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Why is the sea salty? This seems to be one of the problems that preschoolers have long had. Here, the answer is relatively simple. Rain Water will erode the salt in the rocks, and the river will transport the salt to the ocean and accumulate there. Globally, rivers pour 4 billion tons of salt into the sea every year. But when water evaporates from the ocean to rain, it does not take away salt.

Most of the salt in the ocean is sodium chloride, also known as table salt. But there are also small amounts of magnesium, calcium and potassium ions, all of which chemists classify as "salt". If you take a kilogram of sea water from the ocean today and evaporate it, about 35 grams of all this salt will be left. This simple question and a fairly simple answer lead to a more interesting question: as rivers add all this salt every year, does the ocean become more and more salty?

At some point in the past, scientists thought so. In 1715, Edmund Halley (the man who discovered the comet) proposed to calculate the age of the earth using the difference between the salinity of the oceans and the rivers that flow into the oceans. This is a clever idea. If the river adds a little salt to the ocean every year, then we can go back and find out when the fresh ocean first formed. The final calculation shows that the earth (or ocean) is about 0.25 to 150 million years old.

We now know that this number is wrong, and many other evidence tells us that the earth is much older, about 4.5 billion years old, and that the oceans were formed 3.8 billion years ago. The problem is that this calculation does not take into account the way salt is removed from the ocean.

The salinity is balanced. This occurs when the water in an area is isolated from the rest of the ocean and the water evaporates and leaves salt flats. About 5.5 million years ago, the Mediterranean was cut off from the rest of the world and almost completely dried up, taking about 10% of the salt from the world's oceans! The Mediterranean is better now, but today the Dead Sea plays a similar role. Hydrothermal vents also have an effect. When sea water flows through rocks near magma, they heat the water and cause chemical reactions to remove magnesium and sulfate from the water.

Because the total amount of salt in the ocean is large compared to the salt added or removed each year, it is generally balanced, so the ocean does not become more salty over time. However, salinity still varies from place to place over time. That's because salinity is the amount of salt in the water, so if you change the amount of water, you can change the salinity. Rain Water and the river increase fresh water, and evaporation can take away water and increase salinity in specific areas. Ice may also be an important factor, and when sea water freezes into ice, salt remains in a process called salt water repulsion. Therefore, the formation of sea ice increases the salinity of an area, while the melting of ice is the opposite. All of these processes take place on the ocean surface, so they affect the salinity of about 200 meters of sea water.

Add all this up and we'll see some global patterns. When we are near the equator, the surface ocean is more salty because it is warmer and evaporates more than near the poles. The equator is an exception, where there are many Rain Water and low salinity. The Mediterranean Sea and the Red Sea have large evaporation and high salinity. So even if the overall salinity of the ocean is relatively constant, small fluctuations can tell us a lot about the worldwide water cycle.

In addition, the climate crisis has had an impact on salinity. The researchers noticed that the saltiest areas of the ocean are getting saltier, while the lightest areas are getting lighter. This means that, overall, both evaporation and precipitation are on the rise, which could mean more severe land droughts and floods. Salinity itself will also have a direct impact on the climate, salinity and temperature are the main factors that determine the density of sea water. Denser water sinks to the bottom of the sea, and this movement drives the global ocean circulation, which can have a significant impact on the way heat flows around the earth. All this means that it is important to understand these salinity patterns across the ocean and how they change.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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