Methods and procedures to determine the outer limits ... - Springer Link

Acta Oceanol. Sin., 2013, Vol. 32, No. 12, P. 126–132 DOI: 10.1007/s13131-013-0389-y http://www.hyxb.org.cn E-mail: [email protected]

Methods and procedures to determine the outer limits of the continental shelf beyond 200 nautical miles WU Ziyin1,2, LI Jiabiao1,2*, JIN Xianglong1,2, FANG Yinxia1,2, SHANG Jihong1,2, LI Shoujun1,2 1

Key Laboratory of Submarine Geosciences, State Oceanic Administration, Hangzhou 310012, China Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China

2 The

Received 22 May 2013; accepted 10 August 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013

Abstract This paper establishes techniques and methods to determine a variety of boundaries associated with 200 nautical miles beyond the continental shelf. The methods, based on topography, slope and second-derivative profile integrated analysis, are now able to identify automatically the foot of the continental slope (FOS). By analyzing the sedimentary profile, the points of 1% sediment thickness are recognized. Through the intersection, cut, deletion and mergence calculation of the extrapolated data set of fixed-point series, the method succeeds in generating automatically the extrapolated boundaries, including the FOS+60 M line, the 350 M line, and the 2 500 m+100 M line. In addition, based on the automatic analysis of the topographic profile, it can be applied to determine rapidly the points of maximum water depth. Taking the northern Okinawa Trough (OT) as an example, these methods are used to calculate and examine the boundaries included in the Submission by the People's Republic of China Concerning the Outer Limits of the Continental Shelf beyond 200 Nautical miles in Part of the East China Sea (ECS); the boundaries thus derived have a solid scientific and rational basis. Key words: 200 nautical miles, continental shelf, demarcation, methodology, Okinawa Trough Citation: Wu Ziyin, Li Jiabiao, Jin Xianglong, Fang Yinxia, Shang Jihong, Li Shoujun. 2013. Methods and procedures to determine the outer limits of the continental shelf beyond 200 nautical miles. Acta Oceanologica Sinica, 32(12): 126-132, doi: 10.1007/s13131013-0389-y

1 Introduction The demarcation of the continental shelf beyond 200 nautical miles is one of the most significant problems associated with marine science, especially for the areas including the Arctic, the East China Sea (ECS), and the western Pacific regions. It is anticipated that in the next 10 years, the rights to about 75×106 km2 of sea area will be claimed by coastal states, which exceeds half of the continental land area, and 15×106 km2 of this sea area are located beyond 200 nautical miles (Peter and Carleton, 2000). It is stated in Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS) (UN, 1982) that: if a coastal state will delineate the outer limits of its continental shelf beyond 200 nautical miles from the baselines from which the breadth of the territorial sea is measured, they should submit relevant information on the delineation to the Commission on the Limits of the Continental Shelf (CLCS) under Annex II, on the basis of equitable geographical representation. The Commission will then make recommendations to the coastal state on matters related to Article 76 and Paragraph 4 of Annex II of UNCLOS. This is the primary legal framework for coastal states to claim rights to the continental shelf outside the 200-nautical mile limit. It should be noted that the term “continental shelf” here has a different definition as compared the same term used in the subject of marine geology. On December 20, 2001, according to the requirements of

UNCLOS, Russia submitted to the CLCS, through the SecretaryGeneral of the United Nations, the first case for the demarcation of this outer limit. By December 2012, the United Nations (UN) Official Website had released 63 copies of the executive summary and 45 copies of the preliminary information on demarcation (UN, 2013). In 2009 and 2012, China submitted her Preliminary Information and Partial Demarcation to the UN Secretariat. Around the same time, South Korea and Japan also presented their materials for demarcation. Compiling a comprehensive case for demarcation is a complex project that requires relevant scientific evidence and other materials, including demarcation texts, supporting data, an executive summary, and related maps (only the executive summary is released on the UN Official Website). The key evidence that must be provided to the CLCS for review pertain to the following boundaries: the foot of the continental shelf (FOS), the formula lines (i.e., the FOS+60 M line and the line of 1% sediment thickness), the limitary lines (i.e., the 350 M line and the 2 500 m+100 M line), and the external boundaries1). Therefore, maritime demarcation requires a multi-disciplinary collaboration, encompassing fundamental marine investigation and research and information technology, to process the acquired data and to form the various types of boundary points. From literature, there are only a limited number of scientific and technological achievements on maritime demarcation in

Foundation item: Fundamental Project of Science and Technology under contract No. 2013FY112900; Public Science and Technology Research Funds Projects of Ocean under contract No. 201105001; the National Natural Science Foundation of China under contract No. 40506017. * Corresponding author, E-mail: [email protected] 1) FOS, FOS+60 M line, line of 1% sediment thickness, 350 M line, and 2 500 m+100 M line are all demarcation boundaries of the continental shelf beyond 200 nautical miles, which are commonly used internationally. M denotes the unit of nautical miles.

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China that are related directly to the methodology of demarcation beyond 200 nautical miles outside of the mainland (Peng and Wand, 2002; Peng et al., 2005; Liu et al., 2007). Therefore, to improve China's autonomy in the field of maritime demarcation, it is necessary to study the fundamental methodology of this subject. As a part of the complex study, in the present contribution we concentrate on the discussion about the methods associated with the generation of the boundaries. 2 Basic technological steps of demarcation Compiling a case for demarcation comprises five basic steps (Fig. 1): (1) Consider whether a coastal state has its continental shelf extending beyond 200 nautical miles according to UNCLOS. A state with a narrow-shelf does not necessarily have the condition to apply for continental shelf beyond 200 nautical miles because its available continental shelf is less than 200 nm in extent, whereas a state with a wide-shelf has the possibility to apply for rights to the continental shelf beyond 200 nautical miles. (2) Determine the position of the FOS. (3) Build two formula lines: the FOS+60 M line (M refers to the unit of nautical miles that is generally used in the demarcation of the continental shelf) and the line of 1% sediment thickness. (4) Build two limitary lines: the 350 M line and the 2 500 m+100 M line. (5) Integrate the formula lines and the limitary lines to form the outer limits of the continental shelf and then compile the demarcation. 3 Main technology and methodology of demarcation Six types of boundary points/lines can be applied to the outer continental shelf, which can be determined via the following four methods: (1) determine the FOS and the maximum waterdepth point through topographical integrated profile; (2) determine the point of 1% sediment thickness from a sediment profile; (3) determine the FOS+60 M line, the 350 M line and the 2 500 m+100 M line via fixed-point extrapolation arc; and (4) determine the final external boundary through integrated analysis of multiple boundaries. Among these, the FOS+60 M line, the 350 M line, and the 2 500 m+100 M line are generated in the same way; therefore, in this study, these lines are referred to collectively as the extrapolation boundaries. 3.1 Method to build the FOS and the maximum water-depth point The FOS is the starting point for the series of boundaries on the continental shelf beyond 200 nautical miles. It affects the coordinates of the final external boundary and the area delineated, which if misidentified will severely affect the demarcation of the final external boundary. According to Article 76 of UNCLOS, in the absence of evidence to the contrary, the FOS shall be determined as the point of maximum change in the gradient at its base (UN, 1982). The determination of the FOS can be divided into two steps: (1) the determination of the basal region of the continental slope; and (2) locating the point with maximum change of gradient at the base of the continental slope. According to longterm research in the change from continental crust to oceanic crust and the natural extension boundary markers of coastal

1

The continental shelf extends beyond 200 M?

2

Determine the position of the FOS

3

Build two formula lines

4

Build two limitary lines

End

1% sediment thickness FOS+60 M line 350 M line 2 500 m+100 M line

5

The outer limits of the continental shelf

compile the demarcation

Fig. 1. Basic technological flowchart to determine the outer limits of the continental shelf.

states, Hedberg (1976) suggested that the outer edge of the continental margin would be best defined as the external boundary of the topographical continent, which usually is defined accurately as the bottom of the continental slope. In the ECS, the combination of “continental shelf-continental slope-the Okinawa Trough (OT)” exhibits the typical characteristics of the continent-ocean boundary, with the basal region of the continental slope located between the lower shelf and the OT. The most concise method to determine the FOS is to build a series of topographical profiles vertical to the strike direction of the continental slope and to analyze their changes. In addition, the gradient and the second derivative of these topographical profiles can be calculated and the point with the maximum gradient change analyzed. By using the Douglas and Peucker (1973) algorithm to perform the topographical fitting and to obtain a simplified profile curve that contains representative points of the original topographical features, the influence of local small-scale topographic variations and measurement errors can be eliminated. Further, the patterns of maximum gradient changes are analyzed, which leads to the final derivation of the FOS. Figure 2 shows a typical topographical profile that crosses through the northern OT of the ECS, with the FOS position determined by the calculation of the gradient profile, the profile of second derivative, and Douglas-Peucker fitting profile. China has submitted Preliminary Information and an Executive Summary of Demarcation, both of which have proposed to take the connecting line of maximum water-depth points in the OT as the outer limits of the ECS continental shelf beyond 200 nautical miles, which updates and develops the methodology of outer-continental-shelf demarcation. Similar to the method for determining the FOS, the determination of the line with maximum water-depths can be realized as follows. First, a series of topographical profiles, vertical to the strike of the OT axis, is built. Then, the water-depth data of the points that are on the topographical profile and inside the axial region of the OT are analyzed automatically. Subsequently, the maximum depth point (MDP) of each profile is obtained; the points are connected together to form the maximum water-depth line. 3.2 Method to build the line of 1% sediment thickness The line of 1% sediment thickness, also named the formula

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−10.00

A:BOS

topographic profile

O 250

−7.00

Depth/m

D-P profile −4.00

second derivative profile

500

−1.00 gradient profile 750

2.00

E

B:FOS 0

50

Distance/km

100

150

Fig. 2. An integrated profile of FOS determination. line of sediment thickness, was first proposed by the Irish engineer Cardinar in 1978; thus, it is also known as the Cardinar line. This is a line a certain distance away from the FOS, where the sediment thickness is equal to 1% of the distance from the line to the FOS. This formula line combines geophysical evidence, submarine resources, and outer-continental-shelf demarcation; seabed oil and gas resources are usually related closely to the thickness of sediment. In Fig. 3, the point A is the FOS, the point B has a sediment thickness of d, and the distance between B and A is D. When d/D is equal to 1%, B is the point of 1% sediment thickness in consideration. To determine the point of 1% sediment thickness requires high-resolution stratigraphic acoustic profile data, such as those obtained from multichannel seismic profiles. However, these seismic data are expensive, and owing to the uncertainty of seabed acoustic velocity, so far many submissions for demarcation have not provided the location of the points of 1% sediment thickness. In open waters,

especially in Atlantic-type continental margin areas, the line of 1% sediment thickness would help greatly in the determination of the outer limits of the continental shelf. 3.3 Method to build the extrapolation boundaries The FOS+60 M line, also known as the distance formula line, is a continuous curve formed through intersecting, cutting and splicing the envelopes extrapolated in a spherical radius of 60 nautical miles around the FOS point. According to UNCLOS, whichever method is used for demarcation, the outer limit of the continental shelf on the seabed does not exceed 350 nautical miles (i.e., the line of 350 M) from the baseline from which the extent of the territorial sea is measured, or it does not exceed 100 nautical miles from the 2 500 m isobath (i.e., the line of 2 500 m+100 M). These two types of boundary lines are also known as the limitary lines of demarcation beyond 200 nautical miles. These limitary lines

Thickness/m

seafloor

sediment unit

1% sediment thickness point

FOS

B

A

D

reflection base

Distance/km

Fig. 3. Sketch map to show the point with 1% sediment thickness.

d


have a definite purpose, which is to restrict the location of the outer limit points of the continental shelf beyond 200 nautical miles to avoid over-extending. Particularly for a continental margin with massive deposits, the outer limit points generated by the formula line might over-extend far into the ocean, which would benefit the applicant state, but reduce the common area for the other states. At the moment, the 2 500 m+100 M line is widely applied to sea regions with an oceanic ridge, where the traditional international seabed area has been sharply reduced because of the misuse of the regulations regarding the oceanic ridge. For example, the Arctic region has been almost completely partitioned by its surrounding states under the rule of oceanic ridge; similarly, in the Philippines Sea region, Japan has attempted to expand greatly the extent of its outer continental shelf by using the Kyushu-Palau Ridge (KPR). Therefore, the limitary line of 2 500 m+100 M is being used carefully by CLCS in dealing with the demarcation claims of coastal states in sea areas with oceanic ridges. The same technology is used in generating the lines of FOS+60 M, 350 M, and 2 500 m+100 M, which is to form series of extrapolation arcs around a given center with a certain spherical radius, and to obtain the specified boundaries by intersection and mergence calculations of the extrapolation arcs. All the relevant calculations outlined above should be carried out under the spherical coordinates instead of the projection coordinates because the buffering surfaces of both the points and the lines are intensely deformed in the latter (Peng and Wang, 2002).

1

Input Data Limits Type

Data G

FOS+60 M 2 500 m+100 M 350 M

Direction D

E, S, W and N

Radius r 2

Create Arc

Data Arc0

3

Intersect

Data Arc1

4

Cutting

Data Arc3

5

Del Arc

Data Arc4

6

Del Arc

Data Arc5

7

Merge Arc

Data Arc6

ľ FOS+60 M

Ŀ350 M

Data Arc2

ŀ2 500 m+100 M

Final Limit

Fig. 4. Technical flowchart of the generation of an extrapolation line.

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An extrapolation boundary can be generated as follows (Fig. 4). (1) Input the variables. The initial variables include the starting point G of the extrapolated arc, the extrapolated radius r, and the extrapolated direction D. The starting point of an extrapolated arc is also its circle center, which is given by users, and a series of them is obtained from the initial data set G. The extrapolated radius r is based on the type of extrapolated arc, and its values for the lines of FOS+60 M, 350 M, and 2 500 m+100 M are 60, 350, and 100 nautical miles, respectively. The extrapolated direction D is the extension direction of the extrapolated arc, and all the lines of FOS+60 M, 350 M and 2 500 m+100 M are extrapolated seawards. (2) Generate extrapolated arcs. Create a circle around each point of data set G at the same value of spherical distance r, which forms the initial data set of spherical extrapolation. (3) Intersect extrapolated arcs. Perform an intersection calculation of the initial extrapolated arcs by double cycling through the data set. (4) Cut extrapolated arcs. Cut an extrapolated arc into a series of smaller segments by double circulation. (5) Delete internal arcs. Delete internal sub-segment arcs by double circulation, i.e., a sub-segment arc that is fully included inside an initial arc will be deleted. (6) Delete redundant arcs. All the lines of FOS+60 M, 350 M and 2 500 m+100 M are extrapolated seawards, but not all of them are sealed envelope arcs, which differs greatly from the buffer in Computer Graphics. Generally, the extrapolated direction D of the lines of FOS+60 M, 350 M and 2 500 m+100 M is supposed as east, south, west, or north. The value D is an external input variable. The aim of this step is to go through the entire data set and to delete redundant data. (7) Merge extrapolated arcs. Double cycle through data set to check and connect automatically the neighboring extrapolated arcs to form a continuous extrapolated curve. 3.4 Comprehensive construction of final external boundaries The outer limits of the continental shelf beyond 200 nautical miles are composed of formula lines and limitary lines. According to the provisions of Article 76 of UNCLOS, the outer limits are defined by coordinates of latitude and longitude with straight lines of neighboring fixed points, without exceeding 60 nautical miles in length (UN, 1982). As for different continental margin settings and different coastal states, the final outer limits of the continental shelf vary greatly, which might be a combination of either two formula lines or two limitary lines, or might be a complex combination of all these types of boundaries. When considering simultaneously a variety of complex factors, such as politics, neighboring states and historical rights, the external boundary applied may be further complicated. Figure 5 shows the possible composition of the outer limits of the continental shelf for a coastal state with a typical wide continental shelf. Here, the northern shelf is narrower than 200 nautical miles, whereas the southern shelf is wider than 200 nautical miles, where the depth contour of 200 m is used as the dividing line between the continental shelf and the continental slope. In this hypothysised region, the deposition is weak in the northern region and the line of 1% sediment thickness lies between the line of the FOS and the line of FOS+60 M. However, the deposit is well developed in the southern region and the line of 1% sediment thickness lies outside the limitary line of 2 500 m+100 M. Based on analysis of the formula lines of FOS+60 M

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Legend

1% sediment thickness point

350 M

2 500 m+100 M

territorial sea base point

200 m isobath

F1

200 m

2 500 m

2 500 m isobath F2

F3

200 M

60

M FOS+60 M

FOS

200 M

baseline

F4

0M

10

2 500 m

2 500 m+100 M

200 M

350 M

350 M

FOS line FOS+60 M 1% sediment line EEZ 350 M 2 500 m+100 M outer limits

200 m F5

F6

target area

Fig. 5. Sketch map of relevant boundaries in the continental shelf region beyond 200 nautical miles. and 1% sediment thickness, and the limitary lines of 350 M and 2 500 m+100 M, the outer limits of this continental shelf obtained is composed of five curved segments of different properties: Segment F1–F2 is the FOS+60 M line; Segment F2–F3 is the line of 1% sediment thickness; Segment F3–F4 is the 350 M line; Segment F4–F5 is the 2 500 m+100 M line; and Segment F5–F6 is the 350 M line. This is an ideal situation for the outer limits of this continental shelf because there are no neighboring states in the demarcation area and the outer limits of the continental shelf can extend far into the deep ocean. As a limitary line, the 2 500 m+100 M line has potential impact, which benefits some states that might seek to misuse the rule of the oceanic ridge and isolated islands and reefs in the ocean to extend significantly the breadth of the outer continental shelf. Such situation has occurred during the demarcation of 2012. 4 Case study and verification The continental shelf and slope of the ECS, together with the OT, constitute a typical continental margin (Jin, 1992; Xu and Le, 1988; Li, 2008) (Fig. 6). The ECS continental shelf and the OT vary greatly in water depth, and have very different topographical features. However, the continental slope connecting them shows typical characteristics of a continental slope with varied water depth and relatively steep seabed slope (Li, 2008; Wu et al., 2004). In addition, existing geological and geophysical evidence indicates that the continental crust thins dramatically to around 13 km in the southern OT (Gao et al., 2004; Han et al., 2007; Hao et al., 2004), where new oceanic crust might have developed locally (Li et al., 1997; Liang, Wang, et al., 2001; Huang et al., 2006). Furthermore, an early form of linear magnetic striping has been identified in the central and southern OT (Wang et al., 1998; Liang, Wang, et al., 2001; Liang, Wu, et al., 2001). In general, the multiple pieces of evidence from topography, lithology, and geophysics indicate that the continental shelf of the ECS extends naturally eastwards, but terminates in the OT.

Based on a grid of merged water-depth data, a series of topographic profiles vertical to the strike direction of the continental slope in the northern ECS can be built. According to the method described in the Section 2.1, the FOS location of the northern OT can be determined by analyzing the integration of topography, slope, second derivative, and the D-P profile (Fig. 7). Then, according to the Section 2.3, the line of FOS+60 M can be generated from the available FOS. By constructing and analyzing the series of topographical profiles vertical to the axial region of the

Fig.6. Sketch map of the relevant boundaries of the continental shelf of northern OT.


A:BOS −4.00

250

−3.00

Depth/m

−2.00 −1.00 0.00

500

able if the regional geological structure is taken into account. In general, taking the points of maximum water depth as the external boundary of the outer continental shelf beyond 200 nautical miles in the ECS is a scientific choice, which updates and develops the demarcation method of outer continental shelf beyond 200 nautical miles.

1.00 2.00 0

B:FOS 40 60 Distance/km

20

3.00 80

100

Fig.7. A typical profile across the ECS showing the FOS. 500

A

C

750 Depth/m

131

1 000

5 Conclusions (1) According to the relevant provisions of Article 76 of UNCLOS and the relevant technical requirements of CLCS, the determination of relevant boundaries, including the FOS, the formula lines (the FOS+60 M line and the line of 1% sediment thickness) and the limitary lines (the 350 M line and the 2 500 m+100 M line), can be established. (2) The implementation of an autonomous method has demonstrated that the FOS, the external boundaries and the FOS+60 M line, recorded in the Executive Summary of Demarcation of the ECS submitted by China, are all in accordance with the provisions of UNCLOS. Furthermore, it is a scientific and rational decision to take the points of maximum water depth in the OT as the outer limit of the Chinese continental shelf in the ECS.

1 250 B:Max 0

50

100 Distance/km

References 150

Fig.8. A typical profile to determine the MDP (see point B). OT, the MDPs can be obtained (Fig. 8). The FOSs and MDPs are all mutually authenticated by multiple sources of bathymetric data to ensure consistency in the results. The positions of all FOSs and MDPs are similar to the results of Executive Summary of Demarcation (UN, 2013). In the Preliminary Information and Executive Summary of Demarcation submitted to the UN by China in 2009 and 2012 (UN, 2013), China claimed the points and their connecting lines of maximum water depth in the OT as the outer limits of the Chinese continental shelf in the ECS. The outer limits thus established are consistent with the provisions of Article 76 of UNCLOS. All the external boundary points in the Executive Summary of Demarcation are located between the 200 M and the 350 M lines, extrapolated eastwards from the baseline of the Chinese territorial sea, and also between the 200 M line and the distance formula line of FOS+60 M. Therefore, all the external boundary points in the Executive Summary of Demarcation conform to the provisions of UNCLOS, and are effective external boundary points. The OT is a typical Pacific-type continental margin, which differs greatly from an Atlantic-type continental margin. There are a series of marginal sea basins around the Pacific Ocean, some of which have island arcs near the coast; these island arcs limit the ocean-wards extension of the continental shelf (Lee et al., 1980; Kimura, 1985; Sibuet, 1987). As a result, although the ECS has one of the world's broadest continental shelves, as limited by the Ryukyu Island Arc, its external boundary cannot extend as far as the boundary moght extend in the Atlantic continental margins. Hence, taking the points of maximum water depth as the outer limits of the continental shelf not only conforms to the relevant provisions of UNCLOS, but also is reason-

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