【Creative Cultivation Program】Why is sea water salty? Where does the salt come from?

Author: Du Dong Geology

As we all know, the Earth on which we live is a very special planet in the solar system. It is also known as the blue planet because the ocean accounts for 70% of the Earth's surface area. Unfortunately, such a vast ocean cannot be directly used by humans for drinking and irrigation due to its high salinity. Some friends are curious, why is the sea water salty?

To answer this question, we first need to understand what the water cycle is. The water cycle is the water on the surface of the earth, such as rivers, lakes and seas. Under the sun's irradiation, the water in it turns into water vapor through direct evaporation and plant transpiration, then rises to the sky and condenses into clouds. Driven by atmospheric circulation, it moves with the air flow in the air. When certain conditions are reached, part of it turns into rain or snow, and then falls to the surface of the earth or the water. Some of the water that falls seeps into the ground, some is absorbed by plants and the juice evaporates into the atmosphere, and the rest flows into rivers, lakes and finally into the sea along the surface runoff. For billions of years, the water on the earth has been circulating in this constant movement to create the blue planet we have today.

So where does the salt in seawater come from?

Water cycle precipitation constantly washes the surface of the earth, and then gathers together, causing rivers to continuously produce runoff, which in turn carries chemical substances from the ground into the ocean, and this cycle never stops. The result of the water cycle is that the water bodies of rivers are constantly renewed, while the runoff of lakes is renewed more slowly than that of rivers, so in the same area, the content of chemical components in lake water is generally higher than that in river water. The ocean is the destination of the water cycle. Although it has a large volume, because seawater cannot be constantly renewed like river runoff, a large amount of soluble salt has slowly accumulated in seawater over a long geological period. Among the soluble salts in seawater, the most abundant is sodium chloride, which is what we usually call table salt, followed by sulfate. With these ingredients, it is not surprising that seawater is both salty and bitter.

What are the factors that affect the salinity of seawater?

Due to the effects of ocean currents and waves, the salt concentration of seawater in the ocean does not change much, but the concentration is relatively high in sea areas with large evaporation, small precipitation, and no river water inflow. For example, the salt concentration in the northern Red Sea is as high as 42 parts per thousand. On the contrary, the salt concentration is relatively low in sea areas with small evaporation, large precipitation, and many river water inflows, such as the Baltic Sea, where the salt concentration of seawater is less than 10 parts per thousand. In addition, in sea areas where dry and wet seasons obviously alternate, the salt concentration of seawater will have obvious seasonal changes. Take the Yangtze River Estuary in my country as an example, the change with the seasons is very obvious. The salt concentration of seawater in summer is about 25 parts per thousand, while in winter it can reach about 30 parts per thousand.

In this case, will the salt content of seawater become higher and higher? The answer is no. A large number of studies have shown that the volume of ocean water today is similar to that of billions of years ago. This is because the ocean water cycle is stable, that is, the annual evaporation and replenishment of the ocean are basically balanced. However, the total amount of soluble salts brought into the ocean each year is not balanced with the volatile oxygen, hydrogen, carbon dioxide, and a small amount of volatile salts in the atmosphere.

Despite this, the composition of seawater can still maintain a balance mainly due to the regulation of marine organisms. The ocean has a strong self-purification ability. Under the combined effects of physics, chemistry, biology and matter, it is constantly diffusing, diluting, oxidizing, biochemical, reducing and degrading. In particular, marine organisms absorb a large amount of inorganic salts and organic substances from seawater, and play a vital role in regulating the salts in seawater through biochemical reactions and ecological cycles.

However, in recent decades, the massive use of plastics has made every corner of the world full of plastics, and the ocean is no exception. Plastics have become the number one garbage in the ocean. Although the ocean has great self-purification ability, non-degradable plastics will seriously damage the marine ecology. Once the benign cycle of marine ecology is destroyed, the salinity of seawater will change abnormally, and the whole world will be affected. Therefore, if we want to protect our living environment and protect the earth, we need to start with protecting the ocean.

This article is produced by the Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting