Archive of Spotlight Feature Articles

Voyage to the Aleutian Islands: Understanding Alaska's Deep Sea Frontiers

By Sapna Batish and Jennifer Reynolds*,
*Science Director, West Coast & Polar Regions Undersea Research Center

In the summer of 2004, NURP sponsored a pioneering scientific expedition to the waters surrounding a U. S. chain of islands that teem with marine life yet remain largely uninvestigated. The Aleutian Islands, most of which are part of Alaska, are located 2200 km (1200 nautical miles) west of the tip of the Alaska Peninsula. Their waters are home to some of the most productive fisheries in the world; and Dutch Harbor, on Unalaska Island, is currently the largest fishing port in the U.S. The Aleutians' waters also harbor cold-water corals, more commonly referred to as deep-sea corals.

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The Aleutians

The Aleutians' remote and dramatic location has significantly challenged scientific investigation of its waters. The islands sit on top of a continuous submarine ridge that divides the North Pacific Ocean and the Bering Sea. The Aleutians are part of the Pacific Ring of Fire, with approximately twenty-four active volcanoes and frequent earthquakes, caused by the collision of two of the tectonic plates that form the Earth's crust. Although a few good harbors can be found in the archipelago, numerous reefs and almost perpetual fog can prove dangerous to even the most skilled navigators.

Expedition Logistics

In July and August 2004, NOAA's Undersea Research Program (NURP) Center for the West Coast and Polar Regions organized and funded a research cruise to the central Aleutians on the R/V Roger Revelle. Expedition goals were to collect biological and substrate samples, map the sea floor, and capture video of areas never before documented.

The main tool in this research was the Jason II, a deep-diving remotely operated vehicle (ROV) operated by Woods Hole Oceanographic Institution, capable of diving to 6500 m (21,320 f). This was the first operation of a civilian deep-diving vehicle in the North Pacific Ocean or Bering Sea. Other research equipment included a towed deep-sea camera system (TowCam) and a 12 kHz multibeam sonar system on the ship's hull for mapping the seafloor.

Four research teams took part in the expedition. The first two groups departed Dutch Harbor for Leg 1 of the research cruise (July 10 - 24) to study an area south of Unimak Island.

Research Projects

Team 1, headed by Tony Rathburn of Indiana State University, focused on the Ugamak Slide (1), thought to be the site of a massive submarine landslide that may have caused the destructive, Pacific-wide 1946 tsunami. The objectives were to confirm whether the landslide occurred in 1946, as well as to study the recovery of the seafloor ecosystem on the landslide slope.

Multibeam mapping revealed a highly eroded, and therefore, mature seafloor that showed no evidence of a recent landslide. These unexpected findings leave the cause of the 1946 tsunami a mystery and indicate that a great deal more investigation is needed to pinpoint the cause of the tsunami.

(Top left) Bubble gum coral (Paragorgia sp.) shown covered with brittle stars was collected southwest of Amlia Island at 844m. Photo: Sonya Senkowsky, Alaska Science Outreach Reporter. (Top right) Manipulator arm of Jason II closes in on a black coral and deepwater king crab, at a depth of 2160m, just south of Amlia Island. This may be the first documentation of a king crab resting on a black coral colony, which is thought to be an important habitat. Photo: NOAA. (Bottom left) A gorgonian coral (Muriceides sp.) retrieved by Jason II at a depth of 490 m. Photo: Sonya Senkowsky, Alaska Science Outreach Reporter. (Bottom right) Samples of rock and associated invertebrates, retrieved by Jason II near Amlia Island. On the left: igneous dropstone, a rock of volcanic origin transported by a glacier and dropped at that site as floating ice melted. On the right: slab of siltstone, from an outcrop on the seafloor. Photo: Jennifer Reynolds, University of Alaska Fairbanks.

At the same location, biologists collected depth transects that will yield some of the first information about the mosaic of benthic communities on the Alaskan margin. Additionally, they identified both disturbed and undisturbed areas to help assess the biological impact of small-scale disturbance. They also discovered cold methane seeps at a water depth of 3000 m and documented a new type of deep-sea coral habitat that may be associated with methane seepage.

Team 2, headed by Randy Keller from Oregon State University, mapped and explored Derikson and Sirius Seamounts (2), two 50-60 million year old undersea volcanic mountains that are riding on the Pacific Plate as it approaches the Aleutian Trench. They found that the seamounts are being cut by deep, active faults as the plate bends downward into the trench. These faults may act as pathways for water circulation and the venting of warm, chemically altered fluids, which in turn have the potential to support vent-related ecosystems at the seafloor and in the rock fractures. Additionally, new species of deep-sea corals were retrieved with Jason II at this site.

After two weeks, the R/V Revelle returned to Dutch Harbor and exchanged science teams for Leg 2 of the research cruise (July 24 – August 8). NOAA Fisheries biologist, Dr. Robert Stone from the Auke Bay Laboratory in Juneau, Alaska, headed Team 3, which studied deep-sea coral and sponge distributions (3), habitat associations, and species diversity, along a 500-km region between Amlia and Semisopochnoi Islands.

Jason II dives documented a great variety of habitats down to a water depth of 3000 meters, from soft silt ponds to current-swept sandy banks, canyon walls, young volcanic cones, and glacially scoured rock outcrops. Deep-sea corals and sponges were widely distributed at the study sites with an apparent change in density, diversity, and species composition at a depth of approximately 1400 m. The ultimate goal is to construct a model that predicts the distribution and density of coral habitat throughout the Aleutian Islands; this model may be used to directly assist managers in developing methods to minimize fishing interactions with coral habitat.

(Left) Adak Island. Photo: Tom Bolmer, Woods Hole Oceanographic Institution (Right) Gene Yogodzinski, of the University of South Carolina, peruses seafloor maps during the cruise. For several days of the Aleutian research cruise, he led the use of Jason II to find and sample some of Adak canyon's oldest volcanic rocks. Photo: Sonya Senkowsky, Alaska Science Outreach Reporter.

Team 4, led by geologist Gene Yogodzinski from the University of South Carolina, devoted several days to investigating Adak Canyon (4), located immediately southwest of Adak Island. Adak Canyon is a steep-walled, tectonically active rift that appears to form a window into the inner structure of the Aleutian Ridge. New multibeam sonar maps of the seafloor were used to select dive targets, and the ROV dives successfully recovered plutonic rocks, the magmas related to earlier generations of Aleutian volcanoes. Geochemical studies of these rocks will provide information about how the crust of the Aleutian volcanic arc has grown over its 50 million year history, and possibly how magma from this kind of volcanism may have influenced the growth of continents throughout geologic time. Biologists utilized the ROV dives to investigate the distinct ecology of the canyon walls and floor.

For more information on the Aleutian Islands Research Cruise (Leg 1 and 2):
http://www.afsc.noaa.gov/abl/MarFish/coralscruise.htm, http://www.alaskascienceoutreach.com/coralsite/

[12/13/04]

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