The answer to the childish question of why the sea tastes salty is relatively simple. Rains wash salt from the surface of the earth, and rivers deliver it to the oceans – in the amount of about four billion tons annually. Evaporating water, meanwhile, is completely fresh. It returns to land, having rid itself of dissolved substances, so they accumulate in the ocean. A liter of sea water contains an average of 35 grams of salts.
Salinity of surface water
With the childish question, everything seems to be clear, but immediately after it, before we get another, no less interesting. Are the oceans getting saltier as a huge amount of salt is constantly being washed into them? This was once thought to be the case.
In 1715, Edmund Halley suggested that the difference in salinity between the ocean and the rivers flowing into it determine the age of the Earth. The logic here is extremely simple – you need to use ordinary mathematics to calculate when the ocean was completely fresh. The calculation made showed that the Earth is from 25 to 150 million years old.
With all due respect to the great astronomer, this figure is very far from the truth. The evidence now available suggests that the Earth is about 4.5 billion years old, and the oceans on it formed 700 million years later. At the same time, then they were even more salty than today.
It is clear that there is a fundamental mistake in Halley's logical constructions – he did not take into account the mechanisms for removing salt from the oceans. The latter include, for example, deposits of evaporites, which are formed when some area is cut off from the ocean. Water evaporates from it, leaving behind salt marshes.
Approximately five and a half million years ago, the Mediterranean Sea was in this position. It lost its connection with other large reservoirs and dried up almost completely. As a result, about 10% of salt was removed from the ocean. Today, something similar is happening with the Dead Sea.
Hydrothermal springs also play their role. The magma circulating in them heats the water that is in the rocks of the seabed, as a result of which chemical reactions take place that remove magnesium and sulfates. Be that as it may, because the amount of salt in the ocean is very large compared to the volume added and removed, the overall balance remains surprisingly constant.
At the same time, the salinity of water in certain parts of the ocean can change over time. Precipitation and rivers reduce the percentage of salt in the water, while evaporation increases it. A significant factor in this regard is the formation and melting of ice. All these processes occur closer to the ocean surface and affect the salinity of the upper layer up to 200 meters deep. Tracking them, you can notice certain patterns.
So, for example, the salinity of the ocean increases as you approach the equator – it is hotter here and the evaporation rate is higher than at the poles. An exception is the strip immediately near the equator, which is characterized by an increased amount of precipitation. Many rivers flow into the Baltic Sea, so its salinity is below average. In the hot Mediterranean and Red Seas, on the contrary, this figure is higher than usual.
Climate changes also affect the salinity of the ocean. It has been established that in recent decades it has been increasing in the most saline areas, and decreasing where it was already the smallest. This indicates the intensification of the processes of both evaporation and precipitation. The salinity of the water itself can affect the climate. Together with temperature, it determines the density of the liquid filling the oceans, which, in turn, directly affects the circulation of currents in the ocean and heat in the planet's atmosphere.
Understanding these patterns is extremely important. It is for this reason that NASA launched the Aquarius satellite in 2011, which is capable of measuring the salinity of the upper ocean from Earth's orbit. It can update the corresponding parameter for all water bodies of the planet in just a week. This means that specialists working with it have the opportunity to monitor ongoing processes and assess their impact on the climate in virtually real time.