What are pelagic deposits?
Ocean deposits usually consist of unconsolidated sediments, which can come from various sources, and are deposited at the ocean floor. Thickness of these deposits vary greatly from one ocean to another (Fig.1). Ocean deposits can be differentiated on the basis of their composition, source, method of transit, and mode of distribution.
Fig. 1: Total sediment thickness of World’s Oceans and marginal seas
The components of ocean deposits can be transported by rivers, winds, volcanic eruptions, and marine organisms, both plants and animals can also contribute to the transportation and deposit of ocean deposits. According to the characteristics of ocean depth, ocean deposits can be classified as pelagic deposits, and terrigenous deposits.
While terrigenous deposits at mostly the shallower depths can include deposits of mud, sand, gravel, and volcanic materials, derived from denudation of continental rocks, pelagic deposits can consist of organic material in the form of marine plants and animals as well as inorganic material. Pelagic deposits can cover about 75.5 per cent of ocean areas in the form of many types of oozes in a sporadic mix in most cases with other types of ocean deposits. Of the total ocean deposits, red clay covers 31.1 per cent of the ocean floor (Sirisha P., 2017). Pelagic Zone consists four major zones classified according to depth i.e Epipelagic zone, Mesopelagic zone, Bathypelagic zone and Abyssopelagic zone as shown in figure 2.
Fig. 2: Pelagic Zone
As other than volcanic ash, little terrigenous substances are transported into the ocean depths, most of the ocean floor is overlaid with pelagic deposits. Pelagic deposits can consist of both organic and inorganic material, and the organic material can frequently occur as ooze, which is a sort of liquid mud, made up largely of the remains of various marine organisms.
When the organic substance is largely made up of calcium carbonate, the ooze is said to be calcareous. Calcareous ooze can be further classified into globogerina ooze or pteropod ooze. The majority of the ocean floors in the Atlantic and Indian Oceans have pelagic deposits that are made up of calcareous ooze. Ooze can also occur as siliceous ooze, which again can be classified into radiolarian ooze and diatom ooze. The siliceous ooze is present in the southern regions of the Atlantic and the Indian Oceans.
The inorganic material making up pelagic deposits consist mainly of red clay that usually originates from volcanic activity. Red clay is mainly made up of silicon and aluminium dioxide, while the other constituents can include radium, phosphorous manganese and iron. Red clay comprises the most widely distributed specific pelagic deposit and covers more than half of the total ocean floor in the Pacific Ocean (Samiksha S., 2016). The distribution of Pelagic (Clay, Calcareous Oozes and Siliceous Oozes) and terrigenous sediments deposits has been depicted in figure 3.
Fig. 3: Oceanic distribution of pelagic and Terrigenous deposits globally
In 1874, Thomson published a preliminary report on the nature of pelagic deposits, followed by a description by Murray in 1876. This succeeded an expedition by the H.M.S. Challenger in 1872 that carried out pioneer studies of ocean deposits.
These studies established the existence of red clays, globigerina, pteropod, radiolarian and diatom oozes on the ocean floor. This was followed by the publication of many papers investigating the nature of deep ocean deposits. Most studies in this period were interested in paleontological observations in analyzing ocean deposits. Many comparisons were made between pelagic deposits on the ocean floor and soils in terrestrial locations.
Among these observations, Murray’s observations were the most influential, which asserted that sediments were not necessarily located at great depths and oceanic, and deep sea ocean deposits could even be found in environments close to the shoreline. Murray’s observations were popular mainly among the Anglo-American geological diaspora, and many European scientists continued to pursue the presence of oceanic deposits in ancient mountain chains.
Since 1968, with the observations of the Glomar Challenger, a new world of findings opened for geology. The results from their observations conclusively pointed towards a spreading ocean, drifting and colliding continents, etc such that there was a good possibility that ocean deposits could be found within mountain soil.
Mountainous regions indeed were sites of two colliding continents, making them logical sites for pre-ocean deposits (H.C. Jenkyns, 2010). However, studies so far have revealed that pelagic deposits are largely confined to ocean basins, with fresh pelagic deposits no replica of the distant past.
Although investigations of organic pelagic deposits of earlier periods can reveal a great deal about biodiversity in previous eras, not much is known in totality of the complete deep-ocean biodiversity. Scientific knowledge of both pelagic and benthic biodiversity is not complete, and a great deal is still left to be known. In the past, studies have been carried out of deep-ocean biodiversity for example in the Atlantic Ocean, the Arabian Sea, and parts of the Pacific, but these studies have been carried out on a spatially and temporally limited basis on a limited population of taxa.
The deep-ocean floor can exhibit a great variance of topographical characteristics, such as submarine canyons, continental slopes, bathyal or basin plains, and base-of-slope deposits, with a wide occurrence of pelagic deposits that have been mapped. Many features as well can dominate this landscape such as volcanic activity, hypersaline basins, gas releases, underwater landslides, etc. Some parts of the interlinked ocean can be more saline than other parts. A wide range of deep-ocean habitats are possible in these regions.
Deep-ocean currents and habitats are however, not fully documented. Many phenomena can contribute to biodiversity at the ocean floor, such as high oxygen concentrations across the water column down to the ocean floor in winter in the Mediterranean sea, which is not present in late spring and summer, with the seasonal thermocline being 20 to 50 m deep (Danovaro et al., 2010).
Knowing about deep-ocean biodiversity is especially important as we engage in deep ocean activities such as deep ocean drilling and resource extraction from the ocean depths. A number of endemic species might forms habitats in these regions, whose behaviour and movement in deep ocean habitats is not fully studied and monitored. Studies of pelagic deposits might be a great help in this regard to aid studies of anthropogenic activities and the composition of residue at the deep-ocean floor.
Many benthopelagic species exist such as certain species of shrimp whose activities have not been fully documented. Studies on prokaryotic life in deep-ocean habitats is even more limited, and necessary as they can act as primary producers in these deep-ocean habitats. Pelagic deposits, especially organic oozes, can act as suitable habitats for a range of biodiversity to occur, and a fuller study of these habitats can greatly assist both science and human understanding of the effects of anthropogenic activities in the deep ocean.