Chapter 4
Key Concepts
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The polar nature of water accounts for many of its physical properties.
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Seawater contains a number of salts, the most abundant being sodium chloride.
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Salts are constantly being added to and removed from the oceans.
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The exchange of energy between oceans and the atmosphere produces winds that
drive ocean currents and weather patterns.
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The density of seawater is mainly determined by temperature and salinity.
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Vertical mixing of seawater carries oxygen to the deep and nutrients to the surface.
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Waves are the result of forces acting on the surface of the water.
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The gravitational pull of the moon and the sun on the oceans produces tides.
Nature of Water
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Marine organisms are 70 – 80% water by mass, compared to 66% in terrestrial
organisms.
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Physical properties of water
– excellent solvent
– high boiling point and freezing point
– denser in its liquid form than in its solid form
– supports marine organisms through buoyancy
– provides a medium for chemical reactions necessary for life
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Structure of a water molecule
– 2 H atoms bonded to 1 O atom
– polar
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Freezing point and boiling point
– polar water molecules tend to come together forming hydrogen bonds with
one another
– high boiling point reflects energy needed to overcome attractive forces of
hydrogen bonds
– relative high freezing point (0oC) of water is a result of less energy needed to
fix molecules into position to form solid
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Water as a solvent
– polar nature keeps solute’s ions in solution
– water cannot dissolve non-polar molecules, e.g., oil and petroleum products
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Cohesion, adhesion, and capillary action
– hydrogen bonds cause water molecules to be cohesive, accounting for high
surface tension
– adhesion - attraction of water to surfaces of objects that carry electrical
charges
– capillary action - the ability of water to rise in narrow spaces
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Specific heat (Thermal capacity)
– water has a high specific heat due to hydrogen bonds
– ocean can maintain relatively constant temperature
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Water and light
– much light reflected into the atmosphere
– different wavelengths (colors) of light penetrate to different depths
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Chemical properties of water
– pH scale measures acidity/alkalinity
– pH of pure water is 7, considered neutral
– ocean’s pH is slightly alkaline
– organisms’ pH affect metabolism and growth
Salt Water
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Composition of seawater
– most salts present in seawater are present in their ionic form
– 6 ions make up 99% of dissolved salts in the ocean
– trace elements
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Salinity
– seawater = 3.5% salt, 96.5% water
– expressed as in g per kg water or parts per thousand (ppt)
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Salinity
– salinity of surface water varies as a result of evaporation, precipitation,
freezing, thawing, and freshwater runoff from land
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Cycling of sea salts
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Gases in seawater
– gases from biological processes
• oxygen is a by-product of photosynthesis
• release of CO2 from respiration
• oxygen-minimum zone – located just below sunlit surface waters
– solubility of gases in seawater
• seawater has more O and CO2 but less N than the atmosphere
• relative solubility in seawater: CO2 > O > N
• affected by temperature, salinity and pressure
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role of bicarbonate as a buffer
– bicarbonate formed from the solution of CO2
– bicarbonate’s buffering action helps maintain the pH of seawater at a
constant value
Ocean Heating and Cooling
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Earth’s energy budget
– energy input
• sun’s radiant energy heats earth’s surface
• spherical shape + presence of the atmosphere cause the amount of
radiant energy reaching earth’s surface to decrease with increasing
latitude
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Earth’s energy budget
– energy output
• excess energy absorbed by the earth is transferred to the atmosphere
by evaporation and radiation
• accumulation of greenhouse gases can prevent heat energy from
radiating back to space
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Sea temperature
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temperature varies daily and seasonally
affected by energy absorption at the surface, evaporation, currents,
warming/cooling of atmosphere, heat loss
seasonal variations in the amount of solar radiation reaching the earth
because the angle of sun’s rays change dramatically
Winds and Currents
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Winds
– as air heats, its density decreases and it rises; as it cools, density increases
and it falls toward earth
– wind patterns
– Coriolis effect
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Surface wind patterns
– 3 convection cells:
• trade winds
• westerlies
• polar easterlies
– vertical air movement
• Doldrums
• horse latitudes
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Ocean currents
– surface currents
• driven mainly by trade winds
• Coriolis effect
• gyres
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Classification of currents
– western-boundary currents
– eastern-boundary currents
– transverse currents
– biological impact
• western-boundary currents not productive, carry little nutrients, but
increase oxygen mixed in water
• eastern-boundary currents productive, nutrient-rich
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Currents below the surface
– energy transferred from winds to surface water is transferred to deeper water
– deeper-water currents are deflected by the Coriolis effect, down to about 100
m
– friction causes loss of energy, creating an Ekman spiral
– Ekman transport—net movement of water to the 100-m depth
Ocean Layers and Ocean Mixing
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Density—the mass of a substance in a given volume, usually measured in g/cm3
– density of pure water = 1 g/cm3
– density of salt water = 1.0270 g/cm3
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Density increases when salinity increases
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Density increases when temperature decreases
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Characteristics of ocean layers
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– thermocline
– Halocline
– Pycnocline
– seasonal thermoclines
– seasonal thermoclines
Horizontal mixing
– higher density causes water at 30o N to form a curved layer that sinks below
less-dense equatorial surface water and then rises to rejoin the surface at 30o
S
– even denser water curves from 60o N to 60o S below other surface waters
– winter temperatures and increased salinity owing to freezing result in very
dense water at the poles, which sinks toward the ocean floor
Vertical mixing
– vertical overturn
– isopycnal
– vertical mixing
Upwelling and downwelling
– equatorial upwelling
– coastal upwelling
– coastal downwelling
Deepwater circulation
– differences in density cause water movement in deep oceans
– densest water of all is Antarctic Bottom Water
– dense Antarctic water sinks to the bottom and moves slowly toward the
Arctic
– some North Atlantic Deep Water moves into the North Atlantic
– high-salinity Mediterranean Deep Water flows through the Strait of Gibraltar
Waves
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Wave formation
– wave: a flow of energy or motion
– generating force
– restoring force
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Types of waves
– Progressive (forced) waves are generated by wind and restored by gravity,
progress in a particular direction
– deepwater and shallow-water waves
– breakers
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Tsunamis (large seismic sea waves)
Tides
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Tides: periodic changes in water level occurring along coastlines
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Spring and neap tides
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Tidal range
– diurnal tide
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semidiurnal tide
mixed semidiurnal tide