1. Seawater Chemistry

2. 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

3. 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)

5. Salinity
The total quantity (concentration) of dissolved inorganic solids in water
About % by mass in oceans
Average ~3.5% or 35 ppt or 35 %0

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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%

13. 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

14. 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

15. 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

18. 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

19. 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

20. 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

21. 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

23. 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

24. Dissolved Gases Major gases – nitrogen, oxygen & carbon dioxide
Gases dissolve better in cold water
Cold polar water contains more gases that warm tropical water

25. Dissolved Gases
Nitrogen – 48% of the gases in ocean (78% in atmosphere)
Source – diffusion of atmospheric nitrogen
Upper layers saturated with nitrogen gas

26. 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

27. 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

28. 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

29. 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

30. 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)

31. 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)

32. 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