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Cold seeps, which are locations where chemicals -- including methane and other hydrocarbons, brine, hydrogen sulfide, and sometimes carbon dioxide--leak from the seafloor, occur worldwide on both passive and tectonically-active continental margins. Cold seeps are distinguished from hydrothermal vents, which are warm-temperature seeps associated with mid-ocean spreading centers.
Certain microbes at seep sites use chemicals, rather than light, to create food for many species, a process known as chemosynthesis. As the microbes alter the water chemistry through the breakdown of methane, carbonate precipitates out of solution. The carbonate eventually builds up to form special rocks called methane-derived authigenic carbonate (MDAC).
Until 2012, only a few seeps were known from the U.S. Atlantic margin. Since then, hundreds have been discovered, but only a handful has been explored in detail. The ecology of these environments is characterized by dense communities of mussels whose tissues host bacteria that convert chemicals (e.g., methane and/or hydrogen sulfide) into energy. At multiple seeps within this region, vast mussel beds containing several different size classes of mussels have been discovered. This observation indicates that the fuel that the bacteria requires is working continuous “overtime,” enabling these mussels to recruit, grow, reproduce, and thrive. In the densely packed mussel beds, other organisms also are present, including mobile shrimp, sea cucumbers, and various polychaete worms.
Other habitats found at seeps include bacterial mats that create dense layers on the sediment surface, much like a carpet covering the mud, with anoxic/low oxygen environment in the sediments below. The presence of mats is a clue to overall seepage intensity, with high methane fluxes often associated with mats. These environments are extreme and are characterized by high sulfide concentrations that can be toxic to many organisms. Often the sediment fauna are composed of high densities of only a few types of animals that can tolerate these stressful environmental conditions.
Authigenic carbonate created by chemical reactions involving methane, seawater, and microbes, also provides a habitat for encrusting fauna, including deep-sea corals and sponges. These habitats are associated with reduced methane fluxes near seeps. Rock-associated animals typically rely on organic matter produced in the surface ocean for food, transported to them through currents and sinking of particles. The spaces within the rocks are home to many small animals, such as polychaete worms and small crustaceans, some of which graze microbes off the rock surfaces for nutrition.
The patchy distribution of mussel beds, bacterial mats, and authigenic carbonate, interspersed with soft sediments, create a mosaic of habitats, all interconnected in space. They also represent a successional sequence, from active seepage (e.g., bacterial mats) to relatively inactive seeps (e.g., authigenic carbonate). We are interested in the overall distribution of these habitat types and the associated diversity and composition (i.e., who is there and how the communities compare across habitat types) and how the chemistry influences the communities living within seeps.
For this expedition, we will record high resolution video observations of seep communities and make discrete collections of sediments, rocks, megafauna (e.g., mussels), and water to improve our understanding of the biology and ecology of US Atlantic seeps and the processes that influence these unique environments. Rock and mussel shell collections can provide key information on the timing, duration, and origin of fluid venting along the margin. By comparing our results from the U.S. Atlantic margin to other regions where we have conducted seep investigations, including the Gulf of Mexico, we can develop a broader understanding of seep environments and their links to ocean carbon dynamics, continental slope stability and related hazards, and the geographic extent of chemosynthetic communities.
The USGS Gas Hydrates Project is a multidisciplinary effort that is jointly supported by the Coastal and Marine Geology Program and the Energy Research Program at the USGS. Most personnel are based in Woods Hole, Massachusetts (Coastal and Marine) and in Denver (Energy). Additional personnel are located at the Santa Cruz Coastal and Marine Science Center, the Earthquake Program in Menlo Park, and the Minerals Program in Reston.