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CHAPTER 11 The global ocean The global ocean Oceans and ocean basins extend over 71 per cent of Earth’s surface yet receive little attention outside specialist texts, for reasons which are not hard to understand. Oceans possess relatively featureless surfaces and monotonous composition; ocean basin topography is invisible, except at the coastline, in the absence of remotely-sensed images. However, they command our attention through their integration with the supercontinental, rock and hydrological cycles, general atmospheric circulation and as home to a substantial component of Earth’s biosphere. Human interest focuses inevitably on their changing landward margins and the impact there of ocean characteristics. This chapter commences with a review of the tectonic history of modern ocean basins and principal sea-bed architecture, stressing their geological youth and constantly-evolving shape and volume. The processes and significance of isostatic and eustatic controls on basin volume and sea level complete the link between geological and oceanic processes. A profile of ocean geochemistry and physical structure based on salinity, density and temperature outlines the highly diverse character of sea water in its shallow surface layers – which exchange mass and thermal energy with the atmosphere and landsurface – overlying a broadly homogeneous deep ocean. The chapter concludes with an explanation of the nature and significance of surface, wind-driven circulation and the profoundly slower deep ocean or thermohaline circulation driven by temperature–density differences, and their modern significance for climate change. Chapter Summary Evolution of Earth’s ocean basins • Modern ocean basins are young by Earth’s time scale, with a mean ocean crust age of only 55 Ma and none older than 200 Ma. Their present form and distribution are traceable to the start of rifting of the supercontinent Pangaea c. 220 Ma ago and subsequent fragmentation of the Panthalassic super-ocean and its partially enclosed Tethys Sea. • Sea-floor spreading opened the Atlantic Ocean, at the expense of the Panthalassic Ocean and Tethys Sea. The Atlantic Ocean did not open uniformly and many failed rifts or aulacogens now floor major continental basins around its margins. • Rifting of Gondwanaland, Pangaea’s southern arm, closed the Tethys Sea as Africa, Arabia and India moved north and opened the Indian and Southern Oceans in their wake, as Australia moved east and away from Antarctica. • Late Cenozoic opening of the Scotia arc between South America and Antarctica completed the circumpolar Southern Ocean and more recent closure of the Panama isthmus between North and South America shut off Atlantic–Pacific tropical circulation. Africa continues to rift apart. • Ocean architecture comprises a spreading mid-ocean ridge, deep abyssal plain and coastal–offshore slope system straddling a fluctuating coastline. Coast-parallel trenches and associated volcanic arcs occur at subducting margins. 1 Ocean basin geometry and sea levels • The position of the coastline at a point in time is determined by water volume, driven by eustatic controls, and ocean basin geometry, driven by isostatic and tectonic controls. Over 97 per cent of global water is stored in oceans. • Eustatic control depends on water volume, and subtle changes may occur through the steric effects of temperature or salinity on water density. Larger changes depend on the balance of two-way transfers between oceans and continents through the global hydrological cycle. • Late Cenozoic sea level has been particularly unstable, fluctuating globally owing to the eustatic effects of the growth and decay of Quaternary ice sheets. Later Quaternary sea levels have fluctuated by ± approximately 150 m with a further 60– 80 m of potential rise if modern ice sheets were to melt. • Isostatic control depends on more localized crustal movement to restore gravitational equilibrium between crustal lithosphere of different density and thickness, after tectonic displacement or crustal loading/unloading. • Quaternary glaciation has stimulated frequent isostatic adjustment as ice sheets and related transfers of water and sediment have loaded and unloaded continental and oceanic crust. Composition and structure of ocean waters • Ocean water contains dissolved and suspended sediment, derived from seawater– crust interaction, terrigenous sources and rainfall. It is also a reservoir of dissolved atmospheric gases. • Dissolved solids raise the average density of sea water to 1.03 gm cm–3. Salinity, or the expression of solute content measured in parts per thousand (o/oo) by weight, is dominated by NaCl and lesser amounts of nine other elements. • The bulk average density of 34.5 o/oo and chemical homogeneity are found below a halocline or salinity gradient separating a shallow surface layer, with marked variations in salinity at points of sediment or water influx and efflux from the deep ocean. • Density is also sensitive to temperature, which ranges between 0° C and 30° C at the ocean surface before settling to a stable mean temperature between –1° C and +5° C below a shallow thermocline. The pycnocline, or zone of density increase below a shallow surface layer, is initially more sensitive to changes in salinity but ultimately controlled by the greater range of temperatures. • Variations in salinity, temperature and density lead to stratification of ocean waters, which are also marked by a shallow surface photic zone which sunlight can penetrate. 2 Ocean circulation • Ocean circulation occurs through the movement of surface water currents around seasonally mobile gyres or cells and deep water currents. Transient wave motion and oscillatory tidal motion are not considered to be part of this circulation. • Surface currents are relatively swift (1–5 km hr–1) and driven by wind. Coriolis deflection creates clockwise circulation in each northern hemisphere ocean and anticlockwise circulation in each southern hemisphere ocean. • Coriolis deflection also pushes water into the core of each major gyre, raising shallow domes. Water is also stacked against windward coasts by westward equatorial currents, leading to gravity-induced equatorial easterly counter-currents. • Deep water currents are very slow (2–7 km yr –1) and driven by temperature and density differences at depth. Known as the thermohaline circulation, or Global Ocean Conveyor, they are fed by deep bottom waters generated in cold, salineenriched sea-ice waters in the sub-Arctic North Atlantic and Southern Oceans. Tides and waves • Tides transfer surface water mass around the global ocean in response to the competing gravitational fields of Earth, Moon and Sun. • Moon and Sun create tidal bulges in the plane of their maximum pull as Earth rotates about its axis and Moon rotates about Earth. With periodicities of 12.0 and 12.42 hours, they generate a semi-diurnal tidal pattern in equatorial and midlatitudes and a dominant or single tide in high latitudes. • Twice during each monthly lunar cycle, Moon and Sun are in line and drive higher or spring tides; and at intervening times, when they are at right angles, they drive lower or neap tides. • Each tidal wave passes unseen in mid-ocean and oscillates between opposite coasts around amphidromic points with zero or very small tidal range. Tidal range is amplified on contact with shallow and indented coastlines and by high onshore winds. • Waves drawn by wind motion across the ocean surface transmit energy by very little mass in the direction of dominant or transient winds, becoming refracted where they meet discordant coastline. • A wave train of particular wavelength, wave period and wave height develops as a response to a given wind field. These parameters change as waves enter shallower water and eventually break in the inshore zone. The energy and style of the breaking wave determine its geomorphic impact. CASE STUDY : The Life and Death of Oceans Aims and objectives Chapters 10-13 focus on terrestrial geological, geomorphic and oceanic environments and their processes of rock formation, deformation and denudation. This Case Study reveals more of the obscure nature of ocean basins long since gone ~ but whose rocks are represented in geological terranes in northwest 3 England covered in website Case Studies supporting Chapters 12 and 13. These Cases can also be read in association with sections of Chapter 1, describing Wharfedale and Chapter 10 describing the geological development of Wales and the Geological Evolution of Britain. We are reasonably familiar with Earth’s current oceans but there are also references in the text to others ~ the Iapetus, Rheic and Tethys Oceans ~ whose former presence and significance rests solely in those modern terrestrial terranes which incorporate their rocks and fossils. This Case Study throws some more light on the palaeogeography and palaeobiology of the older pair of oceans. They were closely bound up with the assembly of a recognizably “British” crust by the early Mesozoic era 250 Ma ago (see website Case Studies supporting Chapters 12 and 13). This focused on the southern hemisphere ancient micro-continent of Eastern Avalonia which ‘flirted’ first with Gondwanaland and then Laurussia, before being trapped between those continental giants as they formed Earth’s most recent supercontinent, Pangaea (Figure 11.1). Figure 11.1 This is a composite with 3 elements, taken from Figure 22.5.1 (c), (d) and (e) from: Pickering, K.T and Smith, A.G., The Caledonides, In: Van der Pluijm, B.A. and Marshak, S, (2004) (eds.) Earth Structure: an introduction to structural geology and tectonics, 2nd edition, New York: W.W. Norton & Company. pp. 593-606. 4 Locally, “southern” Britain docked with “northern” Britain as the Iapetus Ocean closed along the Iapetus Suture c. 380 Ma ago, just north of the Lake District. Closure of the Rheic Ocean came as Gondwana collided with Laurussia c. 100 Ma later; Britain lay close to the northern edge of the Variscan suture zone. The Iapetus and Rheic oceans are named after the father of the Greek God, Atlas, and Iapetus’ sister Rhea. So what was the global status of our two ancient oceans ? They did not match the Panthalassic super-ocean which encircled Pangaea in area, as fragmented oceans amongst fragmented continents in the mid-stage of a supercontinental cycle, the larger Iapetus ocean would not have looked out of place alongside the Atlantic and even Pacific oceans today. As with the Atlantic ocean, first the Iapetus and then the Rheic oceans opened as earlier continents or supercontinents rifted apart. The Iapetus opened as the southern polar supercontinent Vendia broke up during the Late Neoproterozoic era. Separating Laurentia (“north America”) from Gondwanaland (“south America”, “Africa” and “Antarctic” etc.) between c. 750 Ma - 510 Ma, it widened to 5,000 km before closing again between 460 Ma – 380 Ma. Avalonia, bearing the developing “southern British” crust, rifted late from Gondwanaland (c. 550 Ma) and was drawn northwards towards Laurussia as the ocean began to close. First Avalonia, then Baltica and Siberia docked with Laurentia to form Laurussia on its northern margin. In performing this manouevre, Avalonia opened the smaller Rheic ocean to the south ~ providing a key element in closing one ocean and opening another. It also closed in due course, c. 290 Ma ago, with the unified “British” crust close to the heart of the collision which formed Pangaea. India provides a more recent example of this late detachment from a parent continent, rifting from Africa between 130 – 100 Ma ago and closing part of the Tethys Ocean as it collided with Asia at c. 37 Ma, whilst widening the Indian ocean in its wake. The age and paucity of evidence makes it difficult to map these former oceans accurately but Chris Scotese and colleagues have produced a very useful series of palaeo-tectonic global reconstructions (see Websites below). We might not give a second thought to either ocean’s biospheres ~ assuming them, perhaps, to mirror life in the modern ocean ~ until we appreciate that Earth’s biosphere probably developed in aquatic/shallow-marine environments first and that our oceans span > 450 Ma of Late Neoproterozoic (younger evident or recognisable life) Late Palaeozoic (early life) evolutionary history between them. Moreover, this long period of continental and ocean reconfiguration coincided with the stabilization of large areas of continental crust, a dramatic rise in atmospheric oxygen levels ~ from as little as < 5 % towards its modern 21% volumetric composition ~ and a number of glacial periods due to continuing occupation of southern polar regions by Gondwanaland. The marine biosphere at the start of this period would be scarcely recognizable today; fish just made it before Iapetus ocean closure and amphibians only began to emerge from the Rheic ocean as it closed ! The very earliest life-forms were bacterial, prokaryotic single-cell organisms, developing as early as 3.6 Ga, and yielding only microscopic fossil evidence. Whole colonies, forming bacterial mats or stromatolites occupying shorelines, made for larger fossils but the earliest multi-cellular, nucleusbased organisms ~ eukaryotes, from which all non-bacterial life evolved ~ did not appear until 1.5 Ga. One of the earliest faunal assemblages recognized today is the softbodied Ediacaran fauna of creatures similar to modern jellyfish, marine worms and soft corals. It developed c. 570 Ma ago in tropical seas on the eastern coast of 5 Gondwanaland, named from its type-site in the Flinders Range of modern. onshore South Australia. Shortly afterwards, Earth’s first hard-bodied organisms evolved at the start of the Palaeozoic era and greatly improved their survivability in fossil form. The Iapetus ocean and its counterparts were therefore amongst the first habitats, witnesses and sources of Earth’s great explosion of life-forms. Some of the most evocative fossils of early marine life flourished in its waters, including trilobites (arthropods) which scurried across the sea-bed and graptolites, floating as colonial planktonic animals. Fossil trilobites provide evidence of quite different evolutionary forms on either side of the Iapetus ocean, coming together on the “British” crust only when the ocean closed. Simple molluscs, brachiopods and bivalves formed an evolving shellfish community, sharing the sea-bed with early corals and seaweeds. Oceans were devoid of fish until the appearance of primitive, jawless and exo-skeletal (armoured) forms as the Iapetus ocean shrank. By contrast, the Rheic ocean saw the evolution of bony and cartiligenous, endo-skeletal fish but the extinction and virtual extinction of graptolites and trilobites respectively. Coral reef and shellfish diversity widened, the latter including more gastropod and cepahalopod (ammonite) taxa, and the first amphibians crawled onto land in a development that would quickly lead to the reptiles and eventually to humans. Meanwhile, vascular plants and primitive trees had evolved onshore as the Iapetus ocean closed, ready to develop into the rich flora of the Carboniferous swamp-forests preceding closure of the Rheic ocean and Palaeozoic era. Learning objectives • understand the respective rôles of, and background influences on, basin geometry and water volume in determining sea level • appreciate the chemistry, structure and global circulation of sea water • outline the composition and structure of the marine biosphere and its spatial patterns of productivity Essay titles 1 Write an account of the origins, development and modern tectonic character of the Atlantic Ocean basin. 2 Identify the principal causes of deep ocean or thermohaline circulation and explain its probable influence on global climate. 3 Account for the broad changes in sea level and coastline shape taking place on coastlines in either north America or northern Europe (a) 20k yr ago, (b) 10k yr ago and (c) today. 6 Discussion topics 1 Consider the principal ways in which oceans moderate global climate. 2 Differentiate between storm surges and seismic sea waves, and explain what happens as they approach the coastline. 3 Is a pulse of glacial meltwater likely to ‘sink’ or ‘float’ on entering the sea? Explain the reasons for your answer. Further reading Bigg, G. R. (1996) The Oceans and Climate, Cambridge: Cambridge University Press. A concise and very readable account of ocean processes as biogeochemical systems. It sets out the nature of ocean interactions with adjacent environments and provides a good basis for understanding current developments in related aspects of global environmental change. Kershaw, S. (2000) Oceanography: an Earth Science perspective, Cheltenham: Thornes. Starting with a conventional view of the nature of ocean basins, sea water, ocean circulation and sedimentation, this text explores the dynamic evolution of all components over long geological time scales before concluding with a review of contemporary human impacts. Redfern, R. (2000) Origins: The evolution of continents, oceans and life, London: Cassell & Co. Far more than covering the oceans alone, this superbly-illustrated book supports this section on the Geosphere and the following Biosphere. Its evolutionary Earth approach proceeds, in essence, through the oceans as the largest single planetary surface. Summerhayes, C.P. and Thorpe, S.A., eds (1996) OceanographyL An illustrated guide, London: Manson. This is a collection of papers which addresses the whole spectrum of oceanographic science, including the geological, hydrological and biological operation of the oceans. It includes an up-to-date review of research and management problems and is rich in colour illustrations. References Gross, M. G. (1990) Oceanography, sixth edition, New York: Macmillan Kearey, P. and Vine, F. J. (2008) Global Tectonics, third edition, Oxford: Blackwell Lalli, C.M. and Parson, T.R. (1997) Biological Oceanography: an introduction, second edition, Oxford: Elsevier Butterworth-Heinemann. Open University Oceanography Course Team (1992) The Ocean Basins: their structure and evolution, Milton Keynes: Open University; Oxford: Pergamon Press Scotese, C.R. (2007) PALEOMAP website. Scotese, C. R., Gahagan, L. M. and Larson, R. L. (1988) ‘Plate tectonic reconstructions of the Cretaceous and Cenozoic ocean basins’, Tectonophys. 155, 27–48 Summerhayes, C. P. and Thorpe, S. A., eds (1996) Oceanography: an illustrated guide, London: Manson 7 Web resources http://www.ocean.com A commercial, new-media channel website in partnership with organizations and academic institutions, which provides access to many areas of scientific and human interest in Earth’s oceans, including direct free access to current news stories in published media, interactive, imaging, video, DVD and other services around the world. It also provides film-sharing opportunities with amateur, as well as professional, interests in Earth’s oceans. http://www.sea-search.net/mdic/welcome.html The International Oceanographic Commission operates under UNESCO and provides a wide range of data, information and current news on ocean monitoring, environmental protection, fisheries and ecosystems, climate change implications and management issues relating to Earth’s oceans. https://www.whoi.edu The Woods Hole Oceanographic Institution is the largest US oceanographic institute, based in Massachusetts and dedicated to research and education, aimed at advancing our understanding of the ocean and its interaction with the Earth system, and communicating this understanding for the benefit of society. It offers a wide range of educational, information, current oceanographic news and imaging opportunities. http://www.scotese.com Or search via the PALEOMAP project. Most textbooks exploring long-term palaeoenvironmental geological history include simple maps with suggested reconstructions of continentental and oceanic distributions, although it is difficult to map them accurately. Chris Scotese and colleagues have produced a very useful series of palaeo-tectonic global reconstructions, accessible through his interactive website http://www.jamestown-ri.info/acadian.htm One of a number of useful, often interactive and different perspectives on the Iapetus and Rheic oceans. Others can be found through general searches via Iapetus Ocean and Rheic Ocean. http://www.ahc.gov.au/publications/geofossil/ediacara.html This is one of many websites cover the Ediacaran Fauna and other aspects of Iapetus ocean time. Others include the University of California Berkeley University’s www.ucmp.berkeley.edu/vendian/critters.html A general search via Ediacaran Fauna reveals these and several other useful sites and connections. 8