Download CHAPTER 11 The global ocean

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.
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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.
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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
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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.
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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) ŠLate
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
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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.
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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
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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.
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