Article 76 and the Continental Shelf

Our expedition is focused on the “continental shelf” as defined by Article 76 of the United Nations Convention on the Law of the Sea. Article 76 gives the term a definition different from that used by geographers and geologists. Here is a brief explanation of the two definitions, traditional and legal, and of the criteria outlined in Article 76 for determining the outer limits of the (legally defined) continental shelf.

The traditional definition

The continental shelf off Los Angeles, California, varies in width. The widest area of shelf in this image extends about 12 nautical miles (nm) from the shore to the shelf break, where the water depth is about 100 m.

The continental shelf off Los Angeles, California, varies in width. The widest area of shelf in this image extends about 12 nautical miles (nm) from the shore to the shelf break, where the water depth is about 100 m. Click image for larger view.

In his classic book Submarine Geology (3rd edition, 1973, Harper and Row), Francis Shepard defines continental shelves as “the shallow platforms or terraces that surround most of the continents and are terminated seaward by a relatively sharp break in slope, called the shelf edge or shelf break.” Definitions in introductory geology texts are much like Shepard’s, some of them adding that the continental shelf is the flooded edge of the continent, exposed during ice ages and submerged during interglacial periods. The term works well visually: on many coasts, these extensions of the continent do indeed look like flat, smooth shelves.

General shape of continental shelf, slope, and rise.

General shape of continental shelf, slope, and rise. Click image for larger view.

Shepard goes on to define underwater features beyond the shelf, which figure in the legal definition of “continental shelf” discussed below: “…The continental slope is the relatively steep descent from the shelf break to the deep-sea floor, usually a very irregular area…. At the base of the continental slope, in some areas, a smooth gently sloping apron of sediments merges into the deep-sea floor. This is called the continental rise.”

The Article 76 definition

Article 76 states that each nation has a “continental shelf” that extends to the outer edge of its continental margin or to a distance of 200 nautical miles (nm) from its coastal baselines if the continental margin does not extend that far. The continental margin consists of the continental shelf (as traditionally defined), the slope, and the rise.

The alternate definition of “continental shelf” for legal purposes evolved in the 1900s as nations claimed sovereign rights over ever-larger areas of seabed adjacent to their shores. In 1945, for example, President Harry S. Truman, responding in part to pressure from domestic oil interests, unilaterally extended U.S. jurisdiction over all natural resources—including oil, gas, and minerals—from a narrow belt of sea surrounding the coastline to the entire U.S. continental shelf. Many other nations followed suit, but they did not all use the term “continental shelf” in the same sense. According to a history of the U.N. Convention on the Law of the Sea posted on the U.N. Web site, tthe term “became no more than a convenient formula covering a diversity of titles or claims to the seabed and subsoil adjacent to the territorial seas of States.”

In the 1950s, a number of attempts were made to define the “continental shelf” and coastal nations’ jurisdiction over its resources. By 1973, when lawyers, scientists, and diplomats assembled in New York to convene the Third United Nations Conference on the Law of the Sea (the effort that eventually led to the 1982 adoption of the U.N. Convention on the Law of the Sea), there was a strong consensus in favor of extending coastal nations’ control over ocean resources out to 200 nm from shore, to satisfy nations whose geographic continental shelves were narrow or nonexistent. Thus the Convention set 200 nm as the boundary of the “continental shelf” for seabed and subsoil exploitation.

To satisfy nations with broad continental margins, the Convention provided a way to demonstrate that a continental shelf (as legally defined) extends beyond 200 nm from a nation’s shores. Article 76 spells out how to identify what we have come to call “extended continental shelf.”

A country may use the sediment thickness formula or the bathymetric formula to define the outer limits of its continental

A country may use the sediment thickness formula or the bathymetric formula to define the outer limits of its continental shelf. Click image for larger view. Credit: continentalshelf.gov

 

The continental shelf off Los Angeles, California

A country may use either constraint line to define the outer limits of its continental shelf: either 350 nautical miles seaward of the baseline, or 100 nautical miles seaward of the 2,500-meter depth contour (isobath). Click image for larger view.
Credit: continentalshelf.gov

Determining the limits of extended continental shelf

According to Article 76, a nation may use one of two formulas in any combination to determine the outer limit of its continental shelf: it can place the boundary along a line where the thickness of sediment on the seafloor is at least 1 percent of the distance to the foot of the slope (Formula 1), or it can place the boundary 60 nm beyond the foot of the slope (Formula 2).

The Article also provides two constraint lines beyond which a country cannot claim extended continental shelf; these constraint lines ensure that a common heritage area under the high seas (defined elsewhere in the Convention) is not carved up into nations’ continental shelves. The first constraint line is 350 nm from the nation’s shore, and the second constraint line is 100 nm seaward of the 2,500-m isobath. A country can use any combination of those constraint lines to maximize the extent of its continental shelf.

Gathering data to make the determinations outlined above is a major mission of the 2010 U.S.-Canada Arctic Continental Survey. Defining a continental-shelf limit using Formula 1 requires knowing the thickness of the sediment on the seafloor, which will be provided by seismic-reflection data collected by the Canadian Coast Guard Ship Louis S. St-Laurent. Defining an outer limit using Formula 2 and identifying a constraint line seaward of the 2,500-m isobath both require detailed knowledge of the depth and shape of the seafloor, which will be provided by multibeam bathymetric data collected by the U.S. Coast Guard Cutter Healy.

For additional details, you can read the full text of Article 76, which is shorter than this essay and is posted on the U.N. Web site: click “Part VI” in the bar on the lefthand side.