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Why is the ocean salty?

In this article we will explain why the ocean is salty: fina…

Written by

IOC-UNESCO

Publishing Date

13 June 2022

Reading time

4 min

In this article, we will examine the sources of salt in the sea and answer the question: why is the ocean salty? But before we answer that question, let’s review some basic concepts:

About 70% of the Earth’s surface is covered by the ocean. Of the total water on Earth, 3% is fresh water and almost 97% of the water is salt water.

The six most abundant chemical elements in the ocean are chloride, sodium, potassium, sulfate, magnesium, and calcium, which make up 99% of sea salts.

The freezing point of salt water is -2 °C; the freezing point of fresh water is 0 °C.

Salt water also has economic importance. For example, the sea salt we use in cooking often comes from the evaporation of sea water, which is a natural source of sodium.

Perché l'oceano è salato_Decennio del Mare — © Pexels

Where does all the salt in the ocean and its seas come from?

Salt in the ocean comes from two main sources:

1.Runoff from the land

Rain carries mineral ions from the ground into the water. Rainwater is slightly acidic because some of the carbon dioxide in the air dissolves in it.

When rain falls on rocks, they release mineral salts, which separate into ions. These ions are then carried by the water and end up in the sea. Over 90% of all ions in salt water are sodium and chloride, the main ingredients of cooking salt.

2.Seafloor vents

Hydrothermal fluids are also a source of salts in the ocean. Water seeps through cracks in the seafloor, where it is heated by lava and magma from the Earth’s interior. The increase in temperature triggers a series of chemical reactions: the water tends to lose oxygen, magnesium and sulphates and to collect metals such as iron, zinc and copper from the surrounding rocks.

Some ocean salts originate from underwater volcanic eruptions, phenomena that release the minerals directly into the sea.

What is the effect of salt on water?

At the same temperature, seawater is denser than freshwater because of the salt it contains. This is because water molecules (H₂O) cluster around salt molecules, resulting in salt water having more molecules overall than freshwater, making it denser and causing it to sink below freshwater or less dense water.

Salts and minerals are also used extensively by marine life, for example by removing iron, zinc, and copper from the water.

Differences in salinity and temperature of seawater in ocean basins create what we know as ocean water masses. These masses of different salinities and temperatures make it possible for water to move and transport nutrients around the world. This phenomenon is called deep ocean circulation, and it plays a key role in regulating currents and transporting heat.

Because of the higher density of salt water in the ocean, people, animals and other objects float more in sea water than in fresh water. Each ocean basin and sea has certain characteristics in terms of salinity, think about what happens in the Dead Sea.