A second clue to how the sea became salty is the presence of salt lakes such as the Great Salt Lake and the Dead Sea. Both are about 10 times saltier than seawater. Lakes are temporary storage areas for water. Rivers and streams bring water to the lakes, and other rivers carry water out of lakes.
Thus, lakes are really only wide depressions in a river channel that have filled with water. Water flows in one end and out the other. All the water that flows into these lakes escapes only by evaporation.
When water evaporates, the dissolved salts are left behind. So a few lakes are salty because rivers carried salts to the lakes, the water in the lakes evaporated and the salts were left behind. After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels.
The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean. It is formed from super-saline water flowing from under the seafloor. Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks. This releases ions that are carried away to streams and rivers that eventually feed into the ocean.
Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not removed, so their concentrations increase over time. Another source of salts in the ocean is hydrothermal fluids, which come from vents in the seafloor. The heat causes a series of chemical reactions. The rain water flows off the land and into the rivers and streams that lead all the way to the sea — carrying the dissolved salts along with it.
The salts in the seas have built up over billions of years, and seawater contains about times more dissolved salts than average river water. To put it another way, every one litre of seawater has 35 grams of salts dissolved in it, while a litre of freshwater would only have 0.
Some salts can also enter the seas from hot vents on the deep ocean floor and from volcanoes on the land and in the sea. Since salt is always flowing from the land to the sea, you might think the sea is getting saltier. But actually, some of this salt is removed by algae and animals that live in the sea, and some is deposited as sediment on the bottom of the ocean.
So the salt going into the sea keeps a balance with the salt being deposited or removed. In warmer, tropical areas, more evaporation occurs, so the water is saltier. Towards the north and south poles, the seawater is diluted by melting ice, so the water is not so salty. This is natural. But these differences in salinity might get bigger in the future, because of climate change.
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