Seawater Chemistry

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 3.5-3.7% 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