Marine Atlas: Maximizing Benefits for Kiribati

VOYAGE TO THE BOTTOM OF THE SEA: GEOMORPHOLOGY

Kiribati’s sea floor is rich in physical features of different shapes and sizes that affect the distribution of biodiversity, fishing grounds and deep-sea minerals.

The nation’s seascape is as diverse under- water as its landscape above, including towering underwater mountains (sea- mounts) that attract migratory species from hundreds of kilometres away, and deep- sea canyons that carry nutrient-rich water from the deep ocean to the shallow areas. Geomorphology (the study and classifica- tion of these physical features) reveals both the geological origin of the features as well their shape (morphology), size, location and slope.

shelf areas. On all these features, areas of steep sea floor (escarpments) are likely to contain hard substrate which, coupled with increased current flow, create ideal habitats for filter-feeding organisms such as sponges and cold-water corals. On the deep-sea floor, there are extensive areas of abyssal plains, hills and mountains. The deep Novo-Canton Trough runs to the north of the Phoenix group. This area of sea floor includes the hadal zone, where the sea floor is deeper than 6,000 metres. The mo- saic of different geomorphic features likely supports a large range of different ecosys- tems. In the absence of detailed information on the distribution of biodiversity, geomor- phology can be used to inform decisions on management of the sea floor in Kiribati.

The geomorphology of the sea floor influences the way the ocean moves (see also chapter “Go with the flow”), the way the wind blows and the distribution of water temperature and salinity (see also chapter “Hotter and higher”). These f actors affect the distribution of biologi- cal communities, resulting in different biological communities being associated with different types of sea-floor geomorphology. For exam- ple, seamounts generally have higher biodi- versity and a very different suite of species to the adjacent, deeper abyssal areas.

Similarly, different economic resources are often associated with different features. Many fisheries operate on certain features, such as the shelf, slope or over seamounts, based on where their target species occur. In Kiribati, important deep-sea snapper is mostly found on outer reef slopes and around seamounts (mainly in depths from 100 to 400 metres; see chapter “Fishing in the dark”). Furthermore, different types of deep-sea mineral deposits are also asso- ciated with different features, such as the sea-floor massive sulfide deposits found along mid-ocean ridges, cobalt-rich ferro- manganese crusts on the flanks of sea- mounts and nodule deposits on some deep abyssal plains (see chapter “Underwater Wild West”). Kiribati’s waters harbour 15 different ge- omorphic features, which are presented in this map and associated figures. The distribution of geomorphology reflects many of the patterns observed in the bathym- etry map, as geomorphology is primarily a classification of the shape of the sea- floor features. Kiribati’s waters include 342 seamounts and 12 guyots. Seamounts are large—over 1,000 metres high—conical mountains of volcanic origin, while guyots are seamounts with flattened tops (see also chapter “Underwater mountains”). There are also numerous ridges and chains of abyssal mountains rising up from the sea floor. The island chains of the Gilbert and Line groups are perched along such ridges. The steep sides of all these features interact with cur- rents and create important habitats for many species. Surrounding the islands is an area of generally narrow shelf, which supports extensive coral reefs. The adjacent areas of slope and the margins of the plateau are incised with numerous large, submarine canyons. These canyons are characterized as areas of high biodi-

The lost babai pit For the longest time, Kiribati’s exclu- sive economic zone (EEZ) was very calm. There was no active seismic activity, volcanic eruption or fracture zone on historical records. Then, in December 1981, a family in Arorae Island—the southernmost of the Gil- bert Islands—awoke to a surprise. In the morning, they realized that their babai, or giant taro pit, had com- pletely closed overnight. This was the effect of a series of undersea earthquakes at a location about 150 kilometres south-east of Arorae. The quakes continued until March 1983. This formerly unknown zone of weakness within the litho- sphere underneath the area near Aro- rae Island is an interesting example of the tectonic activity that ultimately shapes our ocean floor and creates the atolls and islands on top.

Submarine Canyon

0 km

100

200

Shelf

0

Shelf break

Terrace Escarpment

2

Slope

Foot of slope

Slope

km depth

Rise

Continental Crust - Granite

4

Fan

Sediments

Sediment Drifts

0 km

200

400

Ridge

2

Pinnacles

Guyot

Seamounts

Abyssal Hills

Trough

Base map showing the location of the largest earthquake swarm events in the Gilbert Island group (bathymetry contours in metres).

4

km depth

Seamount

Continental Crust - Granite

6

versity due to their steep sides featuring rocky slopes, strong currents and enhanced access to food. They also act as a conduit between the deep-sea floor and the shallow

Sediment

Ocean Crust - basalt

Subduction Zone

Sediment Drifts

Upwelling lava

MAXIMIZING BENEFITS FOR KIRIBATI

SUPPORTING VALUES

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