|Home||Archived February 20, 2019||(i)|
In August and September 2008, a scientific expedition mapped unexplored Arctic Ocean sea floor where the United States and Canada may have sovereign rights over natural resources such as minerals, oil, and gas. Scientists from the U.S. Geological Survey (USGS) and the Canadian Geological Survey jointly conducted bathymetric surveys (mapping of sea-floor depths) and seismic-reflection surveys (imaging of rock and sediment layers beneath the sea floor) in the Beaufort Sea, a part of the Arctic Ocean north of Alaska and western Canada. The data gathered during this highly successful mission will improve understanding of the regional geology and help both countries establish the outer limits of their extended continental shelves according to criteria set out in the United Nations Convention on the Law of the Sea (UNCLOS). The extended continental shelf—the sea floor and sub-sea floor beyond 200 nautical miles from shore that meet UNCLOS criteria—is an area of great scientific interest and potential economic development.
U.S. participation in the partnership was sponsored by an interagency Extended Continental Shelf Task Force, composed of representatives from the USGS, the Department of State, the National Oceanographic and Atmospheric Administration, and several other governmental agencies.
The surveys were conducted from two Arctic icebreakers working in tandem: U.S. Coast Guard (USCG) Cutter Healy, commanded by Captain Frederick Sommer; and Canadian Coast Guard (CCG) Ship Louis S. St-Laurent ("Louis"), commanded by Captain Mark Rothwell. Jon Childs (USGS Coastal and Marine Geology Program [CMGP], Menlo Park, Calif.) served as chief scientist aboard Healy, and Debbie Hutchinson (CMGP, Woods Hole, Mass.) participated as principal investigator and USGS representative aboard Louis, with chief scientist Ruth Jackson of the Canadian Geological Survey (Halifax, Nova Scotia). Tom O'Brien, Bill Danforth, and Ellyn Montgomery (CMGP, Woods Hole) and Peter Triezenberg (CMGP, Menlo Park) composed the USGS scientific staff aboard Healy. The USGS contingent on Healy was rounded out by Jessica Robertson (Office of Communications, Reston, Va.), who served as media coordinator and outreach specialist.
Louis departed Kugluktuk, Nunavut, Canada, on August 22 and collected bathymetric and seismic-reflection profiles for 18 days in the Beaufort Sea before rendezvousing at 82°52' N latitude and 141°54' W longitude on September 9 with Healy, which had departed Barrow on September 6. The two vessels worked in tandem for another 18 days, with Healy escorting Louis during seismic-reflection profiling, and Louis escorting Healy when heavy ice conditions precluded seismic-reflection profiling. The two ships parted company at about midnight on September 27, Louis returning to Kugluktuk and Healy to Barrow.
The surveying systems on both ships rely on acoustic (sound) signals that bounce off surfaces separating materials of different density—for example, the boundary between water and sediment (the sea floor), or the boundaries between rock or sediment layers of different types. The frequency of the acoustic signals, produced by sound sources either mounted on or towed behind the vessel, determines how deeply the sound will penetrate beneath the sea floor and how much detail (resolution) will be revealed in the resulting images. Higher frequencies provide greater resolution but less penetration below the sea floor; lower frequencies yield more penetration but less resolution. (For more information about seismic profiling, see Seismic Profiling systems.)
Louis' seismic-reflection profiling system—used to image rock and sediment layers as deep as 10 km beneath the sea floor—employed airguns to produce acoustic signals and a streamer of hydrophones to pick up the returned signals. The source array consisted of three Sercel G-guns with a total volume of 1,190 in3 ; the receiver was a digital multichannel streamer with 16 channels (6.25 m per channel in two active sections, each 50 m long). The streamer and the airgun cluster were deployed from a robust, weighted sled towed immediately aft of the icebreaker at a depth of approximately 12 m, which placed them beneath the pack ice. During the program, approximately 2,800 km of seismic-reflection profiles were collected, of which approximately 1,500 km were shot with Healy breaking ice for Louis.
Healy is equipped with a Seabeam multibeam echosounder for mapping sea-floor depths. Unlike single-beam systems, which measure depth to a single point directly beneath the ship and produce a line of depths as the ship moves forward, the multibeam system measures depths along a line perpendicular to the ship's path and produces a swath of depth data as the ship moves forward. The width of that swath depends on the water depth; Healy's multibeam system collected a swath of bathymetric data approximately 10 km wide in the 3,800-m-deep Beaufort Sea. The original science plan called for Healy to be the lead vessel throughout the program. Early in the two-ship operation, however, it became apparent that the multibeam data collected under the heaviest ice conditions were unusable because the ice interfered with signal transmission from the hull-mounted echosounder transducers. When Louis took over as lead vessel, both the multibeam data and the data from Healy's 3.5-kHz subbottom profiler (a high-resolution, low-penetration seismic-reflection system, also hull mounted) were greatly improved. Approximately 950 km of multibeam and profile data were collected with Louis breaking ice for Healy.
The data from this program and from an earlier survey conducted in 2007 by the Canadians from a single ship with USGS observer and emeritus Bob Rowland aboard are now being interpreted for the new insights they will reveal concerning the geologic history of the Canada Basin and the entire Arctic region.
To prevent the scientific sound sources from harming marine mammals, each ship carried a team of lookouts, or "observers"—three on Louis and two on Healy—with the sole duty of watching for marine mammals. Most of these observers were from the Canadian and Alaskan Native communities and were experts at spotting and identifying marine animals on the ice. When an animal was nearby, the observer alerted the ship's officers so that they could alter the ship's path and ensure that the animals were not disturbed.
In addition to conducting bathymetric and seismic-reflection surveys, the 2008 expedition collected data on the type and extent of sea-ice coverage and the distribution in Arctic waters of microorganisms called mixotrophs. Pablo Clemente-Colón, chief scientist of the National Ice Center and an oceanographer with the National Oceanic and Atmospheric Administration (NOAA), routinely monitored the ice coverage visually and recorded what types of ice were dominantly present. These data were used to validate satellite remote-sensing observations of ice coverage used by various groups, including the National Weather Service, the U.S. Navy (particularly for submarine Arctic crossings), and the U.S. Coast Guard for safety of navigation, life, and property at sea. The data can also be used for fisheries support and research, oceanographic and atmospheric modeling, and much more. Rebecca Gast (Woods Hole Oceanographic Institution) and Robert Sanders (Temple University) collected and studied mixotrophs—algae that eat bacteria as well as use sunlight for photosynthesis, potentially helping them thrive in the extreme polar environment. Gast and Sanders used a conductivity-temperature-depth (CTD) profiler to collect data and water samples at various depths; then they incubated the water with particles that the mixotrophs can eat, and detected the organisms by microscopic analysis. Their study aims to improve understanding of the food chain, carbon cycle, and nutrient cycle in the Arctic Ocean.
For more information about the expedition, including photographs and video footage, visit Arctic Chronicles.
Plans are underway for a second two-ship expedition with Healy and Louis, currently scheduled for August 7-September 16, 2009.
in this issue:
|Home||Archived February 20, 2019|