by Taylor Heyl, Woods Hole Oceanographic Institution
The continental shelf and slope off the northeastern U.S., the underwater edge of the continent that borders the Atlantic Ocean basin, hosts an incredible diversity of habitats including approximately 70 submarine canyons ranging from depths of approximately 100 meters to 3,500 meters, etched by rivers thousands of years ago when this region was above sea level.
These canyons remain little explored but the topography, currents, and sedimentation in and around these submarine canyons support complex systems including chemosynthetic ecosystems in the form of cold seeps.
Cold seeps are patchily distributed, ephemeral environments that occur most frequently along tectonically active and passive continental margins from intertidal to hadal depths, in areas of the seafloor where hydrogen sulfide, methane, highly saline water, and other hydrocarbon-rich fluids escape into the water column. This process can result in the formation of oil seeps, methane, and gas hydrate seeps; brine seeps; pockmarks; and mud volcanoes, although methane seeps are common along continental margins in areas of high primary productivity where crustal deformation and compaction drive emissions of methane-rich fluid.
Seeps have global significance for the transfer of methane carbon from long-term storage in ocean-floor sediments into the ocean where methane released into the water column can be oxidized to carbon dioxide, often leading to changes in ocean chemistry. The discovery and mapping of deep-ocean seeps is essential to understanding the future global carbon budget and renewable energy resources, including hydrocarbon and gas hydrate reservoirs along the U.S. continental slope.
Seeps are now known to exist throughout almost all depth ranges and in all of the global oceans. In 2012, approximately 50 potential distinct seafloor gas seeps were identified by the Okeanos Explorer multibeam sonar along the Eastern U.S. continental margin and these were targeted sites for follow-on investigation in the spring and summer of 2013. Since these initial discoveries, a substantial number of potential seep sites have been identified in Okeanos Explorer water column mapping data.
Biological communities associated with deep-water seep environments are fueled through the process of chemosynthesis (deriving energy from chemical compounds instead of light) and also chemoautotrophic symbiosis (using a symbiont to derive energy from chemical compounds). In the Western Atlantic, seep communities have been described from mud volcanoes and diapirs between 1,000 and 5,000 meters depth in the Barbados accretionary prism area and from the Blake Ridge diapir at a depth of 2500 m off the coast of South Carolina.
Within the past two years, many more seep locations have been discovered off the Eastern Coast of the United States in the Northern Atlantic (Okeanos Explorer cruises EX1201, EX1204, EX1205L1, EX1205L2, EX1206, EX1301, EX1302, EX1303).
In the spring of 2013, during the Deepwater Canyons cruise, a new seep site previously detected with Okeanos Explorer’s multibeam sonar near Norfolk canyon was investigated and an extensive seepage area of at least a kilometer long and hundreds of meters wide was discovered, hosting large communities of bathymodiolid mussels and red crabs. Sea cucumbers, fish, and shrimp were also seen living associated with the seep.
An engineering field expedition to the North Atlantic canyons with the Okeanos Explorer and new remotely operated vehicle (ROV) Deep Discoverer in May of 2013 yielded several new seep discoveries, including one photo-documented with high-definition imagery near Veatch Canyon off the coast of Cape Cod, Massachusetts.
Cold seeps develop unique topography over time, where reactions between methane and seawater and in many cases, the influence of microbial activity, create carbonate rock formations and habitat for diverse faunal assemblages. When methane seepage decreases, the dominant fauna supported by the reduced chemicals decrease in abundance and as the seep becomes inactive, carbonate deposits can become potential habitats for other non-seep organisms, such as cold-water corals.