Seawater
Salinity, Density, Temperature
Water molecule
unique in structure and properties
 H2O is the chemical formula for water.
 Unique properties of water include:
 Higher melting and boiling point than other hydrogen
compounds.
 High heat capacity, amount of heat needed to raise the
temperature of one gram of water 1oC
 Latent (hidden) heat = energy that is either absorbed or released as
water changes state
 Can dissolve more substances than any other solvent
 Called universal solvent
Water molecules are
asymmetrical in shape
with the two hydrogen
molecules at one end
H+ separated by 105o
when gas or liquid phase
and 109.5o when ice
Interconnections of water molecules
 Polarity causes water
molecules to form weak
(hydrogen) bonds between
water molecules
 Water sticks to itself and to
other substances
 Allows water to be the
universal solvent
Figure 5-3
Hydrogen bonds in H2O
Figure 5-8
Pure water vs Seawater
 pH = 7.0 for pure water
 pH= 8.1 for seawater, slightly alkaline
 Density = 1.000 g/cm3 for pure water
 Density = 1.028 for seawater
 Freezing point = 0 °C or 32° F for pure water
 -1.9 °C or 28.6 °F for seawater
 Boiling point= 100°C or 212°F for pure water
 100.6 °C or 213°F for seawater
Snowflake geometry
 All snowflakes have 6-sided geometry
 Caused by water’s polarity and ability to
form hydrogen bonds
 Why does ice float on water?
The formation of ice
 As water cools to
4°C:
 Molecules slow
 Water contracts
 Density increases
 Below 4°C:
 Hydrogen bonds form
 Water expands
 As water freezes:
 Expands by 9%
Water as a solvent
 Water dissolves table salt
(NaCl) by attracting
oppositely charged
particles
 Pulls particles out of
NaCl structure to dissolve
it
Figure 5-4
Salinity
 Salinity = total amount
of solid material
dissolved in water
 Can be determined by
measuring water
conductivity
 Typically expressed in
parts per thousand ppt
or (‰)
Major dissolved components in seawater:
Constituents of ocean salinity
35 g of salt in 1000 g of seawater
Addition of salt
modifies properties of water
 Pure water freezes at 0oC.
 Adding salt increasingly lowers the freezing point
because salt ions interfere with the formation of the
hexagonal structure of ice.
 Vapor pressure decreases as salinity increases because
salt ions reduce the evaporation of water molecules.
 Density of water increases as salinity increases.
Change in scale
Graph of Density of freshwater:
Seawater density depends on temperature, salinity and pressure!
Therefore, it increases with > salt content at constant temp
high density in cold, salty waters –why is this important?
Surface salinity variation
 Pattern of surface
salinity:
 Lowest in high
latitudes
 Highest in the tropics
 Dips at the Equator
 Surface processes help
explain pattern
Surface salinity variation
 High latitudes have low surface salinity
 High precipitation and runoff
 Low evaporation
 Tropics have high surface salinity
 High evaporation
 Low precipitation
 Equator has a dip in surface salinity
 High precipitation partially offsets high evaporation
Why is surface Atlantic more salty than Pacific?
Salinity map showing areas of high salinity (36 o/oo) in green,
medium salinity in blue (35 o/oo), and low salinity (34 o/oo) in
purple. Salinity is rather stable but areas in the North Atlantic,
South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea,
and Mediterranean Sea tend to be a little high (green). Areas near
Antarctica, the Arctic Ocean, Southeast Asia, and the West Coast
of North and Central America tend to be a little low (purple).
http://www.biosbcc.net/ocean/marinesci/02ocean/swcomposition.htm
Salinity variations
Location/type
Salinity
Normal open ocean
Baltic Sea
Red Sea
Great Salt Lake
Dead Sea
Tap water
Premium bottled water
33-38‰
10‰ (brackish)
42‰ (hypersaline)
280‰
330‰
0.8‰ or less
0.3‰
Decrease Salinity by:
 Precipitation – rain, sleet, snow, falls directly on ocean
 Runoff-rivers carry fresh water to the ocean
 Icebergs melting – when glacial ice breaks off –this is
mostly fresh water
 Sea ice melting with the spring thaw-this has a little
salt but is mostly water
Increase salinity by:
 Sea ice forming in cold ocean areas as water freezes–
30% of salts in seawater are retained in ice
 Evaporation – removes very pure water, all salts are left
behind, occurs in hot climates
Sea water consists of water with
various materials dissolved within it.
 The solvent is the material doing the dissolving
and in sea water it is the water.
 The solute is the material being dissolved.
 Solutes in seawater = organic compounds and
nutrients, ionic salts, dissolved gases and trace
elements
 Salinity = salts dissolved in seawater
Solutes in water: Nutrients and Organics
 Major nutrients in the sea are compounds of nitrogen,
phosphorus and silicon.
 Nutrients are chemicals essential for life
 Because of usage, nutrients are scarce at the surface and their
concentrations are measured in parts per million (ppm).
 Concentration of nutrients vary greatly over time and
because of this are considered a nonconservative property
 Marine organic compounds occur in low concentrations
 consist of large complex molecules, such as fat, proteins,
carbohydrates, hormones and vitamins
 produced by organisms or through decay
Solutes in water: Ionic salts
 Sodium and chlorine alone comprise about 86% of the
salt in the sea.
 major constituents of salinity display little variation
over time
 99% of all the salt ions in the sea are sodium (Na+),
chlorine (Cl-), sulfate (SO4-2), Magnesium (Mg+2),
calcium (Ca+2) and potassium (K+)
Solutes in water: Gases and Trace elements
In order of decreasing abundance the
major gases in the sea are nitrogen, oxygen,
carbon dioxide and the noble gases, argon (Ar),
neon (Ne) and helium (He).

Nitrogen and the noble gases are considered to be inert because
they are chemically non-reactive.
Trace elements occur in minute quantities
and are usually measured in parts per
million (ppm) or parts per billion (ppb).
• Even in small quantities they are important in promoting
life or killing it.
Salinity in the ocean:
steady-state condition because amount of salt added to the ocean (input from source)
equals the amount removed (output into sinks)
 Salt sources include weathering of rocks on land and the
reaction of lava with sea water.
 Salt sinks include the following:
 Evaporation removes only water molecules.
 Remaining water becomes increasingly saline, producing a salty brine
 If enough water evaporates, the brine becomes supersaturated and salt
deposits precipitate forming evaporite minerals
 Wind-blown spray carries minute droplets of inland
 Adsorption of ions onto clays
 Shell formation by organisms
Cycling of dissolved components in seawater
Oceans’ salinity may have increased over time
Gases in Seawater
The solubility and saturation value for gases in sea water increase as
temperature and salinity decrease and as pressure increases.
 The surface layer is usually saturated in atmospheric gases because of direct exchange
with the atmosphere.
 Below the surface layer, gas content reflects relative importance of respiration,
photosynthesis, decay and gases released from volcanic vents.
Gases in Seawater: O2
 Surface layer is rich in oxygen because of photosynthesis
and contact with the atmosphere.
 Oxygen minimum layer occurs at about 150 to 1500m below
the surface and coincides with the pycnocline.
 Sinking food particles settle into this layer and become
suspended in place because of the greater density of the
water below
 food draws large numbers of organisms which respire,
consuming oxygen.
 Decay of uneaten material consumes additional oxygen
Gases in Seawater: O2
 Density difference prevents mixing downward of oxygen-rich




water from the surface or upwards from the deep layer
Deep layer is rich in oxygen because its water is derived
from the cold surface waters which sank to the bottom
Consumption is low because there are fewer organisms and
less decay consuming oxygen
Anoxic waters contain no oxygen and are inhabited by
anaerobic organisms (bacteria)
Oxygen tends to be abundant in the surface layer and deep
layer bottom, but lowest in the pycnocline.
Gases in Seawater: CO2
 Major sources of carbon dioxide are respiration and
decay
 Major sinks are photosynthesis and construction of
carbonate shells
 Carbon dioxide controls the acidity of sea water
 Dissolved CO2 in water acts as a buffer, a substance that
prevents large shifts in pH
Gases in Seawater: CO2
 Dissolution of carbonate shells in deep water results
because cold water under great pressure has a high
saturation value for CO2 and the additional CO2 releases
more H+ ions making the water acid.
 Warm, shallow water is under low pressure, contains
less dissolved CO2 and is less acidic.
 Carbonate sediments are stable and do not dissolve.
Gases in Seawater: CO2
 pH is related to the amount of CO2 dissolved in water
because it combines with the water to produce carbonic
acid which releases H+ ions.

CO2 + H2O  H2CO3  H+ + HCO3- H+ + CO3-2
 H2CO3 is carbonic acid, HCO3- is the bicarbonate ion
and CO3-2 is the carbonate ion.


Adding CO2 shifts the reaction to the right and produces more
H+ ions making the water more acid.
Removing CO2 shifts the reaction to the left, combining H+
ions with carbonate and bicarbonate ions reducing the acidity.
Ocean buffering
 Ocean pH = 8.1
(slightly basic)
 Buffering protects
the ocean from
experiencing large
pH changes
Water samples must be collected in
inert containers and isolated during recovery to
prevent contamination
 Nisken bottle has valves at each
end which are automatically
closed when a weight is sent down
the cable causing the bottle to flip
over and seal itself
 Sample depth can be determined
from cable inclination and length
or with a pulsating sound source
Collection of water samples
Temperature, Salinity, and Water Density
The relationship between temperature,
salinity and density of seawater.
Temperature vs Heat
 Temperature is the measure of how fast the molecules
in a substance are moving
 Temperature measured in Kelvin, Celsius, Fahrenheit
 Heat is a measure of how much energy has to be put
into a system or removed from a system to change its
temperature or state (solid, liquid, or gas)
 Heat has units of Energy (1 calorie, calor = heat; the
amount of heat required to raise the temp. of 1 gram of
water by 1 C°)
Ocean moderates coastal temperatures
 Water has high heat
capacity
 it can absorb or
release large
quantities of heat
without changing
temperature
 Hydrogen bonds
cause thermal inertia
 Moderates coastal
temperatures
Evaporation occurs
at temp < 100° C
But it requires more
energy to do so
Atmospheric transport of surplus heat from low
latitudes into heat deficient high latitudes areas:
Seawater density
 Factors affecting seawater density:
 Temperature ↑, Density ↓ (inverse relationship)
 Salinity ↑, Density ↑
 Pressure ↑, Density ↑
 Temperature has the greatest influence on surface
seawater density
Density and
temperature
variations
with depth
Pycnocline and thermocline
 Pycnocline = layer of rapidly changing density
 Thermocline = layer of rapidly changing
temperature
 Present only in low latitude regions
 Barrier to vertical mixing of water and migration of
marine life
Pyncnocline graphics
 http://www.youtube.com/watch?v=TxdiU3LJlZ8
 From Scripps about 11 min
Thermocline graphics
 Swimming through a thermocline 40 sec
 http://www.youtube.com/watch?v=PP42k7YD-nw
 http://www.youtube.com/watch?v=204cv98oduI
 Thermocline explained related to hurricanes
 http://www.youtube.com/watch?v=aKN7Tq_-uwQ
 15 minutes on oceans and motions
Thermocline
 http://www.youtube.com/watch?v=ovIvtKSQy9Y
 Thermocline circulation at risk
Ocean layering based on density
 Mixed surface layer (surface to 300 meters)
 Low density; well mixed by waves, currents, tides
 Upper water (300 to 1000 meters)
 Intermediate density water containing thermocline,
pycnocline, and halocline (if present)
 Deep water (below 1000 meters)
 Cold, high density water involved in deep current
movement
Salinity variation with depth
 Curves for high and low
latitudes begin at different
surface salinities
 Halocline = layer of
rapidly changing salinity
 At depth, salinity is
uniform
Figure 5-22
Halocline graphics
 Diving through a halocline 45 sec
 http://www.youtube.com/watch?v=EUxjx_f-r9A
 http://www.youtube.com/watch?v=dHn80f3lAUs
 BbC halocline 1 min scuba diving in florida caves
 http://www.youtube.com/watch?v=oBCtuaWKjlU
 Arctic sea ice has grown to a record breaking amount