Transcript Slide 1

Seawater Chemistry
Components of Seawater
• water’s ability to dissolve crustal
material as it cycles from ocean to
atmosphere have added solids and
gases to the ocean
• ~97.2% of 1,370 million cubic
kilometers (329 million cubic miles) is
salt
Components of Seawater
• by mass seawater
– about 96.5% water
– about 3.5% dissolved substances
• if the ocean’s waters could evaporate,
remaining salts would cover the entire
planet to a depth of 45 m (150 ft)
Salinity
• The total quantity (concentration) of
dissolved inorganic solids in water
• About 3.5-3.7% by mass in oceans
• Average ~3.5% or 35 ppt or 35 %0
How do ions modify the physical
properties of water?
–Heat capacity decreases with
increased salinity
• Less heat is needed to raise the
temperature of seawater
How do ions modify the physical
properties of water?
–As salinity increases, the freezing
point of water decreases
• Dissolved salts disrupt hydrogen
bonding
• Sea ice forms at a lower
temperature than freshwater ice
How do ions modify the physical
properties of water?
• Seawater evaporates more slowly
than fresh water
–Dissolved salts attract water
molecules
• Osmotic pressure increases with
increasing salinity
Components of Seawater
• About 3.5% of seawater consists of
dissolved substances
–Boiling 100 kg of seawater yields
3.5 kilograms of residue
• Oceanographers use parts per
thousand (o/oo) or ppt
Major Constituents of Seawater
• Nearly every element present in the
crust & atmosphere is also in oceans
• Water 96.5% total percent by mass
 Oxygen
85.8%
(by mass)
 Hydrogen
10.7%
 Ions
3.4% total percent by mass
Major Constituents of Seawater at
34.4 ppt
 Over 99% of seawater salinity comes
from 6 ions:
 Chloride
55%
 Sodium
32%
 Sulfate
8%
 Magnesium 3%
 Calcium
1%
 Potassium
1%
Sources of Ocean’s Salts
• Weathering and erosion of crustal
rocks accounts for some (not the only
source)
– Salts in the ocean are different concentration
than those in river water
Sources of Ocean’s Salts
• Upper mantle appears to contain
more of the substances found in
seawater (including water itself) than
are found in surface rocks
– their proportions are about the
same as in the ocean
Sources of Ocean’s Salts
• Combination of weathering (ex,
sodium) and outgassing (ex, chloride)
• Differences in expected seawater
concentrations may be the result of
interactions at mid ocean rifts
(hydrothermal vents)
• All the water in the oceans cycles
through the seabed every 1 to 10
million years
Principle of Constant Proportions
• The percentage of salts in seawater is the
same in samples from many places,
regardless of how salty the water is
– Same proportions for 33 ppt and 37 ppt
The Ocean is in Chemical
Equilibrium
• The proportion and amounts of
dissolved salts per unit volume are
nearly constant
– what goes in must go out
The Ocean is in Chemical
Equilibrium
• Ions are added to the ocean at the
same rate the are removed
• Additions from the mantle or from
weathering are balanced by
subtractions being bound into
sediments
Residence Time
• Concept of helps explain why ocean
is not getting saltier
• Chemically active ions have shorter
residence times
• See Table 7.3 page 169
(Oceanography book)
• If an ion remains in the ocean longer
than the ocean’s mixing time (~1600
years) it becomes evenly distributed
Dissolved Gases
• Seawater also contains dissolved
gases
• Most gases in the air dissolve readily in
seawater at the surface
• Plants and animals need dissolved
gases to survive
Dissolved Gases
• Major gases – nitrogen, oxygen &
carbon dioxide
• Gases dissolve better in cold water
– Cold polar water contains more
gases that warm tropical water
Dissolved Gases
• Nitrogen – 48% of the gases in ocean
(78% in atmosphere)
– Source – diffusion of atmospheric
nitrogen
– Upper layers saturated with nitrogen
gas
Dissolved Gases
Living organisms require nitrogen to
build proteins
– Nitrogen gas can’t be used by organisms
until it is attached to oxygen in a process
called nitrogen fixation
– Blue-green algae convert nitrogen gas to a
useable form that animals need for
building proteins and amino acids
Dissolved Gases
• Oxygen – 36% of the gases in the
ocean (21% in atmosphere)
– Average of 6 ppm (6 mg/L)
– Source – photosynthesis and
diffusion of atmospheric oxygen
– Living organisms require oxygen for
respiration
Dissolved Gases
• Carbon dioxide – 15% in ocean (<<1%
in atmosphere
• Sea water CO2 levels range between 45
and 54 ml/L
• Source –
– Respiration of animals
– Very soluble in water - moves quickly
from atmosphere to ocean, slowly
from ocean to atmosphere
Dissolved Gases
– Some CO2 forms carbonate ions that
combine with calcium to form
limestone (a sedimentary rock)
– Most of earths surface carbon –
10,000 times that in mass of all living
things – is stored in sediments
Acid-Base Balance
• Seawater is slightly alkaline ~8.0 pH
– Water + carbon dioxide makes
carbonic acid, which lowers the pH
– If acid is added to the ocean
chemical reactions take place to
remove the excess H+ (less acidic)
Acid-Base Balance
• Seawater is slightly alkaline ~8.0 pH
– Carbonic acid disassociates into
bicarbonate (a base) and hydrogen
which raises the pH (more alkaline)
– If alkaline is added to the ocean,
chemical reactions take place to
remove excess OH- (less alkaline)
Acid-Base Balance
• This behavior acts to buffer the water
preventing broad swings of pH when
acids or bases are added
• Enzyme activities and the shapes of
vital proteins require a stable pH
• Since mollusk’s shells are calcium
carbonate, a decrease in pH could
dissolve shells