Marine Atlas: Maximizing Benefits for Solomon Islands

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 Solomon Islands.

Standing on Solomon Islands’ shore and gazing into an alluring turquoise lagoon, it is hard to imagine how deep the ocean truly is. Less than 2 per cent of Solomon Islands’ national waters are shallower than 200 metres, while the other 98 per cent are up to 8,000 metres deep. Changes in ocean depth, also known as bathymetry, affect many other dimensions of human life and natural phenomena. Bathymetric maps were originally pro- duced to guide ships safely through reefs and shallow passages (see chapters “Full speed ahead” and “One world, one ocean”). Since ocean depth is correlat- ed with other physical 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 warn- ing systems to assess the risk of disaster. The bathymetry of Solomon Islands is complex (see map), reflecting the meeting of two large tectonic plates: the Pacific and Australian plates. Solomon Islands’ archipelago is part of a fragmented island arc running north-west to Papua New Guinea and south-east to Vanuatu. The

Central Solomons Trough is a composite basin separating the country’s two main island chains. This trough is 475 kilo- metres long, 90 kilometres wide and 1,800 metres deep.

The country’s complex geology includes unique systems of underwater troughs, trenches and several active seismic frac- ture areas. A discontinuous trench runs south of the island chains; the western

part of this trench is known as the New Britain Trench, while the eastern part is known as the South Solomon Trench, where the maximum depth reaches over 8,000 metres. The South Solomon Trench connects with the North New Hebrides Trench to the east, with depths reaching over 9,000 metres—the deepest in Solo- mon Islands’ jurisdiction. A less well-de- fined trench system, which includes the West Melanesian Trench, North Solomon Trench, Cape Johnson Trench and Vitiaz Trench, runs to the north and east of the ar- chipelago (Krüger, J. and Sharma A., 2008). North of this trench system is the Ontong Java Rise, an area of elevated sea floor more than 2,000 metres above the abyssal sea floor. East of the Ontong Java Rise, the depth of the abyssal sea floor gradually increases from 3,500 metres to over 5,000 metres. South of the Vitiaz Trench and sev- eral ridges and seamounts, including the shallow Pandora Bank area, the sea floor rises to the surface. This area connects to the Fiji Plateau, where the abyssal depth reaches around 3,500 metres. South of the South Solomon Trench, the abyssal sea floor is generally between 3,000 and 4,500 metres deep. There are also several remote shallow areas, such as the Indispensable Reefs, which reach the surface. Along the western boundary with Papua New Guinea, there is another area of raised sea floor less than 500 metres deep. The Pocklington Trough is directly north of this area, with depths of more than 5,000 metres. Further north is an area of fractured sea floor, extending down to Papua New Guinea.

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currents creating opportunities for up- welling and enhanced marine biodiversity.

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 had- al zone. The sublittoral zone encompasses the sea floor from the coast to the shelf break—the point at which the sea floor rap- idly 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 tempera- ture reaches 4°C. This zone is typically dark and thus not conducive 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, such as the North and South New Hebrides Trenches. Under the United Nations Convention on the Law of the Sea (UNCLOS), a coastal state has specific sovereign rights to the seabed, its subsoil and superjacent waters within its EEZ (article 56). Within areas of extended continental shelf, defined under UNCLOS article 76, a coastal state has sovereign rights to certain natural resourc- es on the seabed and subsoil, but not to superjacent waters.

Shaking Gizo

On the morning of 2 April 2007, resi- dents in the village of Gizo in Solomon Islands awoke to an earthquake, which created a 12-metre-high wave. Fifty-two people lost their lives and 13 villages were destroyed. If the earthquake had struck earlier in the morning, when peo- ple were still asleep, the toll may have been much higher. Thousands of people were left homeless and damage was estimated in the millions. Within 15 min- utes, a tsunami warning was issues for the Pacific, from Australia to Alaska, cre-

ating panic along the eastern Australian coast 2,100 kilometres away. Beaches were closed, some schools and day- care centres were evacuated and ferry services were halted in Sydney Harbour amid fears of a repeat of the 2004 Indian Ocean tsunami disaster. This not only shows how far tsunamis can travel, but also how important bathymetry is for the effect of tsunamis. Fortunately, on this occasion the bathymetry worked in fa- vour for Sydney, which only experienced an “extreme tide”.

The complex bathymetry surrounding Sol- omon Islands interacts with deep ocean

MAXIMIZING BENEFITS FOR SOLOMON ISLANDS

SUPPORTING VALUES

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