Why Is the Ocean Salty but Lakes and Rivers Are Not?

Why Is the Ocean Salty
In a nutshell, the ocean is salty because dissolved salts enter the seas and have nowhere else to go.

Have you ever wondered why the ocean is salty, but rives and most lakes are not? Are the oceans getting saltier over time? The oceans are the lifeblood of the planet, covering more than 70% of the Earth’s surface. Their vast blue expanses are a cocktail of dissolved salts, giving seawater its characteristic salty taste. But why exactly is the ocean salty? This phenomenon depends geological and hydrological processes that add and remove salts from ocean water.

The short answer to the question “Why is the ocean salty?” is that water containing salts enters the ocean and has nowhere else to go. The water evaporates, leaving sodium chloride (table salt) and a variety of other dissolved minerals behind.

Why the Ocean Is Salty

The primary reason for the ocean’s salinity is the constant influx of salts from rivers, underwater volcanoes, and deep-sea vents. Gases from volcanoes (and human activities) make rainwater slightly acidic. As rainwater seeps through the Earth’s crust, it dissolves minerals and salts, which rivers then carry to the oceans. The weathering of rocks on land also adds salts to the water, including sodium and chloride, the primary components of table salt.

Underwater volcanoes and hydrothermal vents are significant contributors too. They release mineral-rich fluids into the sea, adding salts that include magnesium, calcium, and potassium.

Water evaporates into the air from the ocean surface. Salts do not evaporate, so they remain trapped in the water.

The Removal of Salts

While these processes add salts to the ocean, there are also processes that remove salts, ensuring the ocean does not become saltier indefinitely. Some marine organisms use the dissolved salts in their biological processes, incorporating them into their bodies or shells. When these organisms die and sink to the ocean floor, the salts are effectively removed from the ocean.

Another process involves the formation of sea spray. When seawater evaporates, it leaves salts behind. The resulting salty sea spray deposits some salts on land when it is blown by the wind.

Also, there is a maximum concentration of any salt, depending on its solubility. After a certain point, any additional salt precipitates, or falls out of solution as a solid.

Differing Ocean Salinities

While all of the world’s oceans contain salts, their salinity varies significantly. For instance, the Atlantic Ocean is generally saltier than the Pacific Ocean, largely due to differences in evaporation, precipitation, river influx, and sea ice formation. The Red Sea and the Persian Gulf are among the saltiest bodies of water in the world, while the Black Sea is among the least salty due to its substantial freshwater influx.

Sea Surface Salinity
Annual mean sea surface salinity from the World Ocean Atlas 2009 (Plumbago, CC Attribution-Share Alike 3.0)

Why Rivers and Most Lakes Are Not Salty

While rivers carry salts to the ocean, they are generally not salty themselves. This is primarily because rivers continuously receive fresh water from rainfall and melted snow, diluting the salt content.

Most lakes are also not salty for similar reasons. They receive freshwater from rivers and precipitation, which dilutes any salts. However, there are exceptions. Some lakes, like the Great Salt Lake in Utah and the Dead Sea bordering Jordan and Israel, are incredibly salty. These are often endorheic lakes, which have no outlet to the sea. Water in these lakes only leaves through evaporation, concentrating salts and other dissolved substances.

Is the Ocean Getting Saltier?

The average saltiness or salinity of the ocean is around 35 parts per thousand. Currently, there is no evidence that the oceans are getting significantly saltier. The processes that add and remove salts from the ocean largely balance each other out, maintaining a roughly steady salinity level over time. However, regional changes in salinity occur, primarily linked to changes in rainfall and evaporation patterns due to climate change.


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