The Physical Structure of Oceans PDF

CHAPTER 1 The physical structure of oceans This chapter sets the ecological scene by describing some of the major physical fea- tures and zones of the ocean. Some appreciation of the physical marine environ- ment is essential to understand the distribution and ecology of marine communities and org...

CHAPTER 1 The physical structure of oceans This chapter sets the ecological scene by describing some of the major physical fea- tures and zones of the ocean. Some appreciation of the physical marine environ- ment is essential to understand the distribution and ecology of marine communities and organisms. The major oceans and seas, all of which are connected, are shown in Fig. 1.1 and depth and area statistics are shown in Table 1.1. In the deepest parts, the seabed lies more than 10,000 m from the surface and the average depth of the ocean is about 3700 m. Although marine organisms are unevenly distributed, they occur throughout this vast three-dimensional environment and have been sampled or seen even in the deepest places. Recent introductory oceanography texts to which the student can refer for more detailed information are given at the end of this chapter. Arctic Ocean 2 5 10 6 7 1 17 16 8 4 11 9 12 3 Atlantic 13 15 Ocean Pacific Ocean Pacific Ocean Indian 14 18 Ocean Southern Ocean Figure 1.1 Major oceans and seas of the world. (1) Bering Sea; (2) Beaufort Sea; (3) Caribbean Sea; (4) Sargasso Sea; (5) Greenland Sea; (6) North Sea; (7) Baltic Sea; (8) Mediterranean Sea; (9) Red Sea; (10) Barents Sea; (11) Arabian Gulf; (12) Arabian Sea; (13) Bay of Bengal; (14) Timor Sea; (15) South China Sea; (16) Sea of Japan; (17) Sea of Okhotsk; (18) Coral Sea. From Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press. Elements of Marine Ecology r 2022 Elsevier Ltd. DOI: https://doi.org/10.1016/B978-0-08-102826-1.00011-9 All rights reserved. 1 2 Elements of Marine Ecology Table 1.1 Approximate statistics for major oceans and seas. Area (km2) Average Max Max depth site depth (m) depth (m) Oceans Pacific 161,760,000 4080 10,803 Mariana Trench Atlantic 85,133,000 3646 8486 Puerto Rico Trench Indian 70,560,000 3741 7906 Sunda (Java) Trench Arctic 15,558,000 1205 5567 Eurasia Basin Southern (Antarctic) 21,960,000 3270 7075 Sandwich Trench Seas Notes North (Greater) 750,000 95 700 Skagerrak Baltic 386,000 55 449 Off Gotland Mediterranean (inc. 2,967,000 1480 5267 Calypso Deep Black and Azov Seas) Caribbean 2,750,000 2200 7685 Cayman Trench Red 450,000 500 2500 Central trough South China 3,700,00 1419 5016 West of Luzon, DK Notes: Not surprisingly, ocean statistics vary according to their source. The area varies according to the exact ocean boundaries used by the calculating agency. In this table, area and depths for oceans and for the Mediterranean are from the NOAA’s National Geophysical Data Centre (Eakins and Sharman, 2010). Data for other seas are from various sources that do not refer to specific surveys and may be less accurate. Source: Modified from Dipper, F.A., 2016. The Marine World. A Natural History of Ocean Life. Wild Nature Press. Courtesy Wild Nature Press. 1.1 Physical features and topography An accurate and complete map of the ocean floor showing its immensely varied terrain of flat plains, mountain chains, trenches, shelves and other fascinating features is not yet available. New features, including large seamounts, are still being discovered. However, full details of all the known depth contours and named features such as ridges and frac- ture zones, seamounts, basins and ocean trenches are shown on the GEBCO World Map, which can be downloaded and printed or details can be viewed online (http:// www.gebco.net). They also provide access to a gazetteer of undersea feature names. Major topographical features have been mapped over many decades, using soundings in the early years and through sonar from the 20th century onwards. The geographical positions of 53 important main features are shown and listed in Fig. 1.2, and the main types (such as ridges and basins) are described in the following sections (1.1 1.6).  The physical structure of oceans 3 Figure 1.2 Map showing the location of major topographical features of the ocean. Light blue areas indicate Continental Shelf. (Modified from Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press) Key: 1. Aleutian Trench 28. Arabian Basin 2. Northeast Pacific Basin 29. Carlsberg Ridge 3. Hawaiian Ridge 30. Central Indian Ridge 4. Central Pacific Basin 31. Southeast Indian Ridge 5. Tonga Trench 32. Crozet Basin 6. Kermadec Trench 33. Kergelen Plateau 7. Southwest Pacific Basin 34. Mid-Indian Ocean Basin 8. Pacific Antarctic Ridge 35. Ninetyeast Ridge 9. Bellingshausen Basin 36. Sunda (Java) trench 10. East Pacific Rise 37. Wharton Basin 11. Middle American Trench 38. Broken Ridge 12. Cayman Trench 39. Australian-Antarctic Basin 13. Peru-Chile Trench 40. Tasman Basin 14. Puerto Rico Trench 41. Lord Howe Rise 15. Labrador Basin 42. New Caledonian Trough 16. Reykjanes Ridge 43. South New Hebrides Trench 17. Norwegian Basin 44. Manus Trench 18. Rockall Plateau 45. Philippine Trench 19. Mid-Atlantic Ridge 46. Challenger Deep 20. Brazil Basin 47. Mariana Trench 21. Argentine Basin 48. Nansei-Shoto Basin 22. South Sandwich Trench 49. Izu-Ogasawara Trench 23. Angola Basin 50. Japan Trench 24. Cape Basin 51. Kuril-Kamchatka Trench 25. Agulhas Basin 52. Northwest Pacific Basin 26. Weddell Sea Basin 53. Emperor Seamount Chain 27. Southwest Indian Ridge 4 Elements of Marine Ecology BOX 1.1 Seabed 2030. In 2017 a project was launched with the aim of completing the mapping of the ocean floor by 2030. This is an international collaborative project facilitated by GEBCO (General Bathymetric Chart of the Oceans) and the Nippon Foundation. A large number of different organizations collect bathymetric data but what happens to this data and how it is collected varies widely. The initial focus is on assembling existing data via four Regional Data Assembly and Coordination Centres. Using current technology, it is estimated that several hundred ship-years of new mapping effort will also be needed to achieve complete cover- age (Mayer et al., 2018). So the project will also facilitate the development of new technolo- gies to speed up this process. 1.1.1 Coastlines and beaches Coastlines and shores around the world vary hugely with everything from vertical cliffs plunging straight into the sea to miles of intertidal mudflat edged by saltmarsh. There are very few places where the coastline runs straight; instead, it is indented by bays and penetrated by sealochs, river estuaries and other features. Neither is it constant in time because coastline and shores are eroded away by wave action and are built up by material brought in by currents, by waves and from rivers. The status of any particular part of the coastline at any one point in time results from a combination of geographi- cal position, geology, sea level, weather (waves) and currents. It can also be affected by biological factors, for example vegetation that may stabilize sand dunes and mangroves and coral reefs that modify wave action. Evolution of coastlines Erosion and deposition, continued over long periods, gradually change the configura- tion of a coastline, tending eventually to straighten it by wearing away the headlands. Over millions of years, these processes would have reduced long stretches of shore to virtual uniformity, were it not for the changes in relative levels of land and sea, that have occurred from time to time throughout the earth’s history (Steers, 1969). The causes of these changes are complex, but variation in world climate altering the vol- ume of water in the oceans has certainly been one of the major factors during the last million years. During this period, there was a series of ice ages when the world climate was much colder than at present, polar ice caps extended to lower latitudes and more snow remained on the mountains instead of melting and flowing into the sea. During each ice age, a greater proportion of the earth’s water was locked up in frozen form and sea level fell. Conversely, during the warmer interglacial periods, melting ice and snow increased the volume of water in the oceans and raised sea level.  The physical structure of oceans 5 Changes in ocean volume do not produce equal relative changes in land and sea level in all parts of the world. The enormous weight of an ice cap can depress the level of the underlying land. When the ice cap melts, although the sea becomes deeper, that part of the land which has been relieved from the huge load of ice may rise con- siderably more than the sea around it so that the sea level falls relative to the rising land. During the last ice age 20,000 years ago, Great Britain was covered by an ice sheet that was thickest over Scotland. When the ice melted, the landmass gradually tilted back up in the north and consequently down in the south, a process called iso- static adjustment that continues to this day. Depending on the change of relative levels of land and sea, the changes of the coastline may be submergent or emergent. Submergent coasts Where sea level rises relative to the land, deep inlets are formed by the sea flooding the lower parts of river valleys. The headlands between valleys are exposed to wave action and are gradually eroded to form cliffs. Within the sheltered valleys, eroded material is deposited and the inlets gradually fill with sand or silt (Fig. 1.3). This sub- mergent type of coastline is especially evident around the south-west peninsula of the British Isles and the flooded river valleys are called rias. Fjords (or fiords) also result from marine flooding, but these often steep-sided and deep inlets are originally formed through glacial erosion. Norway and New Zealand have many such dramatic examples. If land and sea level remain stable for any long period, the headlands are progres- sively cut back, reducing the depth of inlets, until they eventually become bays con- taining beaches between extensive stretches of high cliff. When the protecting headlands have been completely obliterated, the bays themselves become exposed to erosion and the coastline then tends to become fairly uniform, consisting mainly of cliffs with little if any beach, that is, a mature coastline. Emergent coasts Where and when sea levels are falling, the sea recedes from the land and the margin of the water meets a gentle slope that was originally the sea bottom. Waves tend to break a long way out because the water is shallow for a considerable distance from the shore. In the line where the waves break, the seabed becomes churned up and loos- ened material may be thrown ahead of the breaking waves. This sometimes leads to the formation of ridges or bars of sand or gravel, known as offshore bars. These are often of transient duration, their shape and position changing from tide to tide, but occasionally the process is cumulative so that a bar is eventually built up above sea level. It may then become stabilized and consolidated by plant growth along its crest. The seaward side of the bar is now the new coastline. The lagoon between the earlier 6 Elements of Marine Ecology (A) (B) (C) Figure 1.3 Stages in the evolution of a submerged coast. (A) Early phase: flooded valleys, erosion of headlands and deposition of sand and silt within the inlets. (B) Intermediate phase: headlands cut back further, lengthening the cliff line and exposing the bays to stronger wave action. Beach material becomes coarser and less stable. (C) Mature phase: promontories eliminated and cliffs vir- tually continuous. shoreline and the newly formed bar becomes silted up, forming an area of salt marsh which may later convert into sand dunes and finally into ordinary soil. If sea level continues to fall, this sequence may be repeated several times. Once the sea level becomes constant over a long period, wave erosion is likely to encroach grad- ually upon the land, leading at length to the formation of cliffs and the development of a mature coastline. Due probably to a rise of sea level since the last glaciation, much of the coast of the British Isles is the submergent type in various stages of evolution. However, there  The physical structure of oceans 7 are signs of old cliffs and beaches, some now many feet above sea level and far behind the present coastline, in several places, for example parts of Sussex and Devon, Norfolk and some areas of Scotland (Fig. 1.4). Beach modification by waves The type of beach (ie intertidal regions) found in a particular area depends ultimately on the evolution of the coastline but is also subsequently affected by wave action. As a wave breaks on the beach, it causes an up-rush of water that flows up the beach. This is known as the ‘swash’ or ‘send’ of the wave. Part of this water percolates down through the beach and the remainder flows back over the surface as the ‘backwash’. The effect of breakers on the shore may be destructive or constructive. Destructive breakers are usually formed by high waves of short wavelength; for example those aris- ing from gales close to the coast. As these waves break, they tend to plunge vertically or even curl seawards slightly, which results in a less powerful swash, but pounds the beach, loosening beach material and carrying some of it seawards in the backwash. Constructive waves are more likely to be low waves of long wavelength, such as the swell from distant storms. These waves move rapidly shoreward and plunge forwards as they break, transmitting much power to the swash and tending to carry material up the beach, leaving it stranded. The continual transmission of energy from waves to shore gradually modifies the coastline, either eroding the beach by carrying away the beach material or adding to the beach by deposition. In any sequence of breakers, there may be waves derived from many different sources which combine to form many different heights and wavelengths, some destructive and some constructive. The condition of the shore is, therefore, a somewhat unstable equilibrium between the two processes of erosion and deposition. The balance varies from time to time and differs greatly in different regions. Where the shore is exposed to very violent wave action, erosion usually predominates. The waves break up the shore, fragmenting the softer materials and carrying them away. Harder rocks are left exposed and are gradually fractured into boulders, making the coastline rocky and irregular. Strong currents may assist the process of erosion by carrying away finely divided material. But any material carried away from the shore in one place may be deposited as beach material in another and such shores where waves consistently deposit material are made up largely of pebbles, sand or mud. Beach construction Beaches consist of a veneer of beach material covering a beach platform of underlying rock. In very sheltered situations the beach material may rest on a gentle slope of rock virtually unmodified by wave action, but in wave-washed localities the beach platform has usually been formed by wave erosion. Where the land is gradually cut back, both cliff and beach platform are formed concurrently (Figs 1.5 and 1.6). As the beach 8 Elements of Marine Ecology Figure 1.4 Submergent and emergent coastal features can both be found along England’s south coast. (A) Portsmouth Harbour, a well-known and complex ria (submergent feature) provides shel- ter for industry and marinas, but also extensive and ecologically important areas of mud-flats and tidal creeks. (B) Portland Pleistocene raised beach (emergent feature).  The physical structure of oceans 9 Original surface Beach veneer Tidal Cliff range HW Wave LW eroded notch Eroded beac h platform Figure 1.5 Beach section to illustrate the erosion of the cliff to form the beach platform and veneer. Figure 1.6 The famous layered cliffs at Hunstanton, Norfolk, England. The brownish-red carstone (sedimentary sandstone) at the base was deposited when shallow Cretaceous seas covered the area; the two layers above are both chalk and were laid down when the water was deep. The red chalk is coloured by iron pigments and the white chalk above is layered. Both contain many fossils. The eroded beach platform shows as straight lines of boulders. platform becomes wider, waves crossing it lose power and erosion of the cliff base is reduced. The seaward margin of the beach platform is itself sometimes subject to ero- sion by large waves breaking further out, so the cutting back of the beach platform and the cliff base may proceed together. Cliff erosion is caused largely by the abrasive 10 Elements of Marine Ecology action of stones, sand and silt churned up by the water and hurled against the base of the cliff, undercutting it until the overhanging rock collapses. Where the rocks are very hard, they may not be appreciably worn away but can be cracked along lines of weakness by sudden air compression in holes and crevices when waves strike the cliff. This leads to falls of large pieces of rock and produces a boulder-strewn coastline on which the beach platform is often quite narrow. The range of tidal movement has a considerable bearing on the rate of erosion because the greater the depth of water cov- ering the beach platform at high tide, the more powerful the waves that can cross it to erode the cliff base. Various sources may contribute to the materials covering beach platforms; for example fragments derived from erosion of adjacent cliffs, or churned up from the sea- bed, or eroded from the edge of the beach platform, or carried along the coast from other places by currents or beach drifting (see below), including material carried into the sea by rivers. In sheltered regions, finely divided material carried in suspension in the water may be deposited on the beach as sand, mud or silt. Longshore drift and grading As described above, when a wave breaks, the swash may carry stones or smaller parti- cles up the beach. Prevailing winds may cause waves to approach and break on the shore from an oblique angle. This results in the wave swash carrying materials up the beach at an angle, whilst the backwash and any particles it contains run directly down the slope of the beach. Consequently, each time a wave breaks obliquely, some of the beach material may be carried a short distance sideways (Fig. 1.7). This process, known Backshore Downward slope of beach les bb pe of th Pa av ch d n of win Directio e w roa es liqu f app of tion o ob ec Dir Figure 1.7 Longshore drift: the movement of pebbles along a beach by oblique waves.  The physical structure of oceans 11 as longshore drift, can move huge quantities of material over great distances. On a suitable beach, this can be quite easy to see for yourself using brightly painted stones. Drift of sediments along beaches is also assisted by longshore currents generated by oblique waves. Longshore drift has a sorting effect on the horizontal distribution of beach materi- als. Where cliffs are exposed to strong wave action, the shore is usually strewn with boulders too large to be moved by the waves. Boulders gradually become broken up, and the fragments may then be carried along the shore in a series of sideways hops. A short distance from the original site of erosion, the beach is likely to consist mainly of large stones which only the more powerful waves can move. Further along the shore the particle size of the deposit becomes progressively smaller, partly because the peb- bles gradually wear away as they rub one against another and partly because smaller particles are more easily transported. This is well demonstrated by Chesil Beach in Dorset, United Kingdom, where a pebble barrier cuts off the saline Fleet Lagoon from the sea. The east end near Chesil consists of large cobbles, whilst the west end towards Bridport consists mainly of small pebbles (Fig. 1.8). Figure 1.8 Chesil Beach in Dorset, England consists of graded pebbles. This shingle storm ridge was built up by waves over thousands of years and eventually impounded a saline lagoon behind it. Called the Fleet, it is perhaps the most important example of a saline lagoon ecosystem in the United Kingdom. 12 Elements of Marine Ecology Waves also exert a sorting action on the vertical distribution (top to bottom) of beach material which results in different grades of material at different levels on the beach. This is due to the difference in energy of swash and backwash. The swash has the full force of the wave behind it, but the backwash merely flows back down the beach under the influence of gravity. The backwash also contains less water than the swash because some is lost by percolation through the beach. The swash may move large stones up the beach but the lower energy of the backwash may then not be enough to carry them down again. This often leads to an accumulation of large peb- bles at the back of the beach, which builds up into a steep slope until a gradient is reached at which stones begin to roll back down under their own weight. Smaller pebbles, gravel and sand are more easily carried down by the backwash to the lower parts of the shore. On a purely sediment shore the same grading effect can be seen, with coarse sand high up the beach and fine sand and mud lower down. Around the British Isles, there are many beaches with steeply sloping shingle at the higher levels and flatter sand or mud on the middle and lower shore. Drifted beach material accumulates alongside obstructions crossing the shore, for example headlands, large rocks or groynes. The purpose of erecting groynes is to limit longshore drift by causing pebbles and sand to become trapped between them. Wave action then builds the beach into banks between the groynes, preventing the beach material from being carried away, thus protecting the land against erosion. However, without proper planning, this and other beach protection methods such as sea walls, built to protect one place, can cause erosion at another further along the coast. 1.1.2 Continental shelf Close to land the sea is mostly shallow, the bottom shelving gradually from the shore to a depth of about 200 m. This coastal ledge of shallow sea bottom is the continental shelf, which makes up about 8% of the total ocean floor. Its seaward margin is termed the continental edge or shelf break, a sharp drop beyond which the water becomes much deeper. In some places, such as at the Porcupine Bank off the west coast of Ireland, there is no sharp break. The steeper gradient beyond the continental edge is termed the continental slope (Fig. 1.9). The width of the continental shelf varies very much in different parts of the world from less than 1 km to 1500 km, with an average of about 65 km. The British Isles have a broad shelf which is part of the north-western European Shelf, where the continental edge runs to the west of Ireland and the north of Scotland. The whole of the English Channel, Irish Sea and almost the entire North Sea lie above the shelf and so are rela- tively shallow. The shelf is also broad beneath the China Sea, along the Arctic coast of Siberia, under Hudson Bay (Canada), and along the Atlantic coast of Patagonia (southern end of South America) where the shelf extends out to the Falkland Islands (see Fig. 1.2).  The physical structure of oceans 13 Figure 1.9 Diagrammatic cross-section showing major depth zones and divisions for the seafloor and water column. The depth limits of the different features and zones vary to some degree at dif- ferent geographical locations. From Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press. Many of the shelf areas are of special economic importance because geographically the major fisheries are concentrated here. Over 90% of the fish and other food resources taken from the ocean come from shelf areas. Northern hemisphere temper- ate and sub-polar continental shelves are particularly important in this respect. Shelf areas are also widely exploited as sources of oil and gas. 14 Elements of Marine Ecology Several processes contribute to the formation of the continental shelf. It is formed partly by wave erosion cutting back the coastline. It may be extended sea- wards by accumulations of material eroded from the coast, or by river-borne silt deposited on the continental slope. Parts of the shelf appear to consist largely of material held against the continents by underwater barriers formed by reef- building organisms or by tectonic folding. In other places the shelf has been formed chiefly by sinking and inundation of the land, for example under the North Sea. It is possible that in some regions the shelf has been broadened by increments of materials thrust up the continental slope by pressures between the continental blocks and the deep ocean floor. 1.1.3 Ocean basins and abyssal plains Beyond the shelf break, the continental slope descends at a much steeper gradient down to the floor of the ocean basins meeting the abyssal plain at depths of 4000 5000 m or so. The angle and extent of the continental slope vary with locality, with the average generally stated as being between 4% and 7% (a drop of 70 m in 1 km horizontal distance), but may be as steep as 50% rising to near vertical. As we still do not have a complete bathymetric picture of the ocean floor such averages vary with the data available to the researcher. The slope is seldom an even descent and is much fissured by irregular gullies and steep-sided submarine canyons that cut through the slope to the ocean basins (see Box 1.2). These canyons are one of the main routes by which sediment reaches the deep ocean. Continental slopes are predominantly covered by muds derived over geologic time from the land mass via rivers. At the bottom of the continental slope the gradient becomes less steep due to the accumulation of sediment that has come down from unstable masses accumulated higher up the slope. This changing zone is termed the con- tinental rise. It merges gradually with the deep ocean floor, which in most ocean basins is in the form of abyssal plains, vast flats of accumulated mud extending for hundreds of miles with only slight changes of level. In places the monotony is broken by ocean ridges and the ocean floor rises to form ranges of submarine mountains with many sum- mits ascending to within 2000 4000 m of the surface and the highest peaks breaking the surface as oceanic islands. These submarine oceanic ridges and plateaus are a major feature of the Earth’s crust, covering an area approximately equal to that of the conti- nents. There are other parts where the ocean floor is furrowed by deep troughs, the ocean trenches, in which the bottom descends to depths of 7000 11,000 m. 1.1.4 Ocean ridges and seamounts At one time the seabed was thought to be flat and featureless and it is only in relatively recent times that its mountainous nature has been revealed. A vast system of volcanic  The physical structure of oceans 15 BOX 1.2 The Kaikoura canyon. The Kaikoura Canyon is one of at least 660 documented submarine canyons, a list of which was compiled by De Leo et al. (2010) from various databases. It cuts through the east coast continental slope of New Zealand’s South Island, connecting the shelf with the deep ocean basin beyond. This 1000-m deep slash was surveyed extensively in 2006 and both topo- graphical and biological data were collected (De Leo et al., 2010). The resulting detailed digi- tal bathymetry map shows the complexity of such canyons and this leads to a complicated hydrography (water flow) and movement of sediments. Whilst interesting in its own right, the resulting unusual oceanographic conditions make this canyon and potentially many others, extremely rich in terms of both biomass (pelagic and benthic) and biodiversity. De Leo et al. (2010) showed that the Kaikoura Canyon supports a biomass around 100 times higher than anything previously recorded for deep-sea habitats below 500 m (excluding che- mosynthetic habitats such as hydrothermal vents). Even tourists benefit from the high pro- ductivity of Kaikoura Canyon. Sperm Whales (Physeter macrocephalus) regularly surface here whilst hunting for large squid and sightings are almost guaranteed. mountain chains crosses the ocean floor, delineating the edges of tectonic plates (Section 1.1.6). The major chains, known as mid-ocean ridges, run the north-south length of the Atlantic, Indian and Pacific Oceans, bisecting them into east and west basins (though this is skewed in the Pacific), but their existence and extent was not fully appreciated until the 1950s and 1960s. It should be remembered that these ridges are not narrow lines but wide mountain ranges and defining their route is therefore not an exact science. 16 Elements of Marine Ecology Mid-Atlantic ridge The Atlantic is divided into two basins by the mid-Atlantic ridge (MAR) which runs from 70 N to 55 S. The northernmost part of the MAR runs from the Arctic through Jan Mayen Island and Iceland (Box 1.3), and this part is known as the Reykjanes Ridge. The northern part of the MAR system forms a barrier separating the deep levels of the Arctic basin from those of the Atlantic. Much of the crest of this part of the ridge is within 500 m of the surface, extending from the north of Scotland and the Orkneys and Shetlands, west to Rockall and the Faroes (the Wyville Thompson ridge), and then to Iceland (the Iceland Faroes rise) and across to Greenland and Labrador (the Greenland Iceland rise). The main part of the MAR follows a roughly S-shaped course from north to south through the North Atlantic and then the South Atlantic, touching the surface at the islands of the Azores, St Paul’s Rock, Ascension, St Helena, Tristan da Cunha, Gough and Bouvet. These remote oceanic islands, which rise above the ocean surface, are the highest peaks of the MAR. A branch of the MAR, the Walvis ridge, extends from Tristan da Cunha to Walvis Bay on the west coast of Africa. Mid-Indian ridge The Central Indian Ridge runs south from the Indian and Arabian peninsulas and then divides into the Southeast and Southwest Indian ridges. The latter runs west and links with the eastern extension of the MAR. Pacific Ocean ridges A single Pacific tectonic plate underlies the major part of the Pacific Ocean and there is no mid-ocean ridge bisecting the ocean into two almost equal east and west basins as in the Atlantic Ocean. The major mid-ocean ridge, the East Pacific Rise, runs south from the Gulf of California and then joins the southwest and west running Pacific Antarctic Ridge at around 55 S, 130 W. The latter ridge could also be considered as the southern section of the East Pacific Rise and runs towards Antarctica, south of New Zealand. BOX 1.3 Visible ridges. Whilst the average depth of water above the top of any of the mid-ocean ridges is about 2500 m, there is one place in the world where you can walk along the ridge, flanked on one side by the North American tectonic plate and on the other by the Eurasian plate. This is the fiery island of Iceland, which was formed by volcanic activity along the MAR and to this day is one of the most volcanically active places in the world. It not only lies on the boundary between the two plates but also lies above a hotspot in the Earth’s mantle called the Iceland plume. Thingvellir National park in the south-west is one of the most popular sites, visited by geologists and tourists alike.  The physical structure of oceans 17 Volcanoes, seamounts and guyots Seamounts are (mostly) extinct underwater volcanoes and over half of all those known are found in the Pacific Ocean, the most seismically active of all oceans. Some Pacific islands, such as the Islas Marías Archipelago off Mexico, are peaks of the East Pacific Rise, but the more numerous islands of the Central and West Pacific arise from geologic ‘hot spots’ (Fig. 1.10). These volcanically active areas in the underlying mantle of oce- anic tectonic plates are not associated with ocean ridges but build up into underwater volcanoes. If these break the surface, they form volcanic islands such as those that make up the archipelago of Hawaii. A young underwater volcano called Loihi is destined to become the next Hawaiian island, but not for around 100,000 years. As well as chains of ocean island volcanoes, seamounts can also be found associated with mid-ocean ridges and along continental margins. Many seamounts remain undocumented and estimates suggest there are at least 100,000 sizeable ones. Twenty-first century satellite technology allows seamounts that are 1.5 km or more in height to be detected by measuring sea surface height. This is related to the gravitational force exerted by the seabed and sea- mounts exert a greater force than a flat seabed. Guyots are flat-topped seamounts, formed from volcanic islands worn down by wave action over millions of years. After subsiding, they may lie beneath as much as 800 m of water. 1.1.5 Ocean trenches Ocean trenches (see Figs 1.2 and 1.9) are perhaps the most mysterious and least known parts of the ocean and have always fascinated explorers and scientists alike. These are the deepest parts of the ocean floor below about 6000 m and are found mostly around lagoon sinking island seamount guyot atoll barrier reef sinking island volcanic island main vent crust magma chamber HOTSPOT Figure 1.10 Whilst a hotspot in the underlying mantle remains stationery, tectonic plates do not. Plate movement can carry a volcanic island away from its source and it gradually becomes dor- mant. As the dome collapses and the ocean crust beneath it cools and shrinks, the island subsides to become once again a submerged seamount or flat-topped guyot. From Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press. 18 Elements of Marine Ecology the rim of the Pacific Ocean, for reasons that will become obvious in Section 1.1.6 on plate tectonics. The deepest of all is the Mariana Trench off Guam (a US island terri- tory) in Micronesia in the western Pacific, the deepest part of which is the Challenger Deep at nearly 11,000 m. Only three direct measurements have ever been taken; the first from the bathyscape Trieste (1960) at 10,916 m; the second from the submersible Deep Sea Challenger (2012) at 10,908 m and the third from DSV Limiting Factor (2019) at 10,924 (Box 1.4). However, getting a really accurate depth measurement is difficult and any of these might hold the record for the deepest. As their name suggests, ocean trenches are long, narrow and steep-sided, though these terms are relative. They are volcanic in origin, formed where an oceanic tectonic plate meets a continental plate and subducts beneath it (see Section 1.1.6). They run parallel to island arcs or to continental margins where they are associated with moun- tain ranges. Other deep troughs and trenches in the middle of oceans are not trenches in the sense used here and have disparate origins. The Mariana Trench is part of a great line of trenches that border island arcs from near the equator, north past the Philippines, along the east of Japan and on to the Aleutians and Kamchatka. A com- plex of island arc trenches borders New Guinea and the Solomon Islands (New Britain, Solomon and New Hebrides Trenches), and the Fiji Islands south to New Zealand (Tonga and Kermadec trenches). In the eastern Pacific, marginal trenches run BOX 1.4 Five deeps expedition. In December 2018 a purpose-built full ocean depth submersible ‘Limiting Factor’ completed the first of five explorations to the deepest points in each of the world oceans. This first was the Puerto Rico Trench in the Atlantic Ocean (8376 m). Next was the South Sandwich Trench in the Southern Ocean (February 2019; 7434 m), then the Java Trench in the Indian Ocean (April 2019; 7192 m), the Mariana Trench Challenger Deep in the Pacific Ocean (May 2019; 10,924 m) and finally the Molloy Deep in the Arctic Ocean (August 2019; 5550 m 6 14 m). Within each area, other nearby sites were also visited including the Horizon Deep in the Tonga trench of the South Pacific Ocean. Key to finding the deepest spots to dive was the use of a state-of-the-art multi-beam echosounder (Konsberg EM 124) mounted on the hull of the support vessel. This also produces 3D imagery of the ocean floor. Three ballasted lander instruments that are dropped down to the seabed and later recovered when they rise back to the surface were used in conjunction with the submersible. These were fitted with baited camera traps, water samplers and sediment corers and also store sediment, rock and faunal samples collected by the submersible. The expedition was dual purpose involving both extreme exploration and a full scientific programme. All the expeditions were filmed as a documentary series for Discovery Channel. Finally extensive deep sea exploration has become not only possible but a reality.  The physical structure of oceans 19 along the Americas from California and Mexico (Middle America Trench) south to Chile (Peru Chile Trench). In the Indian Ocean, the Sunda (previously Java) Trench borders Sumatra and is one of the longest at 2800 miles. In the Atlantic, there are trenches associated with island arcs in the eastern Caribbean (Puerto Rico Trench) and between South America and Antarctica (South Sandwich Trench). 1.1.6 Driving forces: plate tectonics The theory of plate tectonics, first formulated in the late 1960s and now almost univer- sally accepted, explains the way in which new seabed is formed and spreads out from mid-ocean ridges (sea-floor spreading) and the way in which continents move and split over time (continental drift) (Fig. 1.11). The current configuration of the continents and ocean basins are believed to have evolved over the past 200 million years or so, as evi- denced by the fossil record and other geological data (look at South America and Africa on a map and visualize pushing them together—they fit like two pieces of a jigsaw). Plate tectonics also explains the presence of other important geological features including ocean trenches, the ring of volcanoes around the Pacific Rim (the so-called ‘ring of fire’), arcs of oceanic islands and mountain chains bordering some continents. Constructive margin Constructive margin Destructive margin Continental Abyssal Abyssal Continental Continental Mountain shelf plain plain slope shelf range Continental slope Ocean ridge Ocean trench Sea surface Continental rise New seabed formed Crust Inner Subduction lithosphere zone Crust remelts; Aesthenosphere volcanoes often formed Upwelling Figure 1.11 Diagram to illustrate the theory of seafloor spreading. Submarine ridges mark lines of tectonic plate separation, where new crust is created as molten mantle material moves to the sur- face. Ocean trenches are formed and crust is lost where continental and oceanic crusts collide. 20 Elements of Marine Ecology According to tectonic plate theory, the 60 mile or so thick outer crust of the earth (the lithosphere) is cracked into about 20 separate lithospheric or tectonic plates (plus several much smaller ones) (Fig. 1.12). Most plates carry continental masses embedded in them plus adjacent ocean floor, whilst some carry only ocean floor. These rigid, cool plates are not fixed in position, but ride on top of the underlying layer of partly molten, but dense material called the asthenosphere. The tectonic plates slide over this, moving continents and ocean basins with them. The main driving force is a com- plex flow of thermal energy deep within the mantle (see Whitmarsh et al., 1996) which is not yet fully understood. Although still described as a theory, there is now considerable evidence supporting sea-floor spreading, from thermal probes, seismic wave velocities, magnetic anomalies and the thickness and ages of marine sediments. Ocean ridges follow the lines where the margins of adjacent tectonic plates are moving slowly apart. As the separating plates are pulled away from each other, molten basalt bubbles up from the aesthenosphere to the surface, welling out and cooling on contact with cold seawater. This volcanic material gradually builds up to form ocean ridges, adding new seafloor laterally to the edges of the plates. As the new crust is formed, the earlier crust spreads away. The spreading rate has been calculated as rang- ing from less than 1 cm per year from the MAR to 16 cm per year from the East Pacific Rise between the Pacific and Nazca plates. A depression or ‘rift valley’ runs along the centre line of each ridge marking the actual line of division from which Figure 1.12 Major tectonic plate boundaries. Arrows indicate relative directions of movement. Ocean ridges develop at separating boundaries, and ocean trenches and mountain ranges at collid- ing boundaries. From Dipper, F.A., 2016. The Marine World: A Natural History of Ocean Life. Princeton University Press (Wild Nature Press), 544pp. Courtesy Marc Dando and Wild Nature Press.  The physical structure of oceans 21 lateral spreading is taking place. Along the MAR this is approximately the depth and width of the Grand Canyon, whereas along the much faster spreading East Pacific Rise, the valley is much smaller. Where the edges of moving plates collide, such as along the western rim of the Pacific Ocean, one plate is forced below the other to form a deep oceanic trench with adjacent volcanic islands. This is known as subduction, where the edge of the plate is forced down into the mantle and resorbed. Where an oceanic plate collides with a conti- nental plate, mountain ranges may be thrust up consisting of volcanic material and folded sediments. Where two oceanic plates collide, a volcanic island arc may develop. Subduction is the basis of the so-called ‘ring-of-fire’ in the Pacific Ocean which is an almost continuous chain of volcanoes surrounding it. The Mariana Trench lies where the immense Pacific Plate subducts beneath the much smaller (and lighter) Philippine Plate. Ocean ridges and trenches are therefore associated with areas of volcanic activity resulting from these movements of the Earth’s crust. The ridges are essentially different from mountain ranges on land because they are formed entirely of volcanic extrusions of igneous rock onto the sea floor, whereas ranges such as the Himalayan Mountains consist mainly of crumpled and folded sedimentary rocks. The Himalayan Mountains were thrust up when the plate carrying the then island of India collided with the Eurasian plate. Plate tectonics and continental drift also underlie the formation of the Atlantic Ocean and parts of the Indian Ocean. On this basis the Atlantic Ocean is thought to have originated when the great supercontinent of Pangea started to break up as tec- tonic plates moved apart around 180 million years ago. It is therefore a younger ocean than the Pacific. The Atlantic Ocean is still enlarging through seafloor spreading along the MAR, at a rate of about 2 cm per year. The Pacific Ocean on the other hand may be shrinking through subduction along its western rim. However, the presence of a previously unknown subduction zone about 120 miles southwest of Portugal has recently been suggested through analysis of seabed mapping projects in the region (Duarte et al., 2013). The developing subduction zone results from a currently forming crack in the Eurasian Plate. This may ultimately cause the Atlantic Ocean to shrink. The margins and relative movements of the major plates are shown in Fig. 1.12. Comparison with Fig. 1.2 will reveal the coincidence of ridges and trenches with lines of separation and subduction between tectonic plates. 1.1.7 Seabed depth and bathymetry survey methods The classic 19th century and earlier method of measuring the depth of the sea was by means of sounding weights and lines. The weight was lowered from the vessel until it struck bottom and the length of the marked line was noted or recorded by mechanical 22 Elements of Marine Ecology meters through which the line passed. These simple methods were effective but restricted to relatively shallow water and whilst tallow (animal fat) added to the weight could bring up bottom sediment, this revealed only basic information about the sea- bed. All this changed with the development of sonic methods which use reflected sound waves to image the seabed. Echo sounders and sonar systems Sonar stands for ‘Sound Navigation and Ranging’ and is the detection of objects underwater using sound. Depth measurements are now predominantly made by echo- sounding. From these measurements nautical charts can be developed, seabeds profiled and objects such as wrecks located and mapped. The sonic (or deep) scattering layers (see Section 2.6.3) were discovered in the course of investigations with sonic equip- ment. A great variety of echo sounders are now available to suit all types of vessels from inflatable boats to supertankers and remotely operated vehicles. Some can even be hand-held by divers in the water. Although the design varies, echo sounders all work on the same principle. An acoustic transducer fitted on the underpart of the ship’s hull or towed at a known depth gives out short pulses of sound at a given frequency. The transducer also receives the reflected sound pulses on their return from the seabed. The frequencies emitted by modern echo sounders lie above the audible range, usually between 15 and 50 kHz. The use of ultrasonic frequencies has several advantages. They can be focused into fairly narrow directional beams, giving a more precise echo than is obtainable from the audible part of the sound spectrum and enabling a more detailed picture of the bottom profile to be drawn. There is also less interference from natural sounds. The depth of water is calculated by timing the interval between the emission of the sound at the surface and its return to the surface as an echo reflected from the sea- bed or other objects. The returning echo vibrates the transducer and sets up electrical signals which can be amplified and recorded. In the past, the trace was recorded on sensitized paper with a printed scale. In today’s digital age, the work is all done by computers. The speed at which sound travels through water is known to be about 1500 m per second and so time can be converted to depth. The speed of sound through seawater varies slightly with temperature, salinity and pressure and so for really accurate sonic sounding, the actual speed needs to be measured. This can be estimated from available hydrographic data, or direct measurements can be made by sending acoustic pulses over a known distance to a reflector. Multibeam echo sounders produce depth measurements of the seafloor over a wider area and are used to produce detailed maps of the bathymetry of the seabed. The sound pulses are emitted in a fan-shape beneath the ship as it moves along and the depth across a strip of seafloor is recorded. The data from adjacent strips can be  The physical structure of oceans 23 Figure 1.13 A 3D image of North Stratford Shoal, Connecticut, United States, produced using mul- tibeam echo sounder technology. Such detailed maps and visualizations of underwater bathymetry can be used to predict the extent and boundaries of different ecological habitats and species assemblages. From NOAA’s Maritime Heritage Program; Collection of LCDR Jeremy Weirich, NOAA Corps. stitched together by computer to map the desired area. A 3D picture can be produced using standardized colours, in general shades of red for shallower depths and blues and purples for deeper parts (Fig. 1.13). Side-scan sonar Standard echo sounders, whether single or multibeam, can only make measurements directly beneath the ship over a limited horizontal swathe. This is adequate when rela- tively small areas need to be covered, but side-scan sonar can cover a wider area in a shorter time. These instruments are generally towed behind a vessel. The technique makes use of conical or fan-shaped pulses of sound directed towards the seabed over a wide angle either side of the instrument and perpendicular to the path of the instru- ment. It can also measure the strength of the returning signal and such data can be interpreted to indicate the type of seabed. So, for instance hard rocks produce a stron- ger return signal than soft sediment. Commercial towed side-scan sonars are now extensively used for surveying broad swathes of the seafloor (Box 1.5). Side-scan sonars can also be attached to deep-towed instrument packages for carry- ing out fine-scale surveys of the seabed. These instruments are now revealing 24 Elements of Marine Ecology BOX 1.5 GLORIA. One of the first side-scan sonar systems capable of recording in deep water was an instru- ment called GLORIA (Geological Long Range Inclined Asdic), about 8 m long and developed at the UK Institute of Oceanographic Sciences. The mark II version operated between 1975 and 1997 and was capable of surveying a strip of seabed about 40 60 km wide in water up to 5000-m deep. Features such as volcanoes, canyons, mud slides and nodule fields could be visualized clearly for the first time, by overlapping several strips. extraordinarily complex structures on the abyssal plains resulting from catastrophic fail- ures of continental slopes. Structures such as meandering channels with levies and ‘flood’-plains created by turbidity currents have been revealed, originating from events happening hundreds or even thousands of kilometres away on the continental margin. A further use of side-scan sonar is in the detection of shipwrecks and many archae- ological finds and valuable cargoes have been recovered in this way. It is also being increasingly used for fish location. Echoes may be returned by objects such as fish, floating or swimming in the water in the path of the sound beam. Modern instru- ments are capable of detecting fish shoals, individual fish and very often, the species. The efficiency of this system has undoubtedly contributed to over-exploitation of some fish stocks (see Chapter 8: Human Impacts 1: Sea Fisheries and Aquaculture). Sonar systems are becoming ever more sophisticated allowing us to ‘see’ the seabed in a way early oceanographers would have thought impossible. ‘RoxAnn’ ultrasonic signal processor Standard echo sounders can provide only limited information concerning the nature of the seabed. However, a system for processing echo sounder signals to give informa- tion on seabed type was developed in the United Kingdom in the late 1980s (Chivers et al., 1990). The package processes information from a straightforward echo sounder to provide simultaneous information on the nature of the seabed. The system is based on the electronic analysis of the first and second echoes visualized on the echo sounder display to indicate bottom hardness and roughness. The system interfaces with Global Positioning System (GPS) and computers running specialized mapping software to produce seabed classifications. Today a variety of RoxAnn products are available suited to different users on small and large vessels. The advantage of this system is its relative cheapness and its ability to interface easily with a wide variety of echo sounders. It has a wide application in shallow water ecological surveys, mapping bottom types and identifying habitats and biotopes such as seagrass beds or coral reefs. It is also used by commercial fishermen.  The physical structure of oceans 25 1.2 Depth divisions and ocean zones The ocean is a three-dimensional environment and as stated in the introduction to this chapter, marine life of one sort or another occurs throughout the water column and both on and within the seabed. However, conditions at different depths and in differ- ent parts of the ocean vary and there are significant changes in temperature, pressure, light and food supply as depth increases. The cold, dark, slow-moving bottom layer of the deep ocean is obviously a very different environment from the well-illuminated, wave-tossed waters of the sea surface, or coastal waters with strong currents and fluc- tuations of temperature and salinity. These differences result in a zonation of marine life, defined by physical conditions and the marine environment can be classified into various divisions on this basis (see Fig. 1.9). There are broadly two ways in which organisms live in the sea; they float or swim in the water or they live on or within the sea bottom. There are correspondingly two major divisions of the marine environment, the Pelagic and the Benthic: the pelagic division comprises the whole body of water that forms the seas and oceans and the benthic division the entire sea bottom. 1.2.1 Pelagic division Shallow, nearshore water usually has a greater variation in water movement, tempera- ture and chemical composition than deep water areas. The pelagic division is therefore divided horizontally into a Neritic Province (the shallow water over the Continental Shelf) and an Oceanic Province (the deep water beyond this). In deep water beyond the edge of the continental slope, conditions change with depth and four major zones are usually recognized as shown below. However, the beginning and end of each zone are not rigid depth boundaries, and the zones merge gradually. The epipelagic zone (sunlit zone): from the surface to 200 m depth. There is often a sharp gradient of illumination and only about 1% of the light at the surface reaches the bottom of the zone. There may also be a temperature gradient and in many areas this is irregular, involving discontinuities or thermoclines (Section 2.1.2). There are also diurnal and seasonal changes of light intensity and temperature. Upper layers may experience strong surface currents and wave action. Most phytoplankton is found in this sunlit zone. The mesopelagic zone (twilight zone): from 200 to 1000 m depth, where very little light penetrates and the temperature gradient is more even and gradual, without much seasonal variation. An oxygen-minimum layer and the maximum concentrations of nitrate and phosphate (see Section 2.2.3) often occur within this zone. In the epipe- lagic zone, such nutrients are used up rapidly and then replenished from the mesope- lagic by mixing and upwelling. 26 Elements of Marine Ecology The bathypelagic zone (dark zone): between 1000 m and 4000 m, where darkness is virtually complete except for bioluminescence from fish, crustaceans, jellyfish and other marine life; the temperature is low and constant and water pressure high. The abyssopelagic zone: below 4000 m, where the only animals are those specially adapted to survive dark, cold, extreme pressures and a dearth of food. Depths below 6000 m occur within ocean trenches and this is often classified as the Hadal Zone (in both pelagic and benthic divisions). 1.2.2 Benthic division The entire sea bottom and the seashore together make up the benthic division which comprises three major zones, the Littoral, the Sublittoral and the Deep Sea zones. The Littoral (Intertidal) zone includes everything between the lowest and the highest spring tide levels together with the wave-splashed region above high tide level (see Section 6.2). The Sublittoral zone is the shallow sea bottom extending from the bot- tom of the littoral zone out to the edge of the continental shelf. The Deep Sea zone lies below the continental shelf and can be subdivided into Bathybenthic and Abyssobenthic zones. The Bathybenthic zone lies between the continental edge and a depth of about 4000 m, comprising mainly the continental slope. The Abyssobenthic zone is the bottom below 4000 m, including the continental rise, abyssal plain and deeper parts of the ocean floor. 1.2.3 Organisms The animals, plants and other organisms found in these various zones are also classified according to their way of life as well as the depths at which they are found. Organisms of the pelagic division comprise two broad categories, plankton and nek- ton, differing in their powers of locomotion. The plankton consist of floating organ- isms which drift with the water and whose swimming powers, if any, serve mainly to keep them afloat and adjust their depth level rather than to carry them from place to place. Marine plankton is covered in Chapter 4, Open Water Lifestyles: Marine Plankton. The nekton comprises the more powerful swimming animals, vertebrates and cephalopods, which are capable of travelling from one place to another indepen- dently of the flow of the water (Chapter 5: Open Water Lifestyles: Marine Nekton). However, some jellyfish are powerful swimmers and may essentially be both plank- tonic and nektonic. The populations of the benthic division, the sessile and attached plants and animals and all the creeping and burrowing forms are known collectively as benthos. The term benthopelagic refers to animals, mainly fish, which live very close to, but not actually resting on, the bottom. Hovering slightly above the sea floor, they are well placed for taking food from the bottom.  The physical structure of oceans 27 1.3 Seabed composition Whilst the chemical composition of the seawater (see Section 2.2) making up the pelagic division remains relatively constant through the mixing effects of oceanic cir- culation, the sea floor (benthic division) is an area of great contrasts, from deep ocean ‘deserts’ of soft sediment to hard, rocky reefs covered in seaweeds and sessile animals. However, in terms of area, by far the greater part of the ocean floor is covered in soft sediments, except where the bottom current is strong enough to sweep sediment par- ticles away from the underlying rock, or where the gradient is too steep for them to lodge. Exposed rock is common in shallow water where it is subject to the scouring action of tidal currents and waves, whereas in deep water, rock only remains uncov- ered on steep sides of oceanic ridges, seamounts, trenches and canyons. So the speed of the bottom current is an important factor in determining what the seabed looks like. With sediments, the type of deposit varies with the speed of the bottom current and the size and density of suspended particles. The faster the water moves, the coarser is the texture of the substratum, because finely divided material is more easily held in suspension than larger particles of the same density. Essentially if particles have the same density, then larger ones will settle faster than smaller ones. A denser particle will settle faster than a less dense one of the same diameter. Settling velocity can be pre- dicted by applying a mathematical equation called Stokes’s Law which applies to small spherical particles in a fluid. Given steady-state conditions, then if the settling velocity is known, the approximate age of ocean sediments will relate to their thickness. This is of course a great over-simplification. Once the settling velocity of particles in a sediment is exceeded, the transport of sediment increases rapidly with increase of current speed. In many areas, parts of the seabed over the continental shelf are kept in motion by strong tidal currents. Other factors which determine the types of materials forming the sea floor include: the geology, including the proximity of land and the geographical and geological features of the coastline such as rock formations and the outflows of rivers or glaciers; the depth; the types of suspended matter in the overlying water, including the pelagic organ- isms; and the type of benthic population (which is itself partly determined by the type of seabed). Marine sediments are classified into two main groups, terrigenous and pelagic deposits, according to their origins, which can be both geological and biological. Rocky areas on the other hand mostly have a purely geological origin, although some surface ‘rock’ can be built up by organisms such as corals and coralline algae. 28 Elements of Marine Ecology 1.3.1 Terrigenous deposits Terrigenous deposits are found near land, covering the seashore, continental shelf and upper parts of the continental slope. The deposits show considerable differences of composition from place to place, varying with the nature of the adjacent coastline, the movements of the water and the contours of the seabed. They range from large boulders close to rocky shores where they have been dislodged by violent wave action, through all grades and mixtures of pebbles, gravel and sands down to fine clay. Much of this material is derived from weathering and erosion of exposed land surfaces worn from the coast by wave action or carried into the sea by rivers or glaciers. Terrigenous deposits contain some organic material, often around 0.01% 0.5% of the dry weight, the finer-texture deposits usually having the greater proportion of organic matter. Recognizable traces of various materials of biological origin can often be seen under the microscope. These include fragments of leaf and wood from terrestrial plants and from many salt-tolerant plants found in saltmarshes, for example cordgrass (Spartina). The mud from shallow creeks and estuaries is often rich in such material. Much of the organic matter also comes from a very diverse collection of marine mate- rial, derived from both benthic and pelagic sources. This includes seaweed fragments, diatom cell walls, sponge spicules, polychaete chaetae and fragments of the shells and skeletons of foraminiferans, hydroids and corals, bryozoans, crustaceans, echinoderms and molluscs. Samples from the surface layers of the deposit often contain micro- organisms, for example flagellates, ciliates, foraminiferans, nematodes and copepods. Superficial deposits from shallow water may also contain a microflora of benthic diatoms. Ecologists often use familiar terms such as pebble, fine sand and mud in a rather loose sense, based on what the sediment they are dealing with looks and feels like and on experi- ence. Field ecologists rarely have time to dry and sieve the sediments. However, sediments can be defined by the specific size range of their particles. The much-used Wentworth scale of particle sizes (see Table 1.2) is geometric, giving smaller intervals towards the finer end of the range. Using this scale, a pebble is a pebble only if its size lies between 4 and 64 mm diameter. A pebble larger than this is defined as a cobble or a boulder. Geologists tend to use a different scale called the phi (φ) scale. This converts the unequal steps of the Wentworth scale into an arithmetic series of equal intervals, thereby simplifying graphical and statistical treatment. The particle diameter in millimetres is written as the equivalent negative of the power of 2. So, for example to find the phi size of a pebble of 64 mm diameter, the diameter is first written as a power of 2, that is 26 (64=26). The power of 2 in this case is 6 so the phi size is the negative of 6, that is a 64 mm diameter pebble has a phi size of 26. Likewise, a very fine sand with grains of 0.125 mm diameter has a phi size of +3. This is calculated as follows: convert 0.125 to a fraction=1/8. 1/8 written as a power of 2 is 223. The negative of 23 is +3.  The physical structure of oceans 29 Table 1.2 Wentworth classification of particle grades and phi (φ) scale. Grade name Particle size range (mm) Phi units (φ) Boulder.256 ,28 Cobble 64 256 26 to 28 Pebble 4 64 22 to 26 Granule 2 4 21 to 22 Very coarse sand 1 2 0 to 21 Coarse sand 0.5 1 +1 to 0 Medium sand 0.25 0.5 +2 to +1 Fine sand 0.125 0.25 +3 to +2 Very fine sand 0.065 0.125 +4 to +3 Silt 0.0039 0.0625 +8 to +4 Clay ,0.0039.8 The sizes of the sediment grains in a sample are found by drying the sample and sieving it through a series of sieves with meshes of decreasing size. The sieves are pre- cision tools, constructed with very accurate mesh sizes relating to the phi scale. The grains that pass through one sieve but not the next have a diameter size range between the two phi sizes of the sieves. Marine sediments are never uniform in composition but contain particles of many grades and types. If the particles are mainly of one size, the sediment is said to be well sorted. If there are many sizes of grains, then it is a poorly sorted sediment. Sediments containing more than 10% dry weight of silt and clay fractions are commonly termed ‘muddy sands’. If more than 30% of the deposit is silt and clay, the term ‘sandy mud’ is applied, and deposits with silt and clay fractions exceeding 80% are generally described as ‘mud’. Some areas of the continental shelf are rich in particular terrigenous deposits that are of use to people. For example sand and gravel is often extracted from the seabed for building materials. This can have serious consequences for benthos living on and in the sediment (see Section 9.6). 1.3.2 Pelagic deposits Pelagic deposits occur beneath deep water beyond the edge of the continental slope, carpeting the deep ocean basins. Much of this material has a fine texture and its nature varies with the depth and with the types of organisms that are prevalent in the overly- ing water. The approximate distribution of deep sediments is shown in Fig. 1.14. At depths of less than about 6000 m, pelagic deposits contain a considerable proportion of material of biological origin, commonly some 30% or more by weight. Although these deposits are termed ‘organic’, they seldom contain much decomposable carbon but consist almost entirely of skeletal fragments of planktonic organisms. The types of 30 Elements of Marine Ecology 180° 180° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° 80° 80° Siliceous Pteropod sediments ooze Calcareous Radiolarian sediments ooze Deep-sea clay Arctic Circle 60° 60° 40° 40° Tropic of Cancer 20° 20° Equator 0° 0° 20° 20° Tropic of Capricorn 40° 40° 60° 60° Antarctic Circle 180° 180° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° Figure 1.14 Ocean bottom deposits. Calcareous sediments include globigerina ooze (mostly fora- miniferan shells), coccolith ooze and pteropod ooze. Siliceous sediments include diatom ooze and radiolarian ooze. Some specific areas of the latter are also shown. plankton mentioned here in this section are all described in Chapter 4, Open Water Lifestyles: Marine Plankton. Organic deposits are of two main types, calcareous and siliceous. Calcareous sediments, rich in calcium carbonate and formed mainly from foraminiferan shells, are common in middle and low latitudes, but only down to an average depth of around 4600 m. Below this depth, hydrostatic pressure causes some forms of calcium carbonate to dissolve. The calcite compensation depth (CCD) is the depth where the supply of calcite raining down from above is equalled by its dissolu- tion. So below this depth, there are no sediments containing carbonate. At high latitudes around the polar belts, great areas of the seabed are covered with siliceous sediments. These are areas of high productivity where the rain of material downwards consists mainly of silica-containing diatoms. Calcareous sediments do occur here, but only down to around 3000 m. Other high-productivity areas such as the equatorial belt and coastal upwellings also have siliceous sediments especially where the seabed lies below the CCD. In general, the deeper organic deposits contain less calcareous material and a larger proportion of silica. The organic deposits are named after the main organisms from whose skeletons they are made up and are classified as follows:  The physical structure of oceans 31 Calcareous oozes Globigerina ooze: This is the most widespread of the deposits over the greater part of the deep Atlantic and much of the Indian and Pacific Oceans, covering nearly 50% of the deep-sea bottom and extending to depths of 6000 m. It contains up to 95% cal- cium carbonate mainly in the form of foraminiferan shells (named after the predomi- nant genus Globigerina). Coccolith ooze: A high proportion of coccolith material (Prymnesiophyceae), some- times amounting to 25% or more of the total weight, is occasionally found in samples of globigerina ooze, chiefly beneath areas of warm surface water. Pteropod ooze: This contains many shells and occurs below subtropical parts of the Atlantic at depths down to 3500 m. Siliceous oozes Diatom ooze: This consists mainly of siliceous material in the form of diatom fragments. It occurs as an almost continuous belt around Antarctica beneath the Southern Ocean, its northernmost limit corresponding closely with the position of the Antarctic conver- gence. There is also a strip of diatom ooze across the northern part of the North Pacific. Radiolarian ooze: This contains many radiolarian skeletons and occurs at depths between 4000 and 8000 m beneath tropical parts of the Pacific and Indian Oceans and is also recorded in the Atlantic. Red clay At 6000 m and below, sediments generally contain less than 10% material of obvious biological origin and at these depths the most widespread deposit is red clay, covering nearly 40% of the deep ocean floor. It is a very finely divided sediment, usually brick- red in colour and consisting mainly of fine-grained quartz (silica) and clay minerals such as aluminium oxide, along with small amounts of various compounds of iron, calcium, magnesium and traces of many other metals. The bulk of this material is derived from ‘aeolian fallout’. That is it originates as fine mineral dust lifted from the ground by wind action and carried through the atmosphere, from which some eventually ‘falls out’ over the sea. These clays accumulate incredibly slowly at the rate of 1 mm or so per 1000 years. Organic oozes accumulate at around 10 30 mm per 1000 years. 1.3.3 Deepsea (polymetallic) nodules A feature of wide areas of the deep ocean floor below about 2000 m and especially between 4000 and 6000 m is the presence of sizeable lumps or nodules lying on or within the top 10 cm of the surface of the sediment. These are known as manganese nodules or more accurately polymetallic nodules and occur in many different shapes, with sizes from a millimetre or so up to about 15 cm in diameter, with occasional 32 Elements of Marine Ecology Figure 1.15 Polymetallic nodules scattered on the seafloor of the abyssal Pacific. The large deep water prawn is a species of Bathystylodactylus, photographed from an AUV. Photo: Courtesy Daniel Jones, Southampton Oceanography Centre (UK). larger ones (Fig. 1.15). They are usually rich in metals especially manganese and iron, along with copper, cobalt, nickel and other rarer metals. They are especially numerous in the Pacific, where parts of the bottom are almost covered with nodules, but they are also found beneath other areas of deep ocean. In the northeast equatorial Pacific, the Clarion Clipperton fracture zone receives considerable interest because it is consid- ered potentially the best area for commercial extraction of such minerals, a somewhat contentious subject. Other well-studied nodule fields are the Peru Basin in the south- east Pacific, the Cook Island region in the southwest Pacific, the central Indian Ocean Basin and the Baltic Sea. The ways in which nodules form and the environmental conditions under which they do so are now reasonably well understood and their detailed structure can be shown through techniques such as absorption spectroscopy. The composition, forma- tion and occurrence of polymetallic nodules is summarized in Kuhn et al. (2017). Nodules are usually made up of thin, concentric layers of material, surrounding a cen- tral nucleus which may be a core of silty material, or sometimes a fragment of rock, a fish tooth, or even a whale ear ossicle. Nodules basically form by metal element precipitation from oxygen-rich surrounding seawater (hydrogenetic) or from low-oxygen pore water within sediments (diagenetic). The chemical composition of nodules varies as a result of these two processes as well as geographical and depth parameters. The process is chemically complex but in simple  The physical structure of oceans 33 terms involves precipitation of colloidal particles, for example manganese dioxide or fer- ric hydroxide, which tend to attract ions of other metals from the water. These particles are electrically charged and may be attracted to electrically conductive objects on the bottom, which become the nucleus of a developing nodule. The chemical processes are described in Kuhn et al. (2017). The availability of suitable nuclei may be one of the causes of their patchy distribution. The nodules form very gradually over a period of several million years. Average growth rates are around 10 20 mm per million years, with hydrogenetic layers growing only 1 5 mm and diagenetic layers much faster. Manganese nodules were dredged up by the Challenger expedition as early as 1873 and found to occur in all oceans. Then they were a novelty, but now they are a poten- tial commercial resource. Although at the time of writing (2020), commercial exploita- tion of nodules is still only at the exploration stage, the technology and the financial incentive to do it are both there. Trials of mining systems have been going on since the 1970s and specialized deep sea mining systems, support vessels, platforms and extraction techniques continue to be developed. Deep seabed mining (outside individual country jurisdictions) is regulated by the International Seabed Authority which has already issued regulations on prospecting and exploration and has licensed exploration areas. Nodule areas can support a varied epifauna, especially of anthozoans and sponges, many of which attach directly to the nodules. Potential long-term effects on these populations from mining nodules are discussed in Section 9.6. 1.3.4 Oil and gas-bearing deposits Oil and gas deposits in the ocean today were formed mostly in the Mesozoic era, 252 66 million years ago. Much of the organic matter, mostly dead plankton, that rains down and settles onto the deep-sea floor is consumed. In the ancient ocean envi- ronment considerable amounts also mixed in with inorganic seabed material. With oxygen present, the majority of the organic material is quickly lost as it is oxidized by bacteria to carbon dioxide and other substances, much of which escapes into the water. However, where conditions are anoxic (no oxygen), the material lasts long enough to become buried by more inorganic sediment. Carbon dioxide produced from organic matter deep within the deposit mainly combines to form calcium car- bonate which cements the substratum into rock. Under the right conditions, silt, clay and organic material, compressed as the layers above build up, gradually becomes organic shale. Once this is buried deeply enough below several kilometres of material, inorganic processes induced as the temperature and pressure increase, change it into oil shale and finally into oil and gas. Gas and oil in rocks tend to migrate through porous layers such as sand or limestone but may become trapped within anticlines or uncon- formities beneath non-porous caps of materials such as salt or shales which prevent their escape to the surface. 34 Elements of Marine Ecology 1.4 Sediment sampling Seabed sediment samples may be collected by scientists working in a variety of disci- plines including oceanography, geology, palaeontology and marine biology. There is considerable overlap in the instruments used. Corers are concerned primarily with the sediment itself and are described briefly here, whilst trawls, dredges and grabs designed for collection of infauna are covered in Section 7.6. However, corers are also used for quantitative sampling of infauna especially from the deep sea and the Usnel box corer is described and its operation illustrated in Section 7.6.3 (quantitative benthic sam- pling). Sediment traps help to quantify how fast and how much sediment and organic matter reaches the seafloor. This is especially important in the deep sea, where benthic animals rely on such material for food. Sediment traps are described in Section 7.3.5. A corer is a tube which can be driven down into the sea floor and then with- drawn, enclosing a core of sediment (and any contained organisms). This can be as simple as a piece of plastic drainpipe pushed into the sediment by hand to sample intertidal sediments or sublittoral sediments within diving depths. Most corers are more sophisticated instruments and are usually deployed from ships and boats. Depending on the water depth and the size of the corer, considerable force may be required to drive the full length of the instrument into the sediment. Typically, a corer is weighted and allowed to descend at speed, penetrating the sediment under its own momentum (gravity corers) or assisted in various ways. The original Kullenberg piston corer, developed in the 1940s, made use of hydro- static pressure to help it penetrate deep into the sediment. It consisted of a weighted coring tube with brass liners, inside which was fitted a sliding piston attached to the lowering cable. The corer was lowered with the piston at the lower end of the tube and the apparatus slung from a release mechanism, held in the closed position by counter-weights suspended below the nose of the coring tube. When these counter- weights touched bottom, the release mechanism opened to let the coring tube fall under its own weight. At this moment, the reduced strain on the lowering cable was indicated on the vessel by a dynamometer and the cable winch stopped immediately so that the piston attached to the cable was held stationary as the coring tube plunged downwards. This created a tremendous suction inside the tube which helped to over- come the resistance of the substratum to penetration. Undisturbed cores over 20 m long have been obtained from very deep water with this device. Modern versions of the Kullenberg corer are made from steel pipes lined with plas- tic tubes. Often a gravity corer is suspended on another wire to one side to sample the upper layers of sediment that are disturbed as the main corer hits the bottom. Other types of gravity corer also employ explosive charges, compressed air or vibrations (vibrocorers) to help drive the core tube down into the sediment (Box 1.6).  The physical structure of oceans 35 BOX 1.6 Deep ocean drilling. In the late 1960s through to the early 1980s, a drilling ship the Glomar Challenger, made drill borings in the deep ocean floor as part of the Deep Sea Drilling Project, studying the structure of the earth’s crust beneath the sea. This was followed by the Integrated Ocean Drilling Program (2003 13) and in turn by the current International Ocean Discovery Program (IODP) operating up to 2023. These research projects sample deep rock and sediment formations and provide data on tectonic processes, ocean basin formation, climate change and other processes. A type of piston sediment corer was used in conjunction with the drilling tube of the dril- ling ship Glomar Challenger. The drilling bits used for boring hard rock disrupt the soft upper- most sediments, but by first dropping piston corers down the drill pipe it was possible to obtain cores of undisturbed sediment up to about 200 m long. These cores contain the remains of planktonic organisms deposited on the seabed over a period of several hundred thousand years and provide data on oceanic conditions in the past. The current IODP is continuing and expanding this early work using ocean-going research platforms to collect sediment and rock cores. The data from such sediment cores are vital in today’s study of climate change. Further reading Joseph, A., 2017. Investigating Seafloors and Oceans: From mud volcanoes to Giant Squid. Elsevier Inc, 612pp. Sverdrup, K.A., Kudela, R.M., 2019. Investigating Oceanography, 3rd Edition McGraw Hill, 528pp., international student edition. Thomas, D., 2012. Introducing Oceanography. Dunedin Academic Press, 160pp.