and the USGS.