Why Is the Ocean Salty?


The separation of water into salty oceans, river beds, and freshwater lakes from the formation of the Earth until its current state is the result of a complex and lengthy process. The formation of Earth began about 4.6 billion years ago as gas and dust clouds within the solar system collapsed. During this process, Earth, a planet orbiting the Sun, attracted the surrounding gas and dust due to gravitational effects, and this material gradually contributed to the formation of the planet.

Key Takeaways: Ocean Salinity

  • The average salinity of seawater is around 35 parts per thousand (ppt), which means there are approximately 35 grams of salt dissolved in every 1,000 grams of seawater.
  • Salinity in the oceans comes from various sources, including the weathering of rocks on land, volcanic activity, and the input of salts from rivers and streams that flow into the oceans.
  • Salinity levels can vary significantly from one region of the ocean to another. Areas with high evaporation rates, like the subtropical regions, tend to have higher salinity, while regions with high freshwater input, like near the mouths of large rivers, can have lower salinity.
  • The movement of ocean water due to differences in temperature and salinity is known as thermohaline circulation. This process helps distribute heat and nutrients around the globe, influencing climate and marine life.

Water was brought to Earth during its formation by icy meteorites and comets. Additionally, a significant amount of water was likely present in the molecular cloud from which the solar system formed. Within this cloud, dust particles acted as nucleation sites for water vapor to condense and freeze, forming solid water ice. Subsequently, these ice-containing planetesimals coalesced, leading to the incorporation of water into bodies like Earth.

The separation of this water into salty and freshwater is a result of Earth’s geological and chemical processes, which have evolved over millions of years. Oceans are inherently salty due to a complex interplay of factors. Surface waters constantly evaporate under the Sun’s heat, leaving behind a concentration of salts and minerals in the air as water vapor. Additionally, the salts in the oceans have their origins in various sources, including the weathering of rocks on land and the release of minerals from underwater volcanic activity.

Rivers and lakes, in contrast, predominantly house freshwater due to their continuous replenishment by precipitation and melting snow. Rainwater and melted snow generally have a lower salt content than ocean water. These freshwater sources play a pivotal role in Earth’s ecosystems, providing habitats for various forms of life and serving as essential resources for human societies.

The formation and evolution of oceans and freshwater sources constitute a profoundly intricate and ongoing scientific exploration. While the general mechanisms are understood, the finer details of these processes continue to be the subject of active research. Factors such as meteoritic contributions, chemical reactions, and the geological transformations of Earth’s surface have all played integral roles in shaping the distribution and characteristics of Earth’s water sources.

Why Is the Ocean Salty?

Gulf of Mexico salt pool
On Dive 02 of the 2014 Gulf of Mexico Expedition, ROV Deep Discoverer found an interesting brine pool. Brine seeps are common in the Gulf of Mexico, but brine pools of this size are not common (up to 10 meters wide and ~100 meters long). Image: NOAA.

Primitive seas were initially somewhat salty, and rainwater falling on Earth progressed across the land, accumulating in rocky areas near the oceans. The primary reason for the salinity of the oceans is the transfer of mineral ions found in rainwater from the land to the oceans. Carbon dioxide in the air transforms rainwater into a slightly acidic state. As rainwater progresses over the Earth’s surface, it accumulates in crevices in rocks, where, over time, both the mineral salts present in the rocks and those in the rainwater dissociate into ions and mix with ocean waters.

 The seep lies within Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico.
A brine seep located at the base of East Flower Garden Bank at a depth of about 240 ft (73 m). The seep lies within Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico. It is formed from super-saline water flowing from under the seafloor. Image: NOAA.

Certain areas of the ocean are saltier than others. This image was captured in an underwater cave beneath the waters of the Gulf of Mexico. Within this underwater pool, rocky ledges house “methane barnacles.” For these barnacles to survive, the salt concentration in the environment needs to be over four times saltier compared to other parts of the ocean.

Measurements have revealed that the salt concentration in the coastal areas of the Gulf of Mexico is significantly higher than in the more distant parts. If we could fit a submarine into the underground water pool in the picture, the salt concentration in the water would be enough to keep the submarine suspended above the seabed.

Different regions of the ocean exhibit varying levels of salinity due to geological and environmental factors. The movement of water, evaporation, and the interaction of seawater with underwater geological formations all contribute to the unique salinity profiles observed in different oceanic areas.

Effect of Rocks on Water Salinity

The primary source of the salt found in ocean waters is rocks. Rainwater, which has an acidic nature, erodes rocks. The eroded rock particles are carried into the water, and the ions that mix with the water are transported to rivers and streams. Aquatic organisms consume the majority of the dissolved ions that are present in ocean waters. Through this process, a significant amount of salt is actually removed from the water.

Rainwater’s acidic properties result from various dissolved gases, including carbon dioxide, which forms carbonic acid when combined with water. As rainwater contacts rocks on the Earth’s surface, it begins to dissolve minerals from these rocks. This mineral-rich water flows into rivers and streams, ultimately reaching the oceans.

In the oceans, marine organisms, such as plankton and shellfish, actively take up and use minerals, including calcium, magnesium, and potassium, which are essential for their biological processes. These minerals originate from the eroded rocks that rivers carry into the oceans. As these organisms die, their shells and skeletal remains sink to the ocean floor, where they accumulate over time. This process is one of the ways that minerals are removed from ocean water, contributing to the overall balance of the oceans’ chemical composition.

While the ocean waters do contain salts, this continuous cycle of erosion, mineral uptake by marine life, and the eventual deposition of minerals onto the ocean floor helps regulate the salt concentration in the oceans. It’s a dynamic and process that maintains the delicate balance of salinity in Earth’s oceans.

Hydrothermal Fluids and Salinity

Hydrothermal fluid pathways.
Hydrothermal fluid pathways. Image: NOAA.

Another source of salt within the oceans emerges from hydrothermal fluids emanating from vents situated on the ocean floor. These vents serve as conduits for ocean water to infiltrate seafloor crevices, where it encounters the heat generated by Earth’s magma core. This elevated temperature initiates a cascade of chemical transformations. Consequently, the water released from these reactions experiences a reduction in oxygen, magnesium, and sulfate levels while simultaneously absorbing metals such as iron, zinc, and copper from the adjacent rock formations. Subsequently, this thermally altered and metal-enriched water is expelled through the seafloor vents. Additionally, certain oceanic salts originate from subaqueous volcanic eruptions, releasing minerals directly into the ocean environment.

Within seawater, the two most prevalent ions are chloride and sodium, together constituting approximately 85% of all dissolved ions present. Magnesium and sulfate collectively comprise around 10% of the total ion content. Other ions remain present in minute quantities. The concentration of salt in seawater fluctuates in response to factors like temperature, evaporation, and precipitation. Salinity levels generally exhibit lower readings at the equator and poles, while they elevate in mid-latitudinal regions. On average, the salinity of seawater stands at about 35 parts per thousand. This translates to approximately 3.5% of seawater’s mass being attributed to dissolved salts.

Imagine amassing all the salt distributed across the Earth; the resultant layer would blanket the entire planet with a thickness of roughly 150 meters.

Certain mineral ions are assimilated and utilized by marine organisms and vegetation in the surrounding water. Furthermore, salt finds its way into the ocean from the activity of underwater volcanoes and hydrothermal vents on the seafloor. Enclosed bodies of water have the potential to intensify their salinity, resulting in hypersaline conditions through evaporation. Examples of such environments include stagnant seas and specific salt flats. The augmented salt content contributes to higher water density, causing individuals to experience enhanced buoyancy in stagnant seas compared to the expansive open ocean.

This Article at a Glance

What is ocean salinity?

Ocean salinity refers to the concentration of dissolved salts, mainly sodium and chloride ions, in seawater. It is typically measured in parts per thousand (ppt) or practical salinity units (PSU).

What is a halocline?

A halocline is a layer in the ocean where salinity changes rapidly with depth. It often forms beneath the surface layer and can influence ocean circulation and marine ecosystems.

How does ocean circulation impact salinity?

Ocean circulation, driven by currents and winds, redistributes seawater around the globe. This can lead to variations in salinity as different water masses with varying levels of salt mix and interact.

How salty is the ocean?

The average salinity of seawater is around 35 parts per thousand (ppt), which means that about 35 grams of salt are dissolved in every kilogram of seawater.

What causes ocean salinity?

Ocean salinity is primarily caused by the accumulation of salts from various sources, including weathering of rocks on land, volcanic activity, and the input of salts from rivers and other sources.

Why is ocean salinity important?

Ocean salinity plays a crucial role in regulating the density of seawater, which affects ocean circulation patterns and currents. It also influences the behavior of marine organisms and the distribution of marine life.