Lecture 6
- 1. The World Ocean The Blue Planet: Chapter 10
- 2. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 3. Ocean Basins and Ocean Water • Seawater covers 70.8% of Earth’s surface • Land comprising the remaining 29.2% is unevenly distributed between the northern and southern hemispheres • Most of the water is contained in three interconnected basins; Pacific, Atlantic and Indian Oceans that, along with the Southern Ocean, make up the “world ocean”
- 4. Ocean Basins and Ocean Water
- 5. Ocean Basins and Ocean Water
- 6. Ocean Basins and Ocean Water • Before the 20th century, not much was known about the depth of the ocean • Water depths were made with a weighted line lowered from a ship – Not very efficient! • In the 1920s echo sounders began to be used, generating a pulse of sound that measured how long it took for the echo bouncing off the seafloor to return to the instrument
- 7. Ocean Basins and Ocean Water • Thanks to echo sounders, the topography of the seafloor and depth of overlying water are known in great detail for most of the ocean basins – The Mariana Trench near Guam is the greatest ocean depth measured at 10,924 m • Recent satellite measurements reveal the average depth of the sea is 3970 m, and the average height of land above sea level is 840 m
- 8. Ocean Basins and Ocean Water • As far back as the oldest sedimentary rocks, more than 4 Ga, Earth has had liquid water on its surface – Oxygen isotopes in zircons suggest that the ocean may be 4.4 billion years old • Where the water came from is still uncertain – Carbonaceous chondrites contain water as hydrous minerals, suggesting some water originated from accretion and volcanic steam – Icy comets bombarded early Earth, bringing with them frozen water
- 9. Ocean Basins and Ocean Water • About 3.5% of average seawater consists of dissolved salts – If these precipitated, they would form a layer about 56 m thick on the seafloor • Average salinity is 33 to 37 per mil – Principally sodium, chlorine and six other ions derived from • Dissolved load in streams • Volcanic volatiles carried by atmospheric water • Submarine volcanism • Aeolian dust • Pollutants
- 10. Ocean Basins and Ocean Water • Salinity of ocean water is related to latitude, but also – Evaporation – Precipitation of rain and snow – Inflow of fresh river water – Freezing of sea ice
- 11. Ocean Basins and Ocean Water
- 12. Ocean Basins and Ocean Water • The ocean differs from land in the amount of heat it can store, for a given amount of heat, water has a lower rise in temperature than land – Has a high heat capacity – Low total and seasonal temperature range – Ocean temperatures affect the climate over ocean and land, which in turn controls distribution of plants and animals
- 13. Ocean Basins and Ocean Water
- 14. Ocean Basins and Ocean Water • Physical properties of seawater vary with depth: ocean is vertically stratified – Seawater becomes denser as temperature decreases and as salinity increases – Gravity pulls denser water down – Drives deep ocean circulation – Three major depth zones • Surface zone: 100-500 m (mixed layer) • Thermocline, halocline or pynocline • Deep zone: contains 80% of ocean water
- 15. Ocean Basins and Ocean Water
- 16. Ocean Basins and Ocean Water • Biotic zones – Plants and animals living in the uppermost water occupy the pelagic zone – Benthonic organisms live on or within the bottom sediment – Floating organisms are planktonic – Plant life is restricted to the photic zone – Free swimmers (reptiles, fish, squid, marine mammals, etc.) are restricted only by their own locomotion
- 17. Ocean Basins and Ocean Water • Oceanic sediment is dominated by skeletal remains of single-celled planktonic and benthonic animals, when more than 30% of bottom sediment consists of these remains it is called – Calcareous ooze (carbonate-based) – Siliceous ooze (silica-based) • Lithic sediment, consisting of rock fragments, mantles continental shelves and slopes
- 18. Ocean Basins and Ocean Water
- 19. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 20. Ocean Circulation • Surface ocean currents are broad, slow drifts of surface water caused by friction between the ocean and air flowing over it – 50-100 m deep – Solar radiation provides heat energy, non uniform heating generates winds, which drive the movement of surface ocean water • Ocean current direction is also influenced by the Coriolis force
- 22. Ocean Circulation • Ekman transport – The balance of the wind on surface water and Coriolis force at depth – Generates a spiraling current pattern – Results in a net direction of water movement about 90˚ to the wind direction – Near coasts this leads to vertical movement of ocean water • Upwelling if net transport is away from land • Downwelling if net transport is toward land
- 25. Ocean Circulation • With geography, ocean current drivers set up the major surface current systems in the world ocean • Each major ocean current is part of a large subcircular current system called a gyre – 2 in the Pacific – 2 in the Atlantic – 2 in the Indian
- 27. Ocean Circulation • On either side of the equator, ocean regions are dominated by westward- flowing North and South Equatorial currents • Along the equator is the eastward- flowing Equatorical Countercurrent • Near 60˚ latitude, the Antarctic Circumpolar Current circles the globe
- 28. Ocean Circulation • North Atlantic Deep Water (NADW) originates at the surface of the north Atlantic, flows downward, and spreads southward to the south Atlantic • Flowing beneath this is the colder, denser Antarctic Bottom Water (AABW) • The sinking of dense, cold, saline surface water propels a global thermohaline circulation system
- 33. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 34. Ocean Waves • Surface waves receive their energy from wind – The size of a wave is determined by wind speed, duration, and fetch (distance) – Important wave dimensions are the height (from crest to trough) and wavelength (from crest to crest or crest to trough) – As waves move, each parcel of water revolves in a loop, returning nearly to its former position once the wave has passed – At a depth of half the wavelength, water motion is negligible, this is the wave base
- 35. Ocean Waves
- 36. Ocean Waves • Toward land, as water depth becomes less than L/2, the circular motion of the deepest water parcels is restricted by the shallow seafloor, flattening the loop • As depth decreases, the wave’s shape is distorted; height increases, wavelength shortens, and the wave front grows steeper, eventually collapsing (breaking) – This is turbulent surf
- 37. Ocean Waves
- 38. Ocean Waves • Approaching shore, waves become refracted to parallel the bottom contours • The path of an incoming wave can be resolved in two directional components – Parallel to the shore: longshore current – Perpendicular to the shore: surf
- 39. Ocean Waves
- 40. Ocean Waves • Tsunami – Technically a seismic sea wave – Generated by sudden movements on the seafloor • Earthquake • Submarine or coastal landslide • Large volcanic eruption – Sudden seafloor displacement causes displacement in overlying water, when this water falls back down it splits into two oppositely moving components
- 41. Ocean Waves
- 42. Ocean Waves • Tsunami – Travel at speeds up to 950 km/h – Have wavelengths measured in kilometers – Wave height is only 1-2 meters • Typically not seen or felt in the open ocean – Periodicity can be from 20 min to 1 hr – As the crest moves on shore, water can pile up rapidly to heights of 30 meters and travel great distances inland, as the trough moves on shore, it causes drawdown
- 43. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 44. Ocean Tides • Rhythmic, twice-daily rise and fall of ocean water along coastlines • Caused by gravitational attraction between the Earth and Moon, and to a lesser extent, the Sun • Generates tidal bulges due to gravitational pull and inertial force – Highest and lowest tides when sun and moon are aligned – Least tidal range when sun and moon are not
- 45. Ocean Tides
- 46. Ocean Tides
- 47. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 48. Where Land and Ocean Meet • Zones of dynamic activity marked by erosion and the creation, transport, and deposition of sediment • At any given moment, the geometry of the shoreline represents an approximate equilibrium between constructive and destructive forces
- 49. Where Land and Ocean Meet • Coastal deposits – Beach: wave crashed sediment along a coast, including the surf zone – Spit: elongate ridges of sand or gravel that project from land and end in open water – Barrier island: long, narrow sandy islands lying parallel to a coast
- 50. Where Land and Ocean Meet
- 51. Where Land and Ocean Meet • Marine deltas – Where surf and currents do not erode all new sediment carried to sea by a stream – Sediment builds outward in a fan shape, size and shape dependent on the balance between sedimentation and erosion – Very fertile regions
- 52. Where Land and Ocean Meet
- 53. Where Land and Ocean Meet • Estuaries – Semienclosed marine embayment diluted with fresh water entering by one or more streams – With associated coastal wetlands, they offer important habitats for an array of plants and animals
- 54. Where Land and Ocean Meet
- 55. Where Land and Ocean Meet • Reefs – Warm-water coastlines are characterized by limestone reefs • Colonies built by corals and other carbonate- secreting organisms that generate intense biologic productivity and diversity – Three principal reef types • Fringing reef: attached to or closely borders land • Barrier reef: separated from land by a lagoon • Atoll: roughly circular reef enclosing a lagoon that forms when a volcanic island subsides
- 56. Where Land and Ocean Meet
- 57. Where Land and Ocean Meet • Coastlines are classic examples of systems in a state of dynamic equilibrium • As soon as the local system becomes unbalanced, a change in the shoreline becomes apparent • Coastal erosion tends to have significant impacts on human interests
- 58. Where Land and Ocean Meet • Responses to coastal erosion are of three main categories 1. Hard stabilization: structural responses – Jetties, breakwaters, seawalls – May accelerate erosion in some areas 1. Soft stabilization: nonstructural approach – Plantings, beach replenishment 1. Retreat: idea to limit human interference – abandonment
- 59. Outline • Ocean Basins and Ocean Water • Ocean Circulation • Ocean Waves • Ocean Tides • Where Land and Ocean Meet • Changing Sea Levels
- 60. Changing Sea Level • On most coasts the level of the sea is changing with respect to the land – Rapid changes due to tectonic or isostatic movements of the crust – Slow changes related to global warming – Long time interval changes - eustatic - due to waxing and waning of continental glaciers and ocean-basin volume as lithospheric plates shift position
- 61. Changing Sea Level • Submergence is the rise of water level relative to the land – Nearly all coasts experienced this in the last 10,000 years due to last ice age • Emergence is a lowering of water level relative to the land • Cycles of emergence and submergence are related to the buildup and decay of vast ice-age glacier systems
- 63. Changing Sea Level • Sea ice, land ice, and sea level – With all the water locked up in the cryosphere, it is estimated that global sea level would be 65 to 80 m higher if the ice sheets were to melt – Melting of the West Antarctic ice sheet alone could contribute 8 m sea level rise
- 64. Changing Sea Level • The ocean and society – The world marine fish catch is over 80 million tons per year – Coastal sedimentary rocks host productive oil-bearing deposits – Travel and transport – Energy – Climate moderation
- 65. Extras