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All water contains dissolved chemicals and minerals that are referred to as ”salts”. If that is true, then why is the ocean salty and not every other body of water on Earth? The answer simply lies in the concentration of salts in the water. For example, when 1 cubic foot of sea water evaporates it yields about 1 kg of salt, but 1 cubic foot of fresh water from Lake Michigan contains only one one-thousandth (0.001) of a kg of salt. If the salt in the sea could be removed and spread evenly over the Earth’s land surface it would form a layer more than 165 meters thick.
The biggest question isn’t why is the ocean salty, but why isn’t it fresh like the rivers that empty into it? Because the saltiness of the ocean is the result of several natural influences and processes, the salt load of the streams entering the ocean is just one of these factors. The oceans were only slightly salty in the early eons of Earth, but as rain has fallen and broken rocks and their minerals have been added to the oceans, they have grown more salty. The sun causes pure water to be evaporated from the oceans surface, leaving behind the salts. Since the ocean has a larger surface to lose water from the salts become more concentrated. The fresh water flowing into the oceans help them to not become even more salty.
Oceanographers report salinity (total salt content) and the concentrations of individual chemical constituents in sea water — chloride, sodium, or magnesium for example — in parts per thousand, for which the symbol o/oo is used. That is, a salinity of 35 o/oo means 35 pounds of salt per 1,000 pounds of sea water. Similarly, a sodium concentration of 10 o/oo means 10 pounds of sodium per 1,000 pounds of water. The salinity of ocean water varies. It is affected by such factors as melting of ice, inflow of river water, evaporation, rain, snowfall, wind, wave motion, and ocean currents that cause horizontal and vertical mixing of the saltwater. The saltiest water (40 o/oo ) occurs in the Red Sea and the Persian Gulf, where rates of evaporation are very high. Of the major oceans, the North Atlantic is the saltiest; its salinity averages about 37.9 o/oo. Within the North Atlantic, the saltiest part is the Sargasso Sea, an area of about 2 million square miles, located about 2,000 miles west of the Canary Islands. The Sargasso Sea is set apart from the open ocean by floating brown seaweed “sargassum” from which the sea gets its name. The saltiness of this sea is due in part to the high water temperature (up to 83º F) causing a high rate of evaporation and in part to its remoteness from land; because it is so far from land, it receives no fresh-water inflow.
It is easy to answer: ”Why is the ocean salty?”, but that leads to several other questions that more complex. There are many good facts on palomar.edu. Here on Universe Today there is a nice article on the human impact on our oceans and another on the possibility that there are oceans on Neptune.