Located in the depths of the Pacific Ocean, The Mariana Trench is the deepest point in the world’s oceans ever recorded. It was first discovered by the HMS Challenger and is located South of Japan and East of the Philippines (11°20’ N, 142° 11’ E) in the Pacific Ocean using sounding equipment on board the ship. It was this sounding method that has allowed the majority of early research to be carried out. Sounding has been carried out by a multitude of different research teams and countries. The Mariana Trench has been the focal point of a lot of research due to its unique depth and the implications it has on the deep sea life within it. Challenger Deep (named after the ship from the 1875) is the deepest point with an approximate deepest point of 10,971m ± 22 metres (35,994 feet ± 72 feet) from the ocean’s surface (11°22.4’N, 142°35.5’E). It should be noted that due to the increased interest and research attention it gains, there has been a lot of debate as to its true depth.

Since discovery, there have been four descents achieved, but there has only been a total of three people to reach the bottom of the Mariana Trench. The first descent into the trench was achieved in 1960 by a United States Navy Bathyscaphe named Trieste, which was manned by Swiss oceanographer Jacques Piccard and United States Navy Lieutenant Don Walsh. This was a notable expedition as it had never been attempted before and the oceanographer on board was the son of the creator of the Trieste, Auguste Piccard. The next two expeditions were by unmanned ROVs (Remotely Operated Underwater Vehicle) that were owned by Japan and The United States and were launched in 1996 and 2009 respectively. The second manned excursion (the fourth descent) into the depths of the Mariana Trench was made by film director James Cameron in 2012 in an Australian submersible. There have been many different unmanned excursions down into the Mariana Trench by a variety of countries, but these people are the only people in history to venture down into the depths. This continued exploration has been ongoing since discovery and will likely proceed for a long time as discoveries are still being made with each individual descent.

The Mariana Trench is part of the “Izu-Bonin-Mariana” subduction system (the process of one of the Earth’s outer layers being pushed under another) at the boundary between two tectonic plates which is the reason for its impressive depth. The western edge of the Pacific plate is subducted (dragged below the other plate into the mantle of the Earth by gravity) below the Mariana Plate. It is the movements of the plates around the world that are responsible for many natural occurrences; tsunamis, earthquakes, and the formation of volcanoes are just a few. The movement of the previously mentioned plates is responsible for the creation of the Mariana Islands. The islands form a archipelago (an island chain) in a crescent shape and consist of 16 named islands, 12 of which are unpopulated. The four remaining islands include Guam (the most populated of approximately 150,000 inhabitants), Saipan, Tinian, and Rota.

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A cross section of the Mariana Trench subduction system. It shows the Pacific plate being dragged underneath the Mariana plate. Image: Hussong and Fryer, Wikipedia Commons.

Just How Deep Is The Mariana Trench?

When the trench was originally discovered in 1875 by the HMS Challenger, it was sounded using a weighted rope which recorded the depth of Challenger Deep as 4,475 fathoms (a dated and archaic measuring scale which is equivalent to 8,184 metres) which was the accepted measurement until 1899. It was then that the USS Nero recorded a depth of 5,269 fathoms (9,636 metres). However, in 1951, Challenger II surveyed the depths of the Challenger Deep using echo sounding. Echo sounding is a much faster form of sounding that uses sonar (sound pulses that are sent and received from an initial point, the time taken to travel to and from the seabed is used to calculate a distance), and is massively more accurate. Challenger II calculated the deepest part of the trench to be 5,960 fathoms (10,900 metres), and this was known from then on, as Challenger Deep.

The true depth of the trench is still unknown and highly debated. For years, the “deepest” point has been remeasured and corrected, and there is still no absolute measurement. The most accurate measurement thus far was collected was in 2009 by the Research Vessel Kilo Moana, and has a possible error margin of ± 22 metres. They used highly accurate sonar multibeam bathymetry (similar to echo sounding, but the sonar is emitted as a fan, so can provide a more accurate “map” through a process called beamforming) of the seabed to establish a deepest point of 10,971m (± 22 metres).

The HMS Challenger, the vessel first responsible for the discovery of The Mariana Trench. Image: Allan C. Green,1878. Wikipedia Commons.

What Lives In The Trench?

The incredible depth of the Mariana Trench means that there is next to no light and an incredible amount of pressure (caused by gravity and the weight of the water). This limits the amount of life that can thrive in the trench. The term for animals that can withstand this incredible pressure is “barophiles”. The sightings reported by Piccard and Walsh of the creatures found in the depths of the trench were called into question due to sheer disbelief and the lack of visibility that they had. Since then, as mentioned, there have been a number of unmanned missions to the bottom of the trench, and Cameron’s manned mission.

Amphipoda are a massively abundant type of crustacean (a type of animal with no backbone and an exoskeleton that include: crabs, lobsters, shrimp, and woodlice) that feed off of decomposing biological matter. They are small, usually only growing to a maximum size of a thumb tip. However, in the depths of the Mariana Trench, they were caught and filled the entire 17cm of the trap, and the ones that didn’t fit were assumed to be 30 centimetres long. Additionally, sea cucumbers were seen on film that were so well camouflaged, Cameron couldn’t see them from his submersible. In the 1950’s, Russian ships trawled some of the sea cucumbers from the trench, but they were much smaller than the ones recorded in 2012.

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An Ampeliscidae amphipod – The ones found in the depths of The Mariana Trench were significantly larger than normal amphipods. Image: Wikipedia Commons

However, the thing found at the bottom of the trench that proved to be most interesting, was microbial. The team responsible for sending Cameron to the bottom of the trench have said that the bacteria that they discovered blurred the line between microscopic and macroscopic – another example of the gigantism found in the deep ocean. Ronnie Glud of the University of Southern Denmark published a paper on the bacteria found on the floor of the Mariana Trench. He calculated that there is more microbial life (ten times more) at the bottom than the upper edges of the trench. This was done by sampling how much oxygen is missing from the thick ooze (made of detritus) found on the seabed – he hypothesised that the walls of the trench function as a funnel for the nutrition needed to support life at that depth. The ongoing exploration of the trench will only continue to uncover its secrets about what lies at the depth, and this exploration isn’t going to stop any time soon.

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