In summer 2013, U.S. Geological Survey (USGS) scientists worked with colleagues from the National Oceanic and Atmospheric Administration (NOAA) and several academic institutions to explore submarine canyons, landslides, methane seeps, and seamounts off the northeast U.S. Atlantic coast. “Telepresence” video technology enabled many of them to participate in the expedition of the NOAA Ship Okeanos Explorer without actually being onboard. The goal of this two-leg, 36-day research cruise was to explore and characterize the diversity of benthic habitats and geologic features on the continental margin in this region (Northeast U.S. Canyons Expedition 2013).

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The USGS participants were Amanda Demopoulos (science lead on leg 2; Southeast Ecological Science Center, Gainesville, Florida); Jason Chaytor, Carolyn Ruppel, Uri ten Brink, Shannon Hoy, and Daniel Brothers (Woods Hole Coastal and Marine Science Center, Woods Hole, Massachusetts); Christina Kellogg (St. Petersburg Coastal and Marine Science Center, St. Petersburg, Florida); and Cheryl Morrison (Leetown Science Center, Kearneysville, West Virginia).

The cruise ran from July 8 to August 16, 2013, using NOAA Ocean Exploration’s two-body remotely operated vehicle (ROV) system, which consists of the Seirios camera sled tethered to the Deep Discoverer (D2) ROV, rated to 6,000 meters (20,000 feet) water depth. During 31 dives, the ROV visited ten major and minor canyons, several landslide scars, methane seeps, and Mytilus Seamount (see map). Multibeam bathymetric data (water depths) and subbottom profiles (images of sediment layers beneath the seafloor) were collected over areas of geologic interest, and full water-column CTD (conductivity [related to salinity], temperature, depth) casts were performed when the ROV was out of the water.

The telepresence capability supplied by the University of Rhode Island (URI) Inner Space Center and NOAA’s Ocean Exploration Research (OER) program enabled most USGS scientists to participate in the ROV’s discoveries from their own offices and homes, or occasionally from the Exploration Command Center located at the URI Bay Campus. During each dive, imagery acquired by the ROV was supplied to ship- and shore-based participants and the public in near real-time by high-bandwidth satellite and Internet connections. OER also facilitated real-time audio and text communication among participating scientists and hosted daily teleconferences to plan dive strategies and summarize findings. USGS scientists were involved in planning specific ROV dives, advising on ROV maneuvers at the seafloor, and identifying geologic and biologic features as they were encountered by the ROV.

The ROV dives marked the first time that many of the environments were ever examined visually. The new observations significantly enhance understanding of Atlantic continental margin areas that the USGS has studied for more than four decades. The data will also be critical to planning future USGS hazard, energy, and biology studies in the region.

Submarine Canyons

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Found throughout the world, submarine canyons are complex seafloor features that cut across continental margins, linking the continental shelf to the abyssal plain and creating scenic seascapes reminiscent of their terrestrial counterparts. The steep walls and hard substrates exposed in submarine canyons serve as habitats for diverse communities of deep-sea corals, fish, anemones, sponges, crabs, and many other invertebrate species. The northeast U.S. Atlantic margin is incised by numerous major and minor submarine canyon systems cutting through sequences of sediment and rock units of Cretaceous to Pleistocene age (approximately 100 million to 2 million years old) that are mantled by a mix of fluvial and glacial sediments transported across the region since the Last Glacial Maximum (approximately 20,000 years ago). The sheer scale, complexity, and interaction of the geologic and biologic systems in these canyons and the adjacent parts of the slope require an interdisciplinary approach, involving geologists, physical oceanographers, biologists, ecologists, and modelers.

Although some canyons visited during the cruise have been previously explored via submersible dives (some conducted by USGS scientists), exploration using NOAA’s D2 ROV system allowed for high-definition imaging of the margin stratigraphy exposed in the canyon walls, the long- and short-term erosion and morphological modification of the canyons, and the complex and multifaceted relationships of their geology and biology. Key observations included the distribution of different rock types; the style, timing, and size of wall collapse and sediment transport; the extensive evidence of biologic disturbance of rock and sediment and its role in erosion; and the documentation of large (tens of centimeters wide, up to meters long) burrows or fluid-transport features, many with open central apertures exposed along sections of the walls within several canyons.

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Exploring and imaging submarine canyons facilitated a better understanding of the distribution, species diversity, habitat affinities, and patchiness of enigmatic deep-sea animals. The canyons were found to contain diverse communities of habitat-forming foundation species (for example, corals and sponges) that host a wide variety of associated fauna, including, but not limited to, squat lobsters, brittle stars, shrimp, crabs, fishes, shark egg cases, and octopuses. Population size and species diversity of canyon fauna appeared to be a function of water depth, the presence and strength of bottom currents, and substrate type and morphology. Thus, the geology, biology, and ecology of submarine canyon environments are tightly intertwined.

Submarine Landslides

Five ROV dives were carried out over features related to submarine landslides being studied as part of a regional assessment of U.S. Atlantic margin landslide and tsunami hazards by USGS scientists at the Woods Hole Coastal and Marine Science Center. These dives provided the opportunity to investigate ongoing destabilization and modification of old landslide scarps and to evaluate the use of visual observations in constraining the relative ages of the most recent landslides. Between dives, the Okeanos Explorer conducted new high-resolution multibeam mapping and subbottom profiling in areas identified by the USGS as critical to the evaluation of landslide hazards in the region. The new bathymetric and subbottom data will aid in constraining the size and approximate age of submarine landslides along this part of the U.S. margin and assist planning for future investigations.

Methane Seeps

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Three dives were conducted in areas hosting multiple methane seeps at more than 1,000-meter (3,000 foot) water depths on the Nantucket margin, about 165 kilometers (100 miles) south of the island of Nantucket, Massachusetts. Such deepwater seeps are always considered anomalous because they occur well within the gas hydrate stability zone, where the pressure-temperature conditions dictate that methane should combine with water to form an ice-like solid. On the southeastern U.S. margin—off the Carolinas, Georgia, and Florida—deepwater seeps occur above salt formations that heat the sediments and break down the gas hydrate, but no such salt deposits are known on the Nantucket margin.

The ROV dives confirmed that water-column anomalies discovered during a November 2012 cruise of the Okeanos Explorer correlated with seafloor gas seeps. The ROV found gas hydrate forming at the seafloor at some seeps. The seeps were also characterized by special carbonate minerals formed as a result of anaerobic methane oxidation, mussel communities that rely on methane or hydrogen sulfide, and widespread bacterial mats. The ROV dives expanded the distribution of known gas hydrates on the northern Atlantic margin and constrained methane flux by collecting imagery that can be used to track gas-bubble sizes and emission rates. This set of dives also provided valuable information on the biodiversity of chemosynthetic communities at the newly discovered seeps and a basis for comparison with seeps that had been previously described on the southeastern U.S. margin.

Mytilus Seamount

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Mytilus Seamount, one of the least explored in the New England seamount chain, was visited during two dives, one on the north side and another on the south side. Seamounts represent oases of life, containing rich environments for deep-sea fauna. By systematically exploring the seafloor and surrounding area, the ROV collected critical baseline information that will be needed for better management and conservation of these environments. Stacked, manganese-coated basalt outcrops characterized the geology of the north side, whereas areas of sediment and talus accumulation were interspersed among basalt outcrops on the south side. No scleractinian (stony) corals were observed at the seamount, but several species of fishes, octocorals (which lack a stony outer skeleton), sponges, and other benthic invertebrates were documented.

More to Come

The breadth of scientific discoveries made during the recent series of ROV dives resulted from the outstanding operational support of NOAA OER and the URI Inner Space Center and the cooperation of many scientists at NOAA, the USGS, and U.S. and international academic institutions. The ROV imagery and associated data sets collected during these recent cruises, and continued collaborations as part of future exploration of the submerged margins of the United States, will provide data to support and inform aspects of the USGS science mission for years to come.