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Seamounts, Submarine Channels, and New Discoveries Benefits of Continental Shelf Surveys Extend Beyond Defining the Limits of the Shelf

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Seamounts, Submarine Channels, and New Discoveries Benefits of Continental Shelf Surveys Extend Beyond Defining the Limits of the Shelf Seamounts, Submarine Channels, and New Discoveries Benefits of Continental Shelf Surveys Extend Beyond Defining the Limits of the Shelf by Andrew Armstrong, Larry Mayer, and James V. Gardner Copyright Journal of Ocean Technology 2015 The Journal of Ocean Technology, VOL. 10, NO. 3, 2015 1 Introduction their academic partners at UNH’s CCOM/JHC Article 76 of the United Nations Convention have led 24 cruises on 12 different vessels on the Law of the Sea provides that “the spending more than 700 days at sea and continental shelf of a coastal State comprises mapping approximately 2.3 million square the seabed and subsoil of the submarine areas kilometres of seafloor on the US continental that extend beyond its territorial sea throughout margins. Mapping is continuing with the natural prolongation of its land territory expeditions to the Atlantic margin and Pacific to the outer edge of the continental margin, Islands this year (2015), and additional or to a distance of 200 nautical miles from cruises to the Pacific Islands in 2016. the baselines from which the breadth of the territorial sea is measured where the outer edge Although the limits of the continental shelf of the continental margin does not extend up have not yet been determined for any US area, to that distance.” The Article further provides many of the regions that have been mapped formulae and constraints for delineation of a certainly have ECS. The presence of ECS state’s continental margin that include, among in some other areas remains uncertain. But others, outer limits based on the location of regardless of whether there may be ECS or not, two bathymetric features – the 2,500-m isobath all the projects have yielded stunning maps and the “foot of the continental slope.” of the seafloor that include new discoveries, provide a source of enhanced scientific insight, In 2001, the Center for Coastal and Ocean support more accurate nautical charts, and offer Mapping/Joint Hydrographic Center (CCOM/ a foundation for improved identification and JHC) at the University of New Hampshire management of living and non-living marine (UNH) was funded by the National Oceanic resources. As significant as the potential for and Atmospheric Administration (NOAA) to delineating extended areas of sovereign rights undertake a desktop study to determine where is, in the long term the additional benefits there might be potential for the US to delineate from the ECS mapping may well exceed its continental shelf beyond 200 nautical miles, those of their primary role in locating Article and what additional data might be required 76-relevant bathymetric features. to support such delineation. In May 2002, the authors of the study – Mayer, Jakobsson, The Bering Sea and Armstrong – identified several areas of In the Bering Sea, the project team mapped two potential extended continental shelf (ECS) and opposing areas of the margin in 2003 with a recommended a campaign of “modern, high- Reson 8150 12-kHz multibeam system mounted density, complete coverage” multibeam echo on the RV Davidson. The two areas include the sounder mapping in those areas where potential south face of the Beringian margin and the north delineation depends on location of either the flank of Bowers Ridge. The Beringian margin foot of the slope or the 2,500-m isobath. Areas (Figure 1) provided the first example from the in Alaska, the coterminous US and the US project of how much more could be seen with Pacific Islands were identified as potential modern full-coverage multibeam mapping than regions of interest. was available from the existing compilations of trackline data and satellite-altimetry-derived The US ECS bathymetric mapping effort that bathymetry, such as those from W.H.F. Smith followed the UNH desktop study constitutes and D.T. Sandwell published by the Scripps one of the largest, if not the largest, civil Institution of Oceanography. ocean mapping projects ever undertaken. The first multibeam mapping cruise took place The Arctic in the Bering Sea, north of Alaska’s Aleutian US bathymetric mapping for ECS purposes Islands, during the summer of 2003. Since in the Arctic Ocean began with a 10-day then, NOAA’s Office of Coast Survey and proof-of-concept cruise aboard the US 2 The Journal of Ocean Technology, VOL. 10, NO. 3, 2015 Copyright Journal of Ocean Technology 2015 multibeam system to acquire useful data in these sonar-challenging conditions. Although other multibeam-equipped icebreakers had collected sounding data previously, these data had typically been collected along whatever track the ship followed. Despite the short duration of this first Arctic cruise, the existing bathymetric compilation of the Arctic, the International Bathymetric Chart of the Arctic (IBCAO), was significantly Figure 1: Detailed bathymetry of the Beringian margin – background bathymetry in muted colours from Smith and Sandwell. The improved and a large, previously unknown background bathymetry shows the resolution and seafloor detail seamount was discovered and mapped (Figure 2). available before the multibeam mapping. In honour of the ship and its namesake, Captain Mike Healy of the US Revenue Cutter Service, Coast Guard Cutter Healy, a medium-duty the new discovery was named Healy Seamount. research icebreaker equipped at the time with In the years since 2003, the NOAA/UNH team a SeaBeam 2112 12-kHz multibeam echo has led eight additional mapping cruises to the sounder. The goal of this initial cruise was Arctic Ocean, all aboard Healy, and has been to demonstrate that a multibeam-equipped one of the largest contributors to the constantly icebreaker could collect bathymetric data improving IBCAO compilation. In 2007, of suitable quality while breaking thick sea the team mapped a large field of pockmarks ice and maintaining a purposeful track – in (Figure 3) and nearby, a field of ice sheet- this case, a track along the 2,500-m isobath. derived, mega-scale lineations and bedforms Since only the general location of the 2,500-m on the Chukchi Plateau. isobath was known in this virtually uncharted region of the ocean, this mission required Pockmarks on the seafloor indicate the the survey team to anticipate the trend of vertical escape vents of fluid and/or gas from the bathymetry and direct the icebreaker’s the sediment. The Chukchi Plateau is known track in real-time. Success of the mission to be continental in origin, and methane is the also depended upon the Healy’s ability to likely gas that caused these pockmarks. The remain on a directed, rather than ice-breaking individual pockmarks have a 200-m diameter optimal, track through the ice and for the and typically are 20- to 40-m deep. They Figure 2: The Healy Seamount abruptly rises 3,000 m from the ocean floor to about 925 m of depth. Copyright Journal of Ocean Technology 2015 The Journal of Ocean Technology, VOL. 10, NO. 3, 2015 3 Figure 3: A perspective view, looking north, of gas pockmarks on the surface of Chukchi Plateau. The vertical exaggeration is 5x. Water depth range in the foreground of the image is 380 m to 900 m. IBCAO bathymetry in wireframe. occur in both random patterns and in linear These data were collected with a Kongsberg and circular patterns. Some appear to have EM 122 12-kHZ multibeam echo sounder that coalesced into larger depressions. When the replaced the SeaBeam system on the Healy in pockmarks were formed, and whether or not 2010. The main channel begins in the western they are still active, is yet to be established. portion of the northern Borderland, and runs The very large field of mega-scale lineations eastward for about 160 km, descending 500 m and bedforms is just north of the pockmarks. downslope to the Canada Basin abyssal plain. The mega-scale lineations (Figure 4) are Tributary channels join in several locations, oriented in a southeast to northwest direction and some tributary channels have hanging and are attributed to formation by an ice sheet walls where joining the main channel, rather than icebergs because of their coherent indicating that they have been cut off by the nature. Iceberg scours are typically random in main channel after formation earlier in time. direction and length, whereas these scours are parallel across the entire field, indicating that The North Atlantic an ice sheet that was at least 500-m thick once Eight cruises have been completed along the covered and moved across this area 750 km US Atlantic margin, from offshore of Florida from the present Alaska shoreline. The large to offshore of Maine. These mapping cruises bedforms of the field are likely related to flow have been led by the NOAA/UNH team aboard associated with the ice sheet that caused the vessels from the US Naval Oceanographic mega-scale lineations. Office, the University National Oceanographic Laboratory System (UNOLS), and NOAA. The Farther north, on the northern slope of the vessels were equipped with either a Kongsberg Chukchi Borderland, between latitude 79o EM120, EM121 or EM122 12-kHz multibeam N and 80o N, the mapping team discovered echo sounder. The Atlantic margin has been what may be the highest latitude, large, deep- much better studied and mapped in the past seafloor channel system on Earth (Figure 5). than most other continental margins in the 4 The Journal of Ocean Technology, VOL. 10, NO. 3, 2015 Copyright Journal of Ocean Technology 2015 area of the surface of Chukchi of the surface Chukchi area 2 Figure 4: Map view of a 300 km 4: Figure Plateau. The bedforms are asymmetrical, have wave heights of wave have The bedforms asymmetrical, are Plateau. The orientation of of 2 to 2.5 km. 1 to 10 m and wavelengths from E to the bedforms that flowed suggests a strong current 2 to 6 m deep and slightly The scours are W across the plateau.
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