Chapter 6: Water and
Seawater
Fig. 6-19
Atomic structure
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Nucleus
Protons and neutrons
Electrons
Ions are charged atoms
Water molecule
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H2O
Two hydrogen, one oxygen
Bonded by sharing electrons
Bend in geometry creates polarity
Dipolar molecule
Dipolar molecule
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Weak negative charge at O end
Weak positive charge at H end
Hydrogen bonds
Weak bonds between water molecules
and ions
Explains unusual properties of water
Fig. 6-3
Two unusual properties

High surface tension
Hydrogen bonding creates “skin”
 Important for living organisms
 Capillarity
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Universal solvent
Electrostatic bond between dipolar
water and ions
 Ocean is salty

Fig. 6.4
Fig. 6-5b
Thermal properties of water
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Solid, liquid, gas on Earth’s surface
Water has high freezing point
Water has high boiling point
Water has high heat capacity
Water has high latent heats
Fig. 6-7
Heat capacity

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Heat absorbed or released with
changes in state
Latent heats of
Melting; freezing
 Vaporization, evaporation
 Condensation
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Global thermostatic effects
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Moderate global temperature
Evaporation removes heat from
oceans
Condensation adds heat to
atmosphere
Heat re-distributed globally
Differences in day and night temperatures
Water density
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Maximum density at 4oC
Ice less dense than liquid water
 Atomic structure of ice
 Ice floats
Increased salinity decreases
temperature of maximum density
Fig. 6-10
Fig. 6-8
Seawater
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Salinity=total amount of solid
material dissolved in water
(g/1000g)
Typical salinity is 35 o/oo or ppt
Brackish (hyposaline) < 33 ppt
Hypersaline > 38 ppt
Measuring salinity
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Evaporation
Chemical analysis
 Principle of Constant Proportions
 Chlorinity
Electrical conductivity (salinometer)
Dissolved substances
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Added to oceans
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River input (primarily)
Circulation through mid-ocean ridges
Removed from oceans
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Salt spray
Recycling through mid-ocean ridges
Biogenic sediments (hard parts and fecal
pellets)
Evaporites
Residence time

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Average length of time a substance
remains dissolved in seawater
Long residence time = unreactive

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Short residence time = reactive
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Higher concentration in seawater
Smaller concentration in seawater
Steady state

Ocean salinity nearly constant through time
Dissolved gases
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Solubility depends on temperature,
pressure, and ability of gas to escape
Gases diffuse from atmosphere to
ocean
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Wave agitation increases amount of gas
Cooler seawater holds more gas
Deeper seawater holds more gas
Conservative vs.
nonconservative constituents

Conservative constituents change
slowly through time
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Major ions in seawater
Nonconservative constituents
change quickly due to biological
and chemical processes

Gases in seawater
Oxygen and carbon dioxide in
seawater
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Nonconservative
O2 high in surface ocean due to
photosynthesis
O2 low below photic zone because
of decomposition
O2 high in deep ocean because
source is polar (very cold) ocean
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CO2 low in surface ocean due to
photosynthesis
CO2 higher below photic zone
because of decomposition
Deeper seawater high CO2 due to
source region and decomposition
Acidity and alkalinity
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Acid releases H+ when dissolved in
water
Alkaline (or base) releases OHpH scale measures acidity/alkalinity
Low pH value, acid
 High pH value, alkaline (basic)
 pH 7 = neutral
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Carbonate buffering
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Keeps ocean pH about same (8.1)
pH too high, carbonic acid releases H+
pH too low, bicarbonate combines with
H+
Precipitation/dissolution of calcium
carbonate CaCO3 buffers ocean pH
Oceans can absorb CO2 from
atmosphere without much change in pH
Fig. 6-17
How salinity changes
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Salinity changes by adding or
removing water
Salinity decreases by
 Precipitation (rain/snow)
 River runoff
 Melting snow
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Salinity increases by
 Evaporation
 Formation of sea ice
Hydrologic cycle describes recycling
of water
Fig. 6-19
Hydrologic cycle
Horizontal variations of salinity
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Polar regions: salinity is lower, lots of
rain/snow and runoff
Mid-latitudes: salinity is high, high rate
of evaporation
Equator: salinity is lower, lots of rain
Thus, salinity at surface varies primarily
with latitude
Fig. 6-20
Vertical variations of salinity
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Surface ocean salinity is variable
Deeper ocean salinity is nearly the
same (polar source regions for
deeper ocean water)
Halocline, rapid change of salinity
with depth
Density of seawater
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1.022 to 1.030 g/cm3
Ocean layered according to density
Density of seawater controlled by
temperature, salinity, and pressure
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Most important influence is temperature
Density increases with decreasing
temperature
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Salinity greatest influence on density
in polar oceans
Pycnocline, rapid change of density
with depth
Thermocline, rapid change of
temperature with depth
Polar ocean is isothermal
Layers of ocean
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Mixed surface
layer
Pycnocline
Deep ocean
End of Chapter 6: Water and
Seawater