Lecture 3
Physical Properties of Ocean Water
- Salinity and Light -
Part 1 - What’s in seawater
Salinity
How much salt is in seawater?
Freshwater
101.07 kg/100 l
Seawater
103.6 kg/100 l
Salt in the ocean
- 5.5 x 109 tonnes
- cover the earth to a depth of 45 m
Component Ions in Seawater
Comparison of fresh and salt water
Ion Concentration
-measured in ppt (parts per thousand or ‰)
Average in seawater is about 35 ‰
(freshwater is about 3.3 ‰)
Salinity – Red Sea
- evaporation
- isolation from open ocean
- little freshwater input
41‰
33‰
The effects of all these ions
Seawater freezes at a lower temperature
Fresh
water
Salt
water
Liquid
Ice
Part 2 – How do we measure salinity?
Forchhammer - Law of Constant Proportions
Ratio of ions  Na+ : Cl- : SO4-- : Mg++: K+ : Ca++ :CO3-Constant - worldwide
Calculation of salinity from chlorinity
chlorinity - wt. of Cl-, Br-, I- ions
Salinity % = 1.80655 x chlorinity
Salinity and conductivity
Salinity and refraction
Refractometer
Part 3 – Where do all these ions come from?
Sources – provide elements to system
Sinks – remove elements from system
Sources of Inputs 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 2 Geochemical Cycle
Cycling of Sea Salt
Volcanic activity
Sediments
uplifted
Cl2
H2S
Rainfall
ClSO4-2
River discharge & runoff
Organisms die
CO3-2
Ca+2
SO4-2
Mg+2
N+
precipitation
Bottom sediments
Ca+2
K+
Mg+2
adsorbed by clays
Leaching from rocks
Hydrothermal Vents: A Source & SINK
Minor source
Consume other elements
Lead, Sulfur, Copper, Iron, Cobalt, Gold
Mostly Gypsum & Zinc
Sinks
• Biological activity
• Interaction with particulate matter: clays
and organic matter absorb dissolved metals
Residence time
Residence time =
AMOUNT OF ELEMENT IN OCEAN
RATE OF ADDITION OR
REMOVAL
Residence Time - Concentration
Element
Res. Time (yrs)
Na
Cl
Mg
K
SO4
Ca
Mn
Fe
60 x 106
80 x 106
10 x 106
6 x 106
9 x 106
1 x 106
7 x 103
0.1 x 103
Concentration
Crust (%)
Ocean (mg/l)
2.4
10 770
0.013
19 500
2.3
1 290
2.1
380
0.026
905
4.1
412
0.5
0.0002
2.4
0.002
Gases in Ocean Water
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!
The Carbonate System in Seawater
CO2 in seawater is controlled by:
1. Exchange with the atmosphere
2. Photosynthesis/Respiration:
6CO2 + 6H2 O  C6 H12 O6 + 6O2
Importance of CO3--
CO3--
Relative abundance of carbonic acid,
bicarbonalte ion and carbonate ion in
seawater
100%
H2CO3
(carbonic acid)
50%
HCO3(bicarbonate
ion)
CO3-(carbonate ion)
Average pH of
seawater
Normal pH
range of
seawater
0%
4
5
6
7
8
pH
9
10
11
12
Light Transmission
Light Transmission
• transparent in visible part of spectrum
• Absorbed as is goes deeper in the
water column
• strongly absorbs infrared (heat) and
ultraviolet (prevents damage to DNA)
Light penetration
Absorption of frequencies of light
Light penetration and location
Photosynthetic Rate (%O2/mim)
Photosynthesis and Light
Irradiance (mE/m2/s
Light
Turbid coastal water
0
5
10
Depth
15
Clear open ocean water
20
25
30
Limit for foliose algae
35
40
45
Limit for Laminaria (kelp)
50
.01
.1
1
10
Percent surface light
100
Next time
Tides