Marine Atlas: Maximizing Benefits for Fiji
VOYAGE TO THE BOTTOM OF THE SEA: GEOMORPHOLOGY
Fiji’s sea floor is rich in physical features that affect the distribution of biodiversity, fishing grounds, deep-sea miner- als and even tsunamis and underwater landslides.
The nation’s seascape is as diverse underwater as its landscape above, including towering underwa- ter mountains (seamounts) 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. Geo- morphology (the study and classification of these physical features) reveals both the geological origin of the features as well their shape (morphol- ogy), size, location and slope. The geomorphology of the sea floor influences the way the ocean moves (see also chapter “Go with the flow”) and the distribution of water temperature and salinity (see also chapter “Hotter and higher”). These factors affect the distribution of biologi- cal communities, resulting in different biological communities being associated with different types of sea-floor geomorphology. For example, sea- mounts generally have higher biodiversity 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 Fiji, 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 min- eral deposits are also associated with different features: cobalt-rich ferromanganese crusts are found on the flanks of seamounts, massive sulfide deposits occur along the mid-ocean ridges and nodule deposits are found on some deep abyssal plains (see chapter “Underwater Wild West”). Fiji’s waters harbour 16 different geomorphic features, which are presented in this map and associated figures. The distribution of geomor- phology reflects many of the patterns observed in the bathymetry map, as geomorphology is primar- ily a classification of the shape of the sea-floor features. Some notable features in Fiji’s waters include 59 seamounts and three guyots. Sea-
The slope of physical underwater features is hugely significant, as can be illustrated by an event that happened 1953. On 14 September, Fiji’s capital, Suva, was struck by a nearby earthquake (see also chapter “Smoke under- water, fire in the sea”). Unlike the 2009 Samo- an earthquake (see chapter “Still waters run deep”), this earthquake did not create a direct tsunami that threatened Fiji or other Pacific Island countries. However, it did cause a coral reef platform to collapse, which induced a submarine landslide. Sixty million m3 of mud,
stone and part of a long-wrecked vessel hurtled to the depths of the Suva Canyon, at the west- ern end of the entrance to the harbour. It was this event that triggered a tsunami, devastating the villages of Nakasaleka and Makaluva, as well as parts of Suva. The tsunami resulted in five deaths and caused a total estimated damage of US$500,000 (at 1953 values), making it the most destructive earthquake in Fiji’s recorded history (Rahiman, 2007; Pacific Disaster Center, 2011). Had the tsunami occurred at high tide, it would have been even more damaging.
Foot of slope
Continental Crust - Granite
mounts 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 numer- ous ridges and chains of abyssal mountains, all of which rise up from the sea floor. The steep sides of all these features interact with currents and create important habitats for many species. The main is- lands of Fiji are perched on a raised plateau, which extends to the east and the south. 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 subma- rine canyons. These canyons are characterized as areas of high biodiversity due to their steep sides featuring rocky slopes, strong currents and en- hanced access to food. They also act as a conduit between the deep-sea floor and the shallow shelf areas. On all these features, areas of steep sea floor (escarpments) are likely to contain hard sub- strate which, coupled with increased current flow, create ideal habitats for filter-feeding organisms such as sponges and cold-water corals.
Continental Crust - Granite
Ocean Crust - basalt
MAXIMIZING BENEFITS FOR FIJI
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