Surely you have noticed that seawater is salty, while river water flowing into it is fresh. This seems like a paradox. Rivers have been carrying dissolved salts to the ocean for billions of years, yet they themselves remain fresh. Meanwhile, the ocean, receiving these salts, maintains a stable salinity level—averaging 35 grams per liter. Why then are the ocean and rivers not becoming progressively saltier? Let’s explore this issue.
Where does the salt originate?
The principal and most potent source of salts to the ocean comes from the terrestrial rock formations. If all the salt from the ocean were evenly spread across the land, it would form a layer over 166 meters thick. But how does salt get from the rocks into the water?
It all begins with rain. When atmospheric carbon dioxide dissolves in rainwater, a weak carbonic acid ($\text{H}_2\text{CO}_3$) forms. This “acid rain” falls onto the ground. $\text{H}_2\text{CO}_3$ slowly but surely interacts with rocks and minerals, breaking down their chemical bonds.
During this chemical weathering process, soluble ions—charged particles which constitute salts—are liberated. These include, for example, sodium, magnesium, and calcium cations, and chloride anions. Rainwater not only physically erodes but also chemically decomposes the rocks. Streams and rivers then collect these dissolved ions and transport them directly to the sea.
Why is the ocean salty, yet rivers are not?
The fundamental reason the ocean is saline while rivers are fresh lies within the hydrological cycle. For eons, rivers deliver both water and tiny fractions of dissolved salts to the ocean. The ocean acts here like a massive distiller—water evaporates from its surface, turning into vapor. Dissolved salts cannot evaporate; they remain in the ocean, and their concentration gradually increases as evaporation proceeds.
The evaporated water forms clouds, which then precipitate over the land as rain. This fresh water replenishes the rivers, diluting them and preventing them from becoming saline. The cycle then repeats: rivers once again gather salts from the stones and carry them to the ocean.
Are there other “suppliers”?
Rivers are the main, though not the sole, source of ions. A second major “contributor” is linked to geothermal processes on the ocean floor, primarily near underwater volcanoes and mid-ocean ridges. Here is how it functions: cold seawater permeates cracks in the oceanic crust. There, it is heated by magma, sometimes to $400^\circ\text{C}$, and enters into chemical reactions with the rocks.
Then this hot, mineral-rich fluid is ejected back into the ocean through hydrothermal vents. These are often termed “black smokers.” They get their dark color from sulfide minerals, such as iron sulfide, which precipitate upon contact with the cold water. These “smokers” supply the ocean with many metals, such as iron, zinc, and copper.
Why doesn’t salinity increase indefinitely?
If rivers constantly bring salts and water evaporates, the ocean ought to become ever saltier. This, however, does not occur. Salinity remains steady because the ocean is in dynamic equilibrium. This means the rate at which salts enter equals the rate at which they are removed.
The key to the solution is the differing chemical makeup of river and sea water. Rivers primarily deliver calcium ions ($\text{Ca}^{2+}$) and bicarbonates ($\text{HCO}_3^-$). In the ocean, sodium cations ($\text{Na}^+$) and chloride anions ($\text{Cl}^-$) are dominant. This divergence happens because different ions have varied “residence times” in the water—the average duration an ion spends in the ocean before being purged in some manner. There are essentially two distinct removal pathways for ions—biological and geological.
Biological (Fast) Removal
Ions abundant in rivers—calcium and bicarbonates—have short residence times. They are actively consumed by marine organisms. Corals, mollusks, plankton, and other life forms construct their shells and skeletons from calcium carbonate ($\text{CaCO}_3$). When they perish, their remains settle on the bottom, forming thick deposits of limestone. Thus, these elements are rapidly extracted from the system.
Geological (Slow) Removal
Ions dominating the ocean (sodium and chloride) are, conversely, quite inert and soluble. Their residence times span tens or even hundreds of millions of years. They are removed from the water mainly through geological mechanisms. For instance, when isolated marine basins (like the Mediterranean Sea) desiccated in the past, massive deposits of common salt ($\text{NaCl}$) formed. Other ions, such as magnesium, are removed via reactions with the oceanic crust in those same hydrothermal vents and through interaction with clay minerals on the seabed.
The Cycle of Salt and Water
In summary, the stable salinity of the ocean results from a complex and dynamic balance maintained over eons. First, carbonic acid in rainwater slowly leaches salts from rocks and carries them to rivers, and subsequently, to the ocean. In the ocean, salts concentrate due to water evaporation, while rivers remain fresh thanks to precipitation. “River” ions are quickly claimed by living organisms that build their shells therefrom. “Oceanic” ions are eliminated very slowly via geological processes.
The ocean is an intricate kinetic system. And its stable salinity is critically important, as its gradients (differences in saltiness) are one of the primary forces driving oceanic currents that regulate our planet’s climate.
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