What if the sea were not salty?

If you taste sea water, it is very salty! Lake water and river water are not.

Why is the sea salty? And what if it were not?

Oceans cover about 70 percent of Earth's surface, and the U.S. Geological Survey says about 97 percent of all the water on and in the Earth is salty. Yet rivers and most lakes taste fresh. So where does the sea's salt come from?

In this thought experiment we imagine the whole sea as fresh as a lake. You can follow three paths: what the sea creatures would do, how ice would form, and what would happen to the ocean's slow, deep currents.

Ocean salinity averages about 35 parts per thousand: about 3.5 percent of seawater's weight is dissolved salts, and sodium and chloride make up over 90 percent of the dissolved ions (USGS). That salt shapes the ocean's chemistry, its physics and its life.

This lab is a thought experiment, not a simulation. It imagines a fresh ocean and follows the evidence for three consequences: osmotic balance in marine organisms, the freezing and density of seawater, and density-driven deep circulation. Consequences are labeled as facts, where sources describe today's ocean, or ideas, where we reason about the imaginary one.

Rain falls on rocks. It washes out tiny bits of salt.

Rivers carry the salt to the sea. The salt stays in the sea and builds up.

The sea's salt comes mostly from rocks on land. Rain is very slightly acidic, so over time it wears away rocks and dissolves tiny bits of their minerals. Streams and rivers carry these to the sea.

Sea creatures use up some of them, but others are left behind, so they build up over a very long time. NOAA says salt also comes from hot springs on the seafloor, called hydrothermal vents, and from underwater volcanoes.

Rain absorbs carbon dioxide and becomes slightly acidic (carbonic acid); it erodes rock physically and chemically, releasing ions that runoff carries to rivers and the sea. Organisms remove some ions; others accumulate over long periods (USGS, NOAA).

NOAA adds hydrothermal fluids, which pick up metals such as iron, zinc and copper as heated seawater circulates through the crust, plus underwater volcanic eruptions and salt domes. USGS estimates that the ocean's salt, spread over the land, would form a layer more than 500 feet (166 meters) thick.

Now imagine the sea with no salt at all. It tastes like lake water.

The fish and crabs of the sea are used to salt. Things would change for them!

Picture the ocean suddenly as fresh as a lake. Most sea animals would be in trouble, because their bodies are built for salty water. OpenStax Biology says about 90 percent of bony fish can live only in fresh water or only in seawater, not both.

Ice would change too. Salty sea water must cool to −1.8 °C before it freezes, but fresh water freezes at 0 °C. And the deep currents that are pushed along partly by salty, heavy water sinking near the poles would lose part of their push.

Three consequences follow from what is known about today's ocean. Physiology: about 90 percent of bony fish are stenohaline, unable to osmoregulate in the opposite environment (OpenStax). Sea ice: NSIDC notes that seawater freezes at −1.8 °C and grows denser as it cools toward freezing, so it sinks before freezing; fresh water does not. Circulation: NOAA describes deep currents driven by density differences set by temperature and salinity.

How such an ocean would actually behave as a whole is not something any source predicts; those consequences are ideas.

Salt makes sea water heavier. Salt also makes the sea harder to freeze.

Sea animals' bodies are made to live with salt around them.

Dissolved salt changes water in two big ways. It makes it denser, heavier for its size, and it lowers the temperature at which it freezes. Both shape how sea ice forms and how deep water moves.

Salt also matters to living things. Most fish are built for just one kind of water. A salmon, which can live in both, changes how it works: in the sea it drinks seawater and gets rid of the extra salt through its gills, and in a river it hardly drinks and makes lots of watery urine.

Dissolved ions raise density and lower the freezing point. In the polar ocean, NSIDC explains, the top 100 to 150 meters must cool to −1.8 °C before ice forms, because cooling salt water sinks rather than freezing at the surface. When ice does form, the salt is left behind, making nearby water saltier and denser so it sinks: NOAA calls this the start of the deep-ocean currents of the global conveyor belt.

For organisms, OpenStax describes the two strategies of euryhaline fish such as salmon: in fresh water they drink little and excrete dilute urine; in seawater they drink and excrete excess salts through gills and urine. Most marine invertebrates are osmoconformers, with body fluids matched to seawater.

Salmon are special fish. They can live in rivers and in the sea!

Most fish can only live in one kind of water.

A few fish can manage both kinds of water. Salmon spend part of their lives in fresh water and part in the sea. OpenStax Biology explains that their bodies switch how they handle salt and water when they move.

In the real world, the sea has been salty for a very long time, and its salt keeps arriving from rocks, rivers and seafloor vents. A fresh ocean is only a thought experiment, but it helps us see how much the salt does: for animals, for ice and for the slow currents that carry water around the whole planet.

Euryhaline species such as salmon show that switching between fresh and salt water is possible, but only with dedicated osmoregulatory machinery; most bony fish lack it (OpenStax). Salt also does physical work: NOAA's global conveyor belt begins where sea ice forms near the poles, leaves salt behind, and dense water sinks, and a parcel of water may take about 1,000 years to complete the circuit.

A fresh ocean is purely hypothetical. Because the salt supply from weathering, vents and volcanoes is ongoing, the question is useful for what it reveals about salt's roles, not as a forecast.