Deep sea floor By. The structures and features of ocean basin are summarized in Fig. 1. Ocean basin

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1 Deep sea floor By Sufia Zaman 1, Tanmay Ray Chaudhuri 2, Prosenjit Pramanick 2 and Abhijit Mitra 1 1 Department of Marine Science, University of Calcutta, 35, B. C. Road, Kolkata , India. 2 Department of Oceanography, Techno India University, Salt Lake Campus, Kolkata , India. About 71% of the surface of the planet Earth is covered by saline water. The water depth averages 3.8 km, a volume of km 3. This is the marine ecosystem and is the reservoir of food, oil, natural gas, minerals and several bioactive substances that have immense importance in the pharmaceutical industries. The ecosystem sustains the livelihood of millions of people. The oceans serve as the main highway for international trade as well as the main stabilizer of the world s climate. The oceanic waters and sediments are also the dwelling place of a large variety of flora and fauna. It is interesting to note that the ocean basin has unique designs and sculptures similar to the land features above it (Fig. 1.). Mountain ranges, canyons, valleys, and vast plains are all the important components of the underwater landscape. These physical features of the ocean bottom are called bathygraphic features, and unlike their counterpart topographic features on land, they change at relatively slow pace. Erosion is slow in the relatively calm recesses of the ocean, and changes are mainly attributed to sedimentation, uplifting, and subsidence. The structures and features of ocean basin are summarized in Fig. 1 Ocean basin Continental margin Continental rise Ocean basin floor

2 Continental shelf Continental slope Abyssal Oceanic Sea mounts Trenches Plains rises Fig. 1. Components of the ocean basin (after Mitra 2000) The entire marine/ocean ecosystem is divided in to benthic (substratum based) and aquatic zone or pelagic system (Fig. 2). Deep sea basically starts from the bathyal zone and features of the deep sea are discussed here. Fig. 2. Benthic compartment of the marine ecosystem Bathyal zone

3 Bathyal zone is generally described as lying between 200 and 2,000 m (660 and 6,600 feet) below the surface. Photosynthesis does not occur in bathyal waters as the zone is characteristically dark except in the clear, virtually lifeless waters of the tropics, where small amounts of sunlight can penetrate as deeply as 600 m (2,000 feet). Temperatures in high latitudes range from about 3 to -1 C (37 to 30 F). Elsewhere, normal temperatures range between 5 and 15 C (41 and 59 F). Salinities typically range between 34 and 36 psu in the bathyal zone, varying with local conditions of water-mass formation. Bathyal fauna are adapted to narrow ranges of temperature and salinity. At bathyal depths, currents are exceedingly slow, and in many areas bathyal waters deeper than 1,000 m (3,280 feet) are essentially stagnant, resulting in low oxygen concentrations and poor faunal biodiversity. Abyssal plains In many places, the ocean floor is a flat plain, known as the abyssal plain. Abyssal plains are found at the base of the continental rise and are relatively flat plains having a gradient less than 1:1000. They are formed due to even deposition of sediments from the continental rise carried down by the turbidity currents. Abyssal hills and seamounts are scattered across the sea floor. Submerged flat-topped seamounts, known as guyots, are found most often in the Pacific Ocean. They may vary from low hills to mountains as high as 1525 m. The example of Bermuda rise is very prominent in this context on which the Bermuda Islands are formed. Seamounts are isolated peaks that rise several thousand meters above the sea floor. These guyots are 1000 m to 1700 m (3300 to 5600 ft) below the surface, with many at the 1300 (4300 ft) depth. Many guyots exhibit the remains of shallow marine coral reefs and the evidence of wave erosion at their summits. This indicates that at one time they were surface features and that their flat tops are the result of past coral reef growth, wave erosion, or both. Hadal zone The hadal zone (named after the realm of Hades, the underworld in Greek mythology), also known as the trench zone, is the delineation for the deepest trenches in the ocean. This zone is found from a depth of around 6,000 meters (20,000 ft) to the bottom of the

4 ocean. The hadal zone has extremely poor biodiversity due to stressful environmental conditions in terms of temperature, pressure and light availability. Trenches are long narrow depressions in the hadal zone that are over 6100 m deep. The deepest known trench in the ocean compartment is the Mariana trench of the western North Pacific that is about 11,000 m deep. Vent communities of the deep sea In addition to existence of life in the deep waters of the ocean, the floors are important habitats for biotic community. In 1977, oceanographers discovered a unique community off the Galapagos Islands along volcanic ridges in the ocean floor. Since this initial discovery, marine scientists have discovered several other vent communities in different regions of the world including off the coast of Oregon, the west of Florida, the Gulf of Mexico and in the central Gulf of California. These communities are usually located around the deep sea hydrothermal vents that bring super- heated water and nutrients from beneath the earth s crust. The submarine hydrothermal vents are associated with the volcanically active zone at the crest of the mid-oceanic ridge system. The base of the oceanic crust is extremely hot (>1000 o C), and its upper boundary is in contact with sediments, due to which the elevation of temperature of the ambient media occurs. Hydrothermal activity accounting for water temperature elevations ranging from a few degrees above ambient to o C is now known to occur at sea floor spreading centres worldwide. On the basis of temperature, 3 main types of hydrothermal vents have been recognized which are: a) Diffuse vents: emitting low temperature, clear waters up to 30 o C. b) White smoker vents: releasing milky fluids with temperature varying from o C. c) Black smoker vents: discharging jets of water blackened by sulphide precipitates at temperatures between 300 and 400 o C. Gail and Hunt (1991) described a fourth kind of smoker, the basal mound variety characterized by sulphide-cemented biogenic tubes.

5 The most common type of basal mound smoker is found on the East Pacific rise. On the basis of geological setting, hydrothermal vents can be classified into two main types (Tivey and Johnson, 1989): (1) those at sedimented spreading centres and (2) those at unsedimented spreading centres. Despite the extremes of temperature and pressure, these self-contained communities are some of the most productive in the sea and stand as the proof of dependence of biotic community on non-conventional energy source. Chemosynthesis forms the foundation of such community and chemosynthetic bacteria occupy the first tropic level. Few common fauna of the hydrothermal vent community are listed here: Giant white clams Calyotogena magnifica Mussels - Bathymodiolus thermophilus Tube worm - Riftia pachyptila Galatheid crab - Munidopsis sp. Enteropneust worm - Saxipendium coronatum Brachyuran crab - Bythograea thermydron Turrid gastropod - Phymorphynchus sp. References 1. Gaill, F., and S. Hunt The biology of annelid worms from high temperature hydrothermal vent regions. Review of Aquatic Sciences 4: Mitra, Abhijit (2000). Foundation of Environmental Science, Narendra Publisher, New Delhi, 185 pp. 2. Tivey, M.A., and Johnson, H.P., High-resolution geophysical studies of oceanic hydrothermal systems. CRC Crit. Rev. Aquat. ScL, 1:

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