1. Chapter 7 Seawater Chemistry Salinity
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

2. Chemical Composition of Sea Water
Probably every known naturally occurring element can be found in the ocean
Only a few in large quantities
Water – 96.5% by weight
Dissolved substances – 3.5% by weight
The most abundant ions dissolved in seawater are chloride, sodium and sulfate.

3. The Components of Salinity
The most abundant components of a kilogram of seawater.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

4. Components of Seawater
Constituent
Parts Per Thousand (ppt)
Percent (%) by mass
Oxygen
857.8
85.8
Hydrogen
107.2
10.7
Cl- (Chloride)
18.980
1.9
Na+ (Sodium)
10.556
1.1
SO42- (Sulfate)
2.649
0.3
Mg2+ (Magnesium)
1.272
0.1
Ca2+ (Calcium)
0.400
0.04
K (Potassium)
0.380
HCO3- (Bicarbonate)
0.140
.01

5. Other components of seawater
Minor ions and other constituents:
All other known elements - .1%
Nutrients
Organic compounds
Radioactive elements
Dissolved gasses

6. Dissolved Gases
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

7. Dissolved Gases Nitrogen
Most abundant gas (48% in seawater; 78% in atmosphere)
Inert (non-reactive)
Enters seawater at the surface from exchange with the atmosphere

8. Dissolved Gases Oxygen
2nd most abundant gas (36% in seawater; 21% in atmosphere)
Required by marine animals & plants for respiration
Enters seawater at the surface only
from exchange with the atmosphere
by photosynthesis (CO2+H2O  O2+ glucose)
Gets used up by respiration (O2+ glucose  CO2+H2O ) & decomposition

9. Dissolved Gases Carbon Dioxide
3rd most abundant gas (15% in seawater; ~1% in atmosphere)
Required by marine plants for photosynthesis
Enters seawater at all depths
at the surface from exchange with the atmosphere
at all depths from respiration (O2+ glucose  CO2+H2O ) & decomposition
Gets used up by photosynthesis (CO2+H2O  O2+ glucose)

10. Dissolved Gases Dissolved gases are required for life in the ocean.
Biological processes modify the concentration of gases at different depths.
Photosynthesis
Cellular Respiration
Decomposition

11. Dissolved Gases Photosynthesis reaction: CO2+H2O  O2+ glucose
Plants use CO2, let off O2
Only occurs at the surface (top 100m)
Why??

12. Dissolved Gases Cellular Respiration reaction: O2+ glucoseCO2+H2O
Animals & Plants use O2, let off CO2
Occurs at all depths

13. Dissolved Gases Decomposition Uses O2, lets off CO2
Occurs at all depths

14. Dissolved Gases
Gas dissolves more readily in cold water than it does on warm water (cold water holds more dissolved gas than warm water)
Less salty water holds more dissolved gas than saltier water
Water under pressure holds more dissolved gas than water under less pressure
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

15. Gases in Seawater Lab