Why is sea water salty, and not lake water?
Why is the sea water salty, and not the water of the big lakes? Is the salt concentration changing over time?
It is thought that the salt in the oceans stems from erosion of bedrock near ocean basins; big lakes and rivers are formed by evaporation and subsequent precipitation/runoff of ocean water, and by melting of freshwater ice. Since evaporated water can't "take the salt with it", even big lakes are generally made of freshwater, whereas the oceans are made of saltwater.
People don't really know what happens to the salt concentration (or salinity) of the oceans over time; Accurate salinity maps of oceans are hard to produce since oceans are so big. As the Earth's temperature rises in the future (both from natural and human effects) - two things will happen to the salinity. First, increased evaporation over oceans will tend to make the salinity rise. Second, increased melting at the poles will bring more freshwater into the oceans, which will decrease the salinity. Which of these two effects dominates the water cycle in the future will determine the change in salt concentration.
For more information about the salinity data that is available (in North America), check out the National Oceanographic Data Center's database.
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