1. How Rivers Carry Salt From Rocks to the Ocean
Rainwater is slightly acidic. When this rain hits rocks, it dissolves small amounts of salts and minerals from them [1]. Rivers carry these dissolved materials into the ocean. The ocean collects salt and minerals from every river that flows into it [2]. The reason river water does not taste salty is that the amount of dissolved salt is very small, and rain constantly replaces the water. As long as rain falls regularly, rivers and lakes tend not to become salty [1].
The ocean is different. When water evaporates from the sea, the salt does not leave with the water; it stays behind [1]. This is how salt has accumulated in the ocean over time. If you heat slightly salty water and let only the water evaporate, the remaining liquid becomes more concentrated. The ocean has been doing this for a very long time. However, because the ocean is wide and deep, it does not become suddenly salty from one summer’s evaporation. Salt stays while water leaves, so the concentration builds up over time [1]. But did rivers bring all the salt?
2. How the Ocean Floor and Volcanoes Add Salt to Seawater
The ocean’s salt does not come only from rivers. Seawater seeps into cracks in the ocean floor, is heated by magma, dissolves components from the rocks, and returns to the ocean through hydrothermal vents. This process also adds salts to the sea [1].
When we look at the chemical components, we see different sources. The main players in ocean salt are chloride ions and sodium ions. These two are the most abundant dissolved salts. Oceanography textbooks explain that much of the sodium comes from the weathering of rocks, while chlorine comes from hydrogen chloride released by volcanic gases and similar sources. River water mainly contains calcium and bicarbonate ions, so its contents differ from seawater. The same textbooks state that the idea that "all ocean salt comes from rivers" is inaccurate [3].
Japanese dictionaries also explain that while the concentration of seawater varies by region, the ratio of components to each other is nearly constant worldwide [5]. Even if the saltiness changes from place to place, the ratio of the salt’s contents remains largely the same. If rivers bring this much salt every year, the ocean should get saltier each year. Is that actually happening?
3. Why the Ocean Isn't Getting Saltier Even Though Billions of Tons of Salt Arrive Each Year
The National Oceanic and Atmospheric Administration (NOAA) estimates that rivers around the world carry about 4 billion tons of dissolved salts each year [2]. This is an estimate, not a precise measurement, and it does not include the salt added by hydrothermal vents from the ocean floor. Despite this, the ocean does not seem to be getting saltier. NOAA explains that roughly the same weight of salt that rivers bring in is thought to accumulate as sediment on the ocean floor. Therefore, the ocean is probably not getting saltier [2]. However, the explanation uses words like "thought to be" and "probably," so it is not a perfect balance.
There is another special circumstance for sodium and chloride ions. These two are less likely to be used by living things and less likely to sink and precipitate out of the water. Because of this, they remain in the seawater for a long time and accumulate easily. Elements like iron, zinc, and copper are taken up by organisms and decrease in the seawater [1]. Many of the materials that make ocean salt have the property of staying in the water without being used. So, is there a way for salt to leave the ocean? One such path remains on land.
4. How Ocean Salt Becomes Layers of Rock Salt on Land
The path for salt to leave the ocean is not just sediment on the ocean floor. When parts of the ocean are trapped on land and evaporate, the water evaporates, leaving only salt, which forms thick layers of rock salt. These layers are called evaporites. Evaporites are one mechanism that removes salt from the ocean [4].
Over long periods, the amount of salt entering and the amount leaving balance out. Textbooks explain that ocean salinity has been maintained this way. It is said that ocean salinity has remained generally stable over long timescales [4].
However, it has not always been the same. Estimates using evaporites suggest that oceans before the Mesozoic Era may have had salinity as high as 5% (50 parts per 1000). In the early Miocene, about 23 million years ago, it was 3.7–3.9%, and today it is about 3.5% [4]. While it has not changed greatly over hundreds of millions of years, it has shifted slightly. Salinity has changed slightly depending on the era. So, what was the salinity when the ocean was born?
5. How Salty the Young Ocean May Have Been Long Ago
According to the Japanese version of Wikipedia, the ocean at its birth was acidic, dissolved rocks in the Earth's crust, and became neutralized by alkali and alkaline earth metals, forming salt. After that, the crust slowly dissolved, and salinity increased [7]. However, researchers disagree on how acidic the primitive ocean was.
Research has shown it might have been even saltier. Researchers at Yale University reported in 2021 that during the first 500 million years of Earth’s history, ocean salinity may have reached up to 7.5% [6]. This is still a hypothesis. The salinity of the newly formed ocean is still being studied. After that, it is thought that the salt entering from dissolving rocks and the salt removed to land moved toward a balance. We cannot simply say the ocean got saltier from the day it was formed. The current view is that there were saltier times, and then it settled down.
6. How to Make Salt Water Close to Seawater in Your Kitchen
Using a kitchen scale, measure 193 g of water and 7 g of table salt and mix them. In a total of 200 g, 7 g is salt, so 7 divided by 200 is 3.5%. This is about the same concentration as average seawater. Taste a tiny bit and rinse your mouth with water immediately. You can feel how salty seawater is with your tongue, not just numbers. Do not swallow it. Next, put a little of this salt water on a plate and place it in a sunny window. Leave it until the water is gone. It may take several days. Only the water flies away, leaving white grains. Do not use fire; place it in the sun instead. When seawater evaporates, the path of leaving salt behind is the same. Compare the grains left on the plate with kitchen table salt. Are the shapes and sizes uniform? Changing the drying place or plate shape might change how the grains look.