Chemical Oceanography: Salinity

What is Salinity? A measure of the amount of salt in seawater, measured in parts per thousand (ppt) or percentage (%o). A measure of the amount of salt in seawater, measured in parts per thousand (ppt) or percentage (%o). Seawater has an average salinity of about 35 ppt. or 3.5 %. Seawater has an average salinity of about 35 ppt. or 3.5 %.

Salinity Varies by Latitude and Across the Different Oceans

Cations: Cations: Sodium (Na + ) – 31% by weight Sodium (Na + ) – 31% by weight Magnesium (Mg 2+ ) – 4 % by weight Magnesium (Mg 2+ ) – 4 % by weight Calcium (Ca 2+ ) – 1% by weight Calcium (Ca 2+ ) – 1% by weight Potassium (K + ) – 1% by weight Potassium (K + ) – 1% by weight Anions Anions Chloride (Cl - ) – 55% by weight Chloride (Cl - ) – 55% by weight Sulfate (SO 4 - ) – 8 % by weight Sulfate (SO 4 - ) – 8 % by weight Salts Dissolved in Seawater:

That’s a lot of salt!!! It has been calculated that if all the salts in the oceans of the world were dried up they would yield approximately 4.4 million cubic miles of rock salt. That's enough salt to cover all the landmasses of the world to a depth of 150 feet.

Where does the salt come from? Weathering of Rocks on Land Weathering of Rocks on Land Volcanic Gases Volcanic Gases Circulation at Deep Sea Hyrdothermal Vents Circulation at Deep Sea Hyrdothermal Vents

Weathering of Rocks Rain is slightly acidic and dissolves rocks and sediments in a slow process called weathering. Rain is slightly acidic and dissolves rocks and sediments in a slow process called weathering. Calcium carbonate is particularly vulnerable to weathering. Calcium carbonate is particularly vulnerable to weathering. Runoff and rivers carry the dissolved minerals from the land to the sea. Runoff and rivers carry the dissolved minerals from the land to the sea. Sphinx

Volcanic Gases Volcanoes spew gas rich in chlorine and sulfate from the Earth’s interior. Volcanoes spew gas rich in chlorine and sulfate from the Earth’s interior.

Deep Sea Hydrothermal Vents Occur along mid-ocean ridges where clouds of mineral- rich superheated water erupt from fissures in the seafloor. Occur along mid-ocean ridges where clouds of mineral- rich superheated water erupt from fissures in the seafloor. Chemical interactions that occur in the underlying molten material cause seawater to lose magnesium and gain calcium. Chemical interactions that occur in the underlying molten material cause seawater to lose magnesium and gain calcium.

Why doesn’t salinity increase with time? There is a Constant inflow and outflow of water and salts. Inflow: Inflow: Dissolved minerals from the land and fresh rain/runoff Outflow: Outflow: Water evaporation and Sea Spray depositing salt on coastlines Water evaporation and Sea Spray depositing salt on coastlines

Ocean Salt Content

Why is the salinity lower at the poles? Why is the salinity lower at the poles? Why is the Salinity higher near the Equator? Why is the Salinity higher near the Equator? Salinity can change near the shore. What can make it higher? What can make it lower? Salinity can change near the shore. What can make it higher? What can make it lower? Why is the salinity stay normal in the Ocean Basins? Why is the salinity stay normal in the Ocean Basins?

Resources in the Oceans Chemical Resources: About 30% of the world’s table salt is extracted from sea water.

Salt Recovery – Warm Climate In warm dry climates seawater is allowed to flow into shallow ponds and evaporate down to a concentrated brine solution. In warm dry climates seawater is allowed to flow into shallow ponds and evaporate down to a concentrated brine solution. More seawater is added and evaporated and the process is repeated several times until a thick brine is produced. More seawater is added and evaporated and the process is repeated several times until a thick brine is produced. Full evaporation is then allowed to take place so that a thick white salt deposit is left on the bottom of the pond. Full evaporation is then allowed to take place so that a thick white salt deposit is left on the bottom of the pond. This salt is then collected and refined to produce table salt This salt is then collected and refined to produce table salt

Salt Recovery - Cold Climate In cold climate areas salt can also be recovered. This occurs in freezing ponds. In cold climate areas salt can also be recovered. This occurs in freezing ponds. The ice that forms on the surface of the seawater is nearly fresh (very little salt) The ice that forms on the surface of the seawater is nearly fresh (very little salt) The salts are concentrated in a brine solution below. The salts are concentrated in a brine solution below. The cold brine is then collected, removed and heated so that the last of the water can be removed and only the salt remains. The cold brine is then collected, removed and heated so that the last of the water can be removed and only the salt remains.

Other Minerals in Seawater - 60% of the world’s magnesium comes from the sea - 70% of the world’s bromine - 10 million tons of gold and 4 billion tons of uranium are dissolved in the world’s seawater, however, the concentration is so low (one part per billion or less) that there is no economic way to recover these minerals.

Resources continued… Fresh Water DESALINATION the process of obtaining fresh water from salt water There are three main desalination methods: 1- processes involving a change of state of water (liquid to solid, or liquid to vapor) 2- processes requiring ion exchange columns 3- processes using a semi-permeable membrane, such as electrodialysis and reverse osmosis