In one cubic mile of seawater, there are 120 million tonnes of salt. But where does all that salt come from? Most salt in the sea comes from land.
Why is the sea salty?
Falling rain absorbs carbon dioxide from the air and therefore becomes a little acidic. When this slightly acidic rain lands, it erodes rock, releasing minerals such as salt into the water.This water, now carrying salt, trickles into rivers and lakes, both of which eventually drain into the sea.
It is estimated that collectively, all the rivers of the world carry four billion tonnes of salt into the sea every year.
Why does the sea have so much salt compared to rivers and lakes?
The sea is about two hundred times saltier than the average river or lake. This is because, unlike rivers and lakes, seawater does not have anywhere to drain, yet freshwater is constantly evaporating from its surface. Salt therefore collects in the sea, because it has nowhere else to go.
Lakes can become salty for the same reason. If there is nowhere for water to drain, then evaporation of freshwater from a lake’s surface means the concentration of salt increases in the water that remains.
The saltiest lakes are even saltier than our oceans. For example, the Dead Sea (which is, rather confusingly, technically a lake) is about ten times saltier than the ocean.
Is the sea getting saltier?
It would be easy to assume that the sea is getting saltier. If rivers deposit salt into the ocean, but then freshwater evaporates from the ocean’s surface and the salt has nowhere to go, wouldn’t the concentration of salt keep increasing?
Salt levels in the ocean are actually very stable. This is because there is a balance between the amount of salt entering the ocean and the amount of salt that is deposited on the ocean floor.
Are all seas equally salty?
Salt levels vary from sea to sea. Equatorial waters and polar waters are less salty than other areas, but for different reasons.
Near the equator, the high volume of rainfall means that the sea is a little more diluted than in other regions, while the poles are slightly more dilute because of the influx of freshwater from melting sea ice.
