Ocean Deposits and Coral Reefs

The sea floor is slowly buried under sediments, and in warm shallow waters tiny animals build some of the largest living structures on Earth. Ocean deposits record the history of the planet, while coral reefs are biodiversity hotspots of great ecological value. Both are favourite exam topics.

The unconsolidated sediments that settle on the ocean floor are ocean deposits. They are of two broad kinds:

• Terrigenous deposits: material derived from the land (worn rock, plus volcanic and organic matter), carried to sea by rivers, wind and waves. They are found mainly on the continental shelves and slopes, and are graded by size into mud, sand and gravel.
• Pelagic deposits: fine material settling over the deep-sea plains, covering about 75% of the ocean floor. They include:
  • oozes: soft mud of the shells and skeletons of marine organisms. These are divided into calcareous ooze (calcium carbonate) and siliceous ooze (silica), and
  • red clay: a fine, reddish, inorganic deposit of volcanic origin that covers more than half the Pacific floor.

The deep ocean floor is not just mud and clay. It also holds valuable mineral resources, and these are a regular exam theme:

• Polymetallic nodules: potato-sized lumps scattered on the deep-sea plains. They are rich in manganese, nickel, copper and cobalt, metals needed for batteries and electronics.
• Rare earth minerals: deep-sea sediments and crusts also contain rare earth elements, which are critical for modern technology.

Much of this seabed lies in international waters, beyond any country's jurisdiction. Mining there is regulated by the International Seabed Authority (ISA), a body set up under the UN Convention on the Law of the Sea (UNCLOS). The ISA, not any other ocean body such as the Global Ocean Commission, grants licences for seabed exploration and mining in international waters. India holds ISA exploration licences, including one for polymetallic nodules in the Central Indian Ocean Basin.

Coral reefs are large underwater structures built from the calcium carbonate skeletons of tiny marine animals called coral polyps. Reef-building (hermatypic, or "hard") corals extract calcium carbonate from seawater to make a hard exoskeleton; as generations pile up, a reef slowly grows.

Corals are delicate and grow only where conditions are right:

• Temperature: warm water, around 20°C or above (found between roughly 30°N and 30°S).
• Depth and light: shallow, clear water (about 45–55 m) where sunlight can reach.
• Salinity: clean, normal-salinity seawater. Corals avoid freshwater and very salty water, so they do not grow near river mouths/deltas.
• Oxygen and plankton: plenty of both, for food and growth.

The Great Barrier Reef off Australia is the world's largest coral reef.

Coral reefs are classified into three types by their position:

• Fringing reefs: grow directly from the shore, close to land, with at most a shallow lagoon. They are the most common type.
• Barrier reefs: large reefs lying offshore, parallel to the coast, separated from it by a wide lagoon. They are the largest type (the Great Barrier Reef is the prime example).
• Atolls: roughly ring-shaped reefs surrounding a central lagoon, often formed on a sinking volcanic island.

Reefs are vital but fragile. They shelter extraordinary biodiversity and are highly vulnerable to warming and human pressure.