Seawater Chemistry and Ocean Structure
Seawater Chemistry
70% of the Earth is covered by ocean water!
Seawater Composition
Ocean water is made up of three things:
◦ 1.) Dissolved Gases
◦ 2.) Organic Molecules
◦ 3.) Salts
Seawater is a solution.
Solution: Made of a solute and a solvent.
Solute: a dissolved substance (salt)
Solvent: the dissolver (water)
Review
Why is water such a good solvent?
Answer: Because it is a polar molecule.
Seawater Compostion
There are 1,370 million cubic kilometers of water with 5,000 Trillion kilograms of salt! (Enough to cover entire planet with
45 meters of salt!!!!)
Seawater is:
◦ 3.5% dissolved solids (salts)
◦ 96.5% H
2
O
Salinity
Salinity: the total quantity of dissolved solids in a solution.
Measured in Parts Per Thousand (PPT).
◦ i.e. 2 PPT of salt would mean there are 2 grams of salt for every 1,000 grams of water.
Ocean water has a salinity of 34-35 PPT.
Brackish water (like the Chesapeake Bay) is about
15-20 PPT
Fresh water is <1 PPT
Hypersaline water (Dead Sea) can be over 40
PPT).
◦ Use Salinometer to measure salinity
Principle of Constant Proportions
States that: Seawater is constantly mixed, which means the ionic composition of openocean seawater is the same at every place and depth, and the ratio of one major constituent to another is the same.
Amount of salts can vary but ratio will always be the same.
For example, a high salinity sample from the
North Atlantic and a low salinity sample from the Antarctic will both have 55.04%
Chloride Ions but they may have a different amount.
Determining Salinity
Due to the previous principle, we can use an easy formula.
Salinity in PPT = 1.80655 x Chlorinity in
PPT.
Chlorinity-A measure of the total mass of halogen ions (flourine, chlorine, bromine, and iodine) which is always 55.04%.
4 Colligative Properties of Seawater
Heat capacity of water decreases with increasing salinity
As salinity increases, the freezing point lowers (freezes at -2 degrees C)
Seawater evaporates more slowly than fresh water
Osmotic pressure rises with increasing salinity (pressure on cell membranes is higher).
Factors that Affect Salinity
1.) Water Temperature. Colder water dissolves less salt.
2.) Evaporation. Warmer waters near the equator are saltier because of evaporation.
3.) Precipitation. More rainfall dilutes the saltwater making it less salty.
4.) Surface runoff from rivers. Again, makes water less salty.
5.) Melting Glaciers and Sea Ice. Makes water less salty.
Application
During what season would sea surface salinity be less saline? Spring or Winter?
Why?
Answer: Spring. More rain means more runoff which would dilute the salt content.
Sources of Salt
Three Major Sources
◦ Volcanic Eruptions
◦ Chemical reactions between sea waters and hot, newly formed crustal rocks.
◦ Weathering of continental crust and erosion due to rivers.
Chemical Equilibrium and Residence
Time
Chemical Equilibrium: Proportion and amounts of dissolved salts per unit volume of ocean water are constant. What is put in also comes out in equal amounts.
Residence Time: The average length of time an atom of an element spends in the ocean.
Residence Time = Amount of element in the ocean/Rate at which the element is added
(or removed) from the ocean.
Dissolved Gases
Atmospheric gases dissolve into the sea at the ocean surface and is facilitated by wind and waves.
Major gases include:
◦ Nitrogen: 78% in Atmosphere, 48% dissolved in seawater
◦ Oxygen: 21% in Atmosphere, 36% dissolved in seawater
◦ Carbon Dioxide: 0.035% in Atmosphere, 15% dissolved in seawater
Amount of dissolved gas is affected by:
1.) Temperature: Colder water can dissolve more gas. (A warm soda gets flat because you lose Carbon Dioxide).
2.) Salinity: Higher Salinity = less dissolved gas.
3.) Pressure of water: Lower pressure = less dissolved gas.
Distribution of gases with depth depends on:
Photosynthesis: More carbon dioxide dissolved. Oxygen output. Important in first
100 meters (photic zone).
Respiration: Oxygen dissolved, carbon dioxide output.
Decomposition: Requires oxygen, produces carbon dioxide. Important for consuming oxygen at greater depths.
Compensation Depth: Depth at which photosynthesis balances rate of respiration.
◦ Oxygen minimum at 500-800 meters.
Seawater pH
Closer to 1 means more Acidic
Closer to 14 means more Basic (a base)
7 is neutral
Seawater pH
pH: Concentration of hydrogen ions in a solution.
Seawater has an average pH of 7.8 so it is slightly basic or alkaline.
Due to formation of bicarbonate which gives more OH- ions.
