Mission Plan
View of USCG HEALY and Canadian Coast Guard Cutter LOUIS S. ST. LAURENT working together. Click image for larger view.
Credit: UNH/NOAA
2010 U.S.-Canada Arctic Continental Shelf Survey
In August 2010, scientists from the United States and Canada will work together for the third consecutive year to map Arctic Ocean seafloor north of Alaska and west of the Canadian Arctic Islands. They are using two icebreakers to collect data that will help determine the limits of the "extended continental shelf." Previous joint missions were conducted in 2008 and 2009.
The five nations with Arctic coastlines—Norway, Russia, Canada, Denmark (Greenland), and the United States—are in various stages of defining the outer limits of their continental shelf in the Arctic in accordance with international law. Knowing where these limits lie is important because coastal states have sovereign rights over the continental shelf for the purpose of exploring and exploiting its natural resources—including those resources on the seabed (such as deep-water coral communities or mineral crusts and nodules) and beneath the seabed (such as oil and gas). Under international law, as reflected in the U.N. Convention on the Law of the Sea, every coastal country has a continental shelf out to 200 nautical miles (nm) from its coastal baselines, or to a maritime boundary with another coastal country. However, the continental shelf of a coastal country extends beyond 200 nm (the "extended continental shelf") if it meets the criteria outlined in Article 76 of the Convention. (Note that this legal definition of "continental shelf" is different from the definition traditionally used by marine geologists.) The criteria for determining the limits of the extended continental shelf relate to the depth and shape of the seafloor and the thickness of underlying sediment layers. The primary mission of the 2010 U.S.-Canada Arctic survey is to gather data so that these criteria can be applied to the U.S. and Canadian continental margins in the Arctic.
Planned tracklines for the 2010 U.S.-Canada Extended Continental Shelf survey. "EEZ" stands for Exclusive Economic Zone, which extends from a nation's coastline out 200 nautical miles (or to a maritime boundary with another nation). nm, nautical miles; km, kilometers. Click image for larger version or download a hi-res pdf. Credit: USGS
The 2010 survey is taking place primarily in the Canada Basin of the Arctic Ocean, in an area bounded approximately by latitudes 71°N to 85°N and longitudes 158°W to 112°W (see trackline map). This will be the first joint expedition to image sub-seafloor sediment layers within 200 nm of the coast in the Arctic Ocean.
The surveying systems on both ships rely on acoustic (sound) signals that bounce off surfaces separating materials of different "acoustic impedance"—the product of the speed of sound in the material times its density. Examples of such surfaces are the boundary between water and sediment (the seafloor) and the boundaries between rock or sediment layers of different types. The frequency and energy 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 seafloor 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 deeper penetration but less resolution.
Ship using multibeam echosounder to map overlapping swaths of seafloor. Credit: Fisheries and Oceans Canada Web page.
3D view of seamount rising from the abyssal plain at a depth of more than 3800 m to a least depth of 2622 m, constructed from multibeam data collected during the 2009 mission. View toward southwest. The background is the best available bathymetry for the region indicating the apparent isolation of this feature. In the far background (approximately 240 miles/440km) away is the Chukchi Plateau. Click image for larger version. Credit: CCOM/JHC University of New Hampshire
The U.S. Coast Guard Cutter Healy is using a hull-mounted multibeam echosounder to measure bathymetry, providing information about the depth and shape of the seafloor. Unlike single-beam echosounders, 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, producing a swath of depth data as the ship moves forward.
Healy's multibeam echosounder sends out a pulse of sound every few seconds that travels to the seafloor and back. The time it takes a pulse to travel to the seafloor and back is converted to depth using accurate measurements of the speed of sound in seawater, which is monitored daily by the researchers. The ship measures bathymetry along a swath whose width depends on water depth. Where the Beaufort Sea is about 3,800 m deep, for example, the swath is about 10 km wide. As the system collects overlapping swaths of depth data, it builds up a 3D image of the seafloor that looks similar to a topographic map of land.
Multichannel seismic-reflection profile from northern Gulf of Mexico (Keathley Canyon), shot during a 2003 cruise aboard the research vessel Gyre to investigate gas hydrates (USGS Open-File Report 2005-1411). CDPs (Common Depth Points, seafloor locations used to process the multichannel data) are 5 m apart. Vertical scale is in seconds of two-way travel time. This profile extends about a kilometer below the seafloor; data collected during the expedition will extend much deeper. Click image for larger version. Credit: USGS
The Canadian Coast Guard Ship Louis S. St-Laurent (Louis) is using a multichannel seismic system to collect seismic reflection and refraction data, which provide information about sediment thickness and other seafloor characteristics. Louis tows an array of three airguns that generate an acoustic signal at regular intervals. The airguns release air bubbles into the water which collapse to produce the acoustic pulses. The sound energy penetrates the seafloor and is refracted (bent) and reflected (bounced back) by geologic layers at and beneath the seafloor. Receivers called hydrophones are also towed behind the ship, in a 16-channel streamer configuration that senses the return signals and converts them to a digital form that can be recorded on the ship. The streamer and the airgun cluster are deployed from a robust, weighted sled towed immediately aft of the icebreaker at a depth of approximately 12 m, keeping them beneath the pack ice. The data collected by this system produce a cross-sectional view of the sediment layers as deep as several kilometers beneath the seafloor. (For detailed information about marine seismic instruments, please visit site)
Onboard Healy is another instrument that profiles sediment layers beneath the seafloor: a high-resolution subbottom "chirp" profiler. The high frequency of the chirp profiler's sound signal results in high-resolution images of shallow sediment layers—potentially down to several 10s of meters below the seafloor surface. Both ships collect gravity data, which delineate density anomalies in the sediments and deeper rocks.
Primary Mission
US and Canadian Coast Gaurd working together. Click image for larger view. Credit: UNH/NOAA
The primary mission of the two-ship expedition is to collect both multichannel seismic data (Louis) and multibeam echosounder data (Healy). In light to moderate ice, Healy leads and breaks ice so that Louis can tow its seismic gear. Where heavy ice makes towing the seismic equipment unsafe, or in areas where multibeam bathymetric data have scientific priority, Louis breaks ice ahead of Healy. Where open water permits, the two ships can operate independently. At these times, Healy may also collect samples of the seafloor, using a variety of devices. A rock dredge collects samples of bedrock from the sloping sides of bathymetric features. Additional sampling may include dart coring of rock outcrop, and piston coring, box coring, gravity coring, or dredge sampling of abyssal seafloor material. Information from these samples will not only help define the limits of the extended continental shelf but will also expand our knowledge of the region's geologic history.
Ancillary Science
As in past years, additional studies are being conducted on Healy to take advantage of the ship's location in the Arctic Ocean. In 2010, the ancillary studies include the following:
ARCTIC OCEAN ACIDIFICATION
As carbon dioxide builds up in the atmosphere, some is absorbed by the ocean, where it decreases seawater pH. This "ocean acidification" reduces the availability of carbonate ions (CO32-), which are important for building the shells and skeletons of calcifying marine organisms, such as corals. Studies indicate that surface waters from the more temperate oceans may have already undergone a reduction of 0.1 pH units (from nearly 8.2 to 8.1) since the beginning of industrial times. Ocean acidification is expected to cause complex interactions between seawater, calcifying organisms, and surrounding carbonate sediments in coastal-marine ecosystems.
USF scientist Sherwood Liu calibrates a Multiparameter Inorganic Carbon Analyzer (MICA) during a test cruise in summer 2008 off west Florida. Click image for larger view.
Credit: USGS
To study ocean acidification in the Arctic Ocean, scientists aboard Healy are collecting and analyzing seawater throughout the mission, both while Healy is underway and while the ship is stopped "on station." Their aim is to document the carbonate chemistry of the Arctic waters and, among other things, to test the waters' saturation state with respect to calcium carbonate. For underway measurements, the researchers are using a specially designed instrument to continually measure partial pressure of CO2 (pCO2), pH, and total dissolved inorganic carbon in seawater, as well as pCO2 in the atmosphere. Approximately every 3 to 4 hours, the scientists are collecting discrete seawater samples from the flow-through system and using benchtop spectrometers to measure pH with extreme precision so that the quality of the data from the flow-through system can be independently assessed.
A CTD is lowered into the water from Healy. Credit: USCG
Occasionally the ship is stopped "on station," presenting an opportunity to conduct seawater sampling and profiling throughout the water column with a CTD rosette. The rosette is a frame fitted with a circle of 12-liter Niskin bottles that are triggered to close at different depths, thus collecting samples throughout the water column. "CTD" stands for conductivity (used to calculate salinity), temperature, and depth. Data from the CTD rosette casts are also being used to calculate the speed of sound in the seawater, information needed to accurately convert raw signals from the multibeam echosounder into water depths.
Seawater samples collected during the expedition will also be analyzed for microbiology and microbiological processes, thereby creating critically important baselines. The resulting data sets will provide comprehensive integrated information on carbonate chemistry and the microbiological communities of Arctic waters.
ICE OBSERVATIONS AND BUOY DEPLOYMENTS
Healy in ice. Click image for larger view. Credit: Continentalshelf.gov
Open-ocean drifting buoys provided by the International Arctic Buoy Programme (IABP) are being deployed in open water during the westernmost tracks of the cruise. Two Airborne eXpendable Ice Buoy (AXIB) seasonal ice-buoy prototypes are being tested and may be deployed in the marginal ice zone or in open water. Depending on need, opportunity, and sea-ice conditions, one of the seasonal buoys may be deployed on multiyear sea ice; such deployments typically take 30 to 45 minutes of on-the-ice time. A pre-cruise analysis of sea-ice conditions in the Beaufort Sea and Chukchi Cap region is being provided by the National Ice Center (NIC) to the Chief Scientist to guide in the fine tuning of the initial cruise tracks. In addition to onboard sea-ice analysis and satellite-imagery cruise support, the NIC personnel are collecting hourly observations of sea-ice characteristic as the Healy navigates ice-covered waters. Recorded observations include estimates of sea-ice thickness and snow depth during icebreaking operations in the ice pack.
MARINE MAMMAL OBSERVATIONS
Polar bears. Click image for larger view. Credit: Continentalshelf.gov
Marine mammal observers will be aboard to make observations of the occurrence and density of marine mammals. Parts of the expedition will be in areas where observations have never or only rarely occurred. These observations aid in understanding the overall distribution of animals and contribute to baseline information about the effects of climate change on Arctic ecosystems.
