Marine Atlas: Maximizing Benefits for Kiribati

STILL WATERS RUN DEEP: OCEAN DEPTH SUPPORTING VALUES

It is important to understand how ocean depth influences both the distribution of life below the surface and the management of human activities along the coasts of Kiribati.

Standing on Kiribati’s shore and gazing into an alluring turquoise lagoon, it is hard to imagine how deep the ocean truly is. Only 0.1 per cent of Kiribati’s national waters are shallower than 200 metres, while the other 99.9 per cent are up to 8,155 metres deep in the Nova-Canton Trough. Changes in ocean depth, also known as bathymetry, affect many other dimensions of human life and natural phenomena. Bathymetric maps were originally produced to guide ships safely through reefs and shallow passages (see chapters “Full speed ahead” and “One world, one ocean”). Since ocean depth is correlated with other phys- ical variables such as light availability and pressure, it is also a determining factor in the distribution of biological communities, either those living on the bottom of the sea (benthic), close to the bottom (demersal) or in the water column (pelagic). In addition, bathymetry significantly af- fects the path of tsunamis, which travel as shallow-water waves across the ocean. As a tsunami moves, it is influenced by the sea floor, even in the deepest parts of the ocean. Bathymetry influences the energy, direction and timing of a tsunami. As a ridge or seamount may redirect the path of a tsunami towards coastal areas, the po- sition of such features must be taken into account by tsunami simulation and warning systems to minimize the risk of disaster. Kiribati comprises three island groups: the Gilbert group, the Phoenix group and the Line group. Each of these groups is charac- terized by extensive areas of deep abyssal sea floor between 4,000 and 6,000 metres deep. The easternmost Gilbert group is a chain of islands rising up from the Gilbert Ridge, which runs north to south through

the EEZ. This ridge is mostly between 3,000 and 4,000 metres deep. The Phoenix group consists of a number of islands and seamounts rising up from the deep ocean sea floor. The Nova-Canton Trough is the deepest part of sea floor found in Kiriba- ti’s waters and lies in the northern part of the EEZ. At its deepest point, this trough measures 8,155 metres and is the result of tectonic movement of the sea floor. The Line group are a chain of islands rising up from a series of ridges running from the south-east to the north-west, including the Boudeuse, Menard and Minneapolis Ridg- es. All the island groups have a significant number of seamounts rising up from the deep abyssal sea floor. The sea floor can be divided into several different zones based on depth and tem- perature: the sublittoral (or shelf) zone, the bathyal zone, the abyssal zone and the

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hadal zone. The sublittoral zone encom- passes the sea floor from the coast to the shelf break—the point at which the sea floor rapidly drops away. The bathyal zone extends from the shelf break to around 2,000 metres depth. The lower limit of the bathyal zone is defined as the depth at which the temperature reaches 4°C. This zone is typically dark and thus not condu- cive to photosynthesis. The abyssal zone extends from the bathyal zone to around 6,000 metres. The hadal zone, the deepest zone, encompasses the deep-sea floor typically only found in ocean trenches.

More space for Kiribati While Kiribati’s land mass is rather small, it has sovereignty over a very large marine area. Why? Because the national area includes the exclusive economic zone (EEZ), with its boundary drawn 200 nautical miles from the coast. And this area may grow further yet. In 2012, Kiribati applied for an extended continental shelf claim for a particular region adjacent to the Line Islands that includes the Line Islands Ridge Com-

plex—a chain of tropical atolls, eleva- tions, submarine ridges and seamounts. These submarine elevations do not form a simple linear chain, but rather com- prise scattered volcanic constructs and therefore constitute a natural prolonga- tion of the continental shelf. For this reason, Kiribati’s EEZ may be extended, showing just how important a good understanding of bathymetry is for establishing maritime boundaries.

MAXIMIZING BENEFITS FOR KIRIBATI

SUPPORTING VALUES

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