A river can flow into the sea without tasting nearly as salty as the water it joins. That difference raises an interesting question: where did the ocean’s salt come from, and why does it stay there?
The journey begins on land
Rainwater can react with rocks and gradually break them down. This weathering releases dissolved substances, including electrically charged particles called ions. Rivers and streams carry some of those ions toward the sea. [1]
Sodium and chloride are especially abundant in seawater. They are also the ions associated with ordinary table salt. However, seawater contains other dissolved substances too; its chemistry is more complicated than a container of water with a single ingredient stirred into it. [1]
The ocean floor contributes too
Land is not the only source. Seawater circulates into cracks in the seafloor, where heat and reactions with rock change its chemistry. Water returning through hydrothermal vents can carry dissolved minerals. The seafloor therefore participates in the exchange rather than acting like an inert bowl beneath the ocean. [1]
Why rivers taste different
Fresh water is not necessarily free of dissolved minerals. River water generally contains them in much lower concentrations than seawater. It is continually moving along its route, while the ocean receives dissolved material from many rivers over long periods. [2]
When seawater evaporates, the water enters the atmosphere but most dissolved salts remain behind. Salt also leaves seawater through processes such as mineral formation and incorporation into sediments. The ocean is part of an ongoing cycle, not simply a basin becoming saltier without limit. [2]
Saltiness is not identical everywhere
Rainfall and river flow add fresh water, while evaporation removes water. These processes help create regional differences in salinity. The answer to “Why is the sea salty?” therefore links a single wave to weather, geology, and water moving around Earth. [1]
Sources and image credit
1. NOAA Ocean Service: Why is the ocean salty?
2. NOAA Ocean Service: Why are rivers not as salty as the ocean?
Cover: Barbara Taylor, NOAA/NMFS/SWFSC / NOAA Photo Library. CC BY 2.0. Resized and cropped or letterboxed to a 16:9 cover. Image source · Usage / license