Mitra Abhijit̲ - Blue of the carbon blue reservoir planet

Springer India 2015

Abhijit Mitra 1

The sculptures on the ocean basin

Can never be created by human machine.

The Authors

The ocean basin has unique designs and sculptures similar to the land features above it. 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 represented in Figs..

Brief descriptions of these features are given here:

Continental Margin

The continental margin includes the continental shelf and slope. At the edge of a continent is the continental shelf (Fig. ). Continental shelves are generally flat areas, averaging 68 km (40 miles) in width and 130 m (430 ft) in depth, that slope gently towards the bottom of the ocean basin. The width of a continental shelf is frequently related to the slope of the land it borders. Mountainous coasts, like the West Coast of the USA, usually have a narrow continental shelf, whereas low-lying land, like the East Coast of the USA, usually has a wide one. Continental shelves are actually part of the continents to which they are attached. The criterion for defining the seaward edge of the continental shelf is a marked change in slope. The continental shelf has a gradient of about 1:1000. Seaward from the continental shelf is the continental slope , which extends to a depth of 1.63.2 km. The continental slope has a gradient between 1:2 and 1:40. The extent of the sloping can vary from a gradual drop to a steep decline into an ocean trench, as illustrated by the slope that occurs off the western coast of South America. Because of the steepness of the angle, the continental slope usually has less sediment.

Some continental slopes have submarine canyons that are similar to canyons found on land. Many of these submarine canyons are aligned with river systems on land and were probably formed by these rivers during periods of low sea level. The Hudson River canyon on the East Coast of the USA is an example of this. Other submarine canyons have ripple marks on the floor, and at the ends of the canyons, sediments fan out, suggesting that they were formed by moving sediments and water. Oceanographers believe that these canyons were formed by turbidity currents. Turbidity currents are swift avalanches of sediment and water that erode a slope as they sweep down and pick up speed. At the end of the slope, the current slows and the sediments fan out. Turbidity currents can be caused by earthquakes or the accumulation of large amounts of sediments on steep slopes that overload the slopes capacity to hold them.

Continental Rise

At the base of a steep continental slope, there may be a gentle slope called a continental rise. A continental rise is produced by processes such as landslides that carry sediments to the bottom of the continental slope. Most continental rises are located in the Atlantic and Indian Oceans and around the continent of Antarctica. In the Pacific Ocean, it is more common to find trenches located at the bottom of the continental slopes.

Oceanographers have studied that sediments from the continental shelf are carried into the deep ocean through the canyons present in the continental slope. At the foot of the continental slope, the suspended materials and sediments settle out that slope gently towards the ocean floor. This region is basically the continental rise and has a gradient between 1:50 and 1:800.

Ocean Basin Floor

The ocean basin floor extends seaward from the continental rise or the continental margins and includes the abyssal plains, oceanic rises, seamounts and trenches. 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. Oceanic rises refer to rather isolated areas, which are elevated above the abyssal floor and are distributed sporadically on the ocean basin floor. They may vary from low hills to mountains as high as 1,525 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 metres above the sea floor. They are volcanic in origin and are found principally in the vicinity of the fault zones. Some seamounts show evidence of coral reefs and surface erosion, suggesting that at one time they were above the surface. Movements of the ocean floor, the natural process of compaction that volcanic material undergoes, subsidence due to cooling of the ocean floor, erosion, and the increased weight of sediments at the top may be the reasons for the sinking of these structures.

Trenches are long narrow depressions in the ocean floor that are over 6,100 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. The PeruChile Trench extends for over 6,120 km (3,600 miles) along the coast of South America and is the longest of the ocean trenches. The Java Trench extends for a distance of almost 4,760 km (2,956 miles) along the coast of the islands of Indonesia. By comparison, the Atlantic has only two, relatively short trenches, the South Sandwich Trench and the Puerto RicoCayman Trench. Trenches are invariably associated with the systems of active volcanoes and are believed to be caused by down wrapping of the oceanic crust beneath the continental crust.

Overview of Estuarine Environment

An estuary may be defined as a transition zone between the freshwater and marine water with unique physico-chemical and biological attributes. According to Odum (), estuaries belong to different class of fluctuating water-level ecosystems. Estuaries generally occupy those areas of the coasts, which are least subjected to marine action and thus are major sites for development of harbours, recreational activities and appropriate aquacultural activities. In estuaries, freshwater collected over vast regions of the land pours into an ocean, which sends salt water upstream far beyond the river mouth.

Each estuary has its own physical features that influence its ecology. These include the amount of river discharge, depth and general topography, specific circulation pattern, climatic regime and vertical tide range. Though the physico-chemical condition in estuaries is stressful, yet, the food availability is so favourable that the ecosystem is packed with life. Organisms living in this habitat therefore exhibit wide range of tolerances (i.e., they are mainly euryhaline). Estuaries rank first among the most productive regions of marine ecosystems as they typically contain high biomass of benthic algae, seagrasses and phytoplankton that support large number of fishes and birds. This is mainly because estuaries are enriched by nutrients that are contributed through land drainage. Moreover, the nutrient retention capacity within the estuary is also unique due to which the flora and fauna thriving within such system never face the scarcity of nutrients.