The abyssal areas of the sea represent over 90% of the benthos as a whole. Some 80% of the ocean lies at depths of over 3,000 m, yet we have scarcely begun to explore it. We know more about the moon than we do about the deep oceans on our planet. Even in Monterey Bay, the best-studied sea floor in the world, over 98% of the benthos remains unexplored by humans. On almost every dive in a deep-sea submersible, marine biologists are finding species that are new to science. It is an exciting time for researchers involved in deep-sea biology.
In the dark and inaccessible regions of the deep sea, the vast tracts of the abyssal plain might seem devoid of life, but they are remarkably diverse areas of the ocean. Animals living in the deep sea endure incredible pressures, extreme cold and live in constant darkness. Although food is a limited resource, the deep sea has many advantages. Low currents, continual dark, temperature and salinity, are steady and constant in contrast to the turbulence of the surface waters. Animals produce light, bioluminescence, to signal to other animals and to lure prey within easy reach.
Humans have long been fascinated with the sea, and yet, even with modern technology, we have barely begun to explore the vast areas of the deep sea. Only 10 km2 of the deep sea floor has been explored. Humans using ordinary SCUBA gear can dive to about 30 m, and this is generally adequate for studying coral reefs and the upper layers of the photic zone. Deeper diving is possible using a commercial atmospheric underwater diving suit called JIM after its inventor. JIM is a carbon fibre reinforced plastic and aluminium suit with tight pressure seals and articulated joints. The diver is always at atmospheric pressure within the suit, which relieves the need for decompression during surfacing (as is required of SCUBA divers). Some also have thrusters to allow movement in the water. JIMs are theoretically capable of taking a diver to 450m.
In 1964, the first successful scientific deep-sea manned submersible, called Alvin, was the first deep-sea submersible capable of carrying passengers. Owned and operated by the Woods Hole Oceanographic Institution, Alvin could dive to depths of about 3 km and later updated versions to about 4 km. Alvin was the first manned submersible to explore hydrothermal vents in the 1970s and has since discovered 24 new vents in the Atlantic and Pacific Oceans. Now Alvin along with other submersibles can reach depths of 6 km. Even so, submersibles have only explored a few hundred metres of the mid-ocean ridges and hydrothermal vents. Mid-ocean ridges stretch for over 45,000 km throughout the major oceans of the world except the North Pacific. They are the most significant geological structures on Earth, some 3,000 m high and 50 km wide. We have a lot yet to discover in these remarkable places.
Remotely operated vehicles (ROVs) are able to explore deeper than manned submersibles and when an inspection of the Titanic was carried out at a depth of 4 km, Alvin was used as a manned submersible but took with it an ROV called Jason which remained linked to Alvin by an umbilical cord but was able to probe into the Titanic without risking the safety of the human divers. The Johnson Sea Link, owned and operated by Harbour Branch Oceanographic Institution, found pieces of the Space Shuttle Challenger after it crashed into the sea. It was possible for investigators to discover the cause of the disaster after the Johnson Sea Link found the rocket booster with a faulty seal.
Cutting edge scientific investigation to the deep sea using manned submersibles and ROVs is being carried out by the Monterey Bay Aquarium Research Institute, Harbour Branch Oceanographic Institution and the Woods Hole Oceanographic Institution in the US. Manned submersibles, such as the Johnson Sea Link, which carry two or three people and a pilot, can reach 1,000 m with just a few ROVs worldwide able to dive even deeper at around 3,000 m. However, even with the incredible achievements and discoveries in the deep sea in recent years, the very deepest part of the ocean, the Marianas Trench, remains inaccessible at over 11,000m.
The continental shelf constitutes just 8% of the world ocean, yet this is where the majority of the fauna and flora of the benthos live. The average depth is just 200 m and lies within the photic zone, the most productive area of the sea. Sea levels vary, and continental shelves change over time with submerged beaches, cliffs, river valleys and the prograding deltas. Submarine processes such as wave action and currents continuously rework sedimentary deposits, and so the continental margins are a dynamic and ever-changing region of the sea.
The continental shelves we see today are the result of continental terraces formed when sea levels were lower during glaciations 15-20,000 years ago. The average slope of the continental shelf varies between 3-200 depending on whether the margin is passive or active. Atlantic type passive margins are deep with a gradual slope caused by continental rifting, while Pacific type active margins are steep and occur at destructive plate boundaries, marked by volcanic activity and earthquakes.
Erosion of rock from the continents continually builds continental shelves seaward by the deposition of sediments carried in rivers from the land or submarine canyons in the continental shelf and slope. These are called submarine fans and occur on all types of the continental slope. Submarine fans are enormous and are measured in tens of kilometres. One submarine fan that lies seaward of the Ganges River, the Ganges Fan, is 2,500 km out to sea and up to 5,000 m deep.
There are primarily two origins for sediments found on the sea floor, terragenous and bioclastic with a small contribution from volcanic activity. Terragenous sediments originate from the erosion of continental rock. Bioclastic sediments are the result of biological activity and include the dead remains of pelagic plants and animals that have sunk to the sea floor. These types of sediment can also be classified as pelagic or deep-sea sediments.
Currents sort Terragenous sediments with the large particles deposited inshore over the continental shelves, and the fine particles carried in suspension to the offshore area. In warm wet latitudes, chemical weathering of rock predominates while in cold, high latitudes, physical weathering produces the bulk of the sediment which is carried out to sea.
Currents move sediments off the edge of the continental shelf and onto the continental slope and rise and into the ocean basin. Fine particles stay in suspension while the coarser material collects and occasionally shifts in what is known as turbidity currents. These are vast heaps of sediment and water that slump down into submarine canyons. One famous turbidity current occurred in 1929 and was started by an earthquake in the Grand Banks east of North America. The turbidity current was so powerful that submarine cables were broken and sediment was carried for 300 miles travelling at speeds estimated at 55 knots.