Chemical Oceanography: Salinity

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 50+ m.

Salt is essential… Virtually every culture has customs and traditions relating to salt: About 4,700 years ago the earliest known treatise on pharmacology was published. A major portion of this writing was devoted to a discussion of more than 40 kinds of salt and healing properties. A far-flung trade in ancient Greece involving exchange of salt for slaves gave rise to the expression, "not worth his salt." Special salt rations given early Roman soldiers were known as "salarium argentum," the forerunner of the English word "salary."

Salt played a key role in the history of West Africa, particularly during the trading empire of Mali in the 13th through 16th centuries. Salt was traded ounce-for-ounce for gold. Venice rose to economic greatness through its salt monopoly. A salt tax (gabelle) was a significant cause of the French Revolution. Thousands of Napoleon's troops died during his retreat from Moscow because their wounds would not heal as a result of a lack of salt. During the War of 1812, salt brine was used to pay soldiers in the field, as the government was too poor to pay them with money.[ The British planted the Great Hedge across India to prevent tax-evading salt smuggling and implemented a salt starvation policy, but were trumped by Mahatma Ghandi’s non-violent resistance, epitomized by his famous march to the sea to make (untaxed) salt.

Where does the salt come from? Weathering of rocks on land Volcanic Gases Circulation at Deep Sea Hydrothermal Vents

Where does the salt come from? Weathering of Rocks Rain is slightly acidic and dissolves rocks and sediments in a slow process called weathering. Calcium carbonate is particularly vulnerable to weathering. Runoff and rivers carry approximately 2.3 billion metric tons of salts into the ocean every year. Sphinx

Volcanic Gases Volcanoes spew gas rich in chlorine, sulfur, sulfates, bromine, and boron 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. 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: Dissolved minerals from the land and fresh rain/runoff Outflow: Water evaporation and sea spray depositing salt on coastlines; precipitation of minerals; organisms building shells or skeletons (CaCO3, SiO2, etc)

Ocean Salt Content

Resources in the Oceans Chemical Resources: ~ 30% of the world’s table salt is extracted from sea water. ~ 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, but the concentration is so low (one part per billion or less) that there is no economic way to recover these minerals.

Salt Recovery – Warm Climate In warm dry climates seawater is allowed to flow into shallow ponds and evaporate down to a concentrated brine solution. Full evaporation forms a thick white salt deposit on the bottom of the pond. This salt is then collected and refined to produce table salt.