Why is the sea salty when the rivers feeding it are not?

Rivers do contain salt, but only a little. They are flushed constantly by fresh rain and flow onward to the sea. The ocean is the end point: it receives the dissolved minerals from all those rivers and keeps them, while evaporation removes water and leaves the salts behind. Over a very long time this has made seawater far saltier than any river. [1] [2]

Rain absorbs carbon dioxide from the air and becomes slightly acidic because of carbonic acid. When it falls on land, it physically erodes rock and chemically breaks it down, dissolving small amounts of salts and minerals as ions. Runoff carries these into streams and rivers and then to the sea. The Conversation adds that rain also contains sulphur dioxide, and that sodium and chloride are among the salts dissolved from rock. [3] [4] [5]

Rivers and lakes are replenished often by new rain, and most have an outlet, so dissolved minerals do not build up to high concentrations. The ocean is the basin that catches the output of the world's rivers. When seawater evaporates as part of the water cycle, the salts stay behind. NOAA estimates that rivers carry about four billion tons of dissolved salts to the ocean each year. [2] [6] [1]

The difference in concentration is large. The Conversation gives about 35 grams of dissolved salts per litre of seawater, against about 0.5 grams per litre of fresh river water, roughly 300 times more in the sea. The USGS puts ocean salinity at about 3.5 percent by weight, with sodium and chloride making up over 90 percent of the dissolved ions. [5] [3]

Rivers are not the only supplier. Seawater seeps into the oceanic crust, is heated, dissolves minerals from the rock and returns through hydrothermal vents. The USGS says the whole volume of the oceans could circulate through such vents every 10 to 20 million years, though some salts also react with the rock and are removed. Underwater volcanic eruptions add minerals in a similar way. [3] [4] [6]

Marine organisms remove many substances, such as iron, zinc and copper. Sodium and chloride are not used up in this way, so they accumulated over time. The early seas were probably only slightly salty. [4] [1]

NOAA says about the same tonnage of salt probably settles out as sediment on the seafloor as arrives, so the ocean today probably has a balanced salt input and output. The Conversation makes the same point, noting that algae and animals also remove some salt. Salinity still varies by place: it is higher in warm regions with more evaporation and lower near the poles, where melting ice dilutes the water. [1] [5]