7 ocean water chemistry

Ocean Water Chemistry



Figure 4.17a



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Distribution of Earth’s Water
• Oceans
• Ice Caps and Glaciers
• Atmosphere
• Rivers and Lakes
• Inland Seas
• Groundwater

97.2 %
2.15 %
0.001 %
0.009 %
0.008 %
0.625 %


Ocean Water is SALTY
• Salinity: Total amount of dissolved solids

• Units: o/oo = 1/1000
• Range: 33 – 37 o/oo

• Increase in salinity:
– Increase in Density
– Decrease in Freezing Point
– Decrease in Vapor Pressure
– Increase in Osmotic Pressure


Origin of Salts in Oceans
• Rivers (largest transport of chemicals to ocean)
– Rain + CO2  H2CO3
– Si, Al, Na, K, Mg
• Volcanoes
– Cl, S, CO2

• Dust / Rain
–

Fe, Si

• Anthropogenic
–

CO2, P


Example Geochemical Cycle

Concept of Steady State

Example 2 Geochemical
Cycle


Residence Time

(T = Ocean amount/Output rate)

• Concentration of elements in seawater is
determined by their removal rate
• Conservative elements:
–
-

Major Elements: Cl, Na, SO4, Mg, Ca, K
Minor Elements: Br, Sr, B, C, F

• Non Conservative Elements
–
–
–
–

Nutrients: N, P, Si
Dissolved gases: O2, CO2, N2
Trace Elements: Fe, Al, Mn
Organic Compounds


Residence Time - Concentration

Element

Res. Time (yrs)

Na 60 000 000
Cl80 000 000
Mg 10 000 000
K 6 000 000
SO4 9 000 000
Ca
Mn
Fe

1 000 000
7 000
100

Concentration
Crust (%)
Ocean (mg/l)

2.4
0.013
2.3
2.1
0.026
4.1
0.5
2.4

10 770
19 500
1 290
380
905
412
0.0002
0.002



Dissolved Gases

Gas Solubility:
Decreases with Temp. and Salinity
Increases with Pressure


Gases in Atmosphere & Oceans
Percent Gas Phase by Volume
Gas Atmosphere

Surface Ocean

Total Ocean

N2

79%

48%

11%

O2

21%

36%

6%

CO2

0.04%

15%

83%



Seawater pH
• Pure water pH = 7
• Seawater pH = 7.5 – 8.1
• Seawater is very well buffered!

CO2(gas)+H2OH2CO3H++HCO32H++CO32

H2O: Universal Polar Solvent


H20: Temperature and Density


H2O: Frozen & Liquid density


H2O: Heat Capacity
• Heat Capacity: heat needed to change the
temperature of a substance
• Water has higher heat capacity than:
– All solids
– All liquids, except liquid ammonia
• Latent heat of Vaporization: heat needed to
evaporate a liquid
– Water has the highest of all substances


Seawater: Temperature and Density