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All water on the Earth contains some amount of salt, measured in parts per thousand parts (ppt). Ocean water usually contains about 35 ppt salinity, which is dissolved salts from mineral deposits. It is likely that while the planet was developing, the water in the oceans was not as salty as it is today; the water that collected from planetary impacts settled into the depressions on the planet, and absorbed soluble elements. Over time, the salinity of the largest bodies of water increased. River water collects salts from its riverbed and brings that salt to the ocean, while underwater earthquakes and other shifting in the rocks releases soluble elements, which dissolve into the ocean water. Rainfall and glaciers melting both are fresh, but can cause erosion of ocean shores into the water; soluble elements dissolve, while insoluble elements fall to the bottom of the ocean to become part of the ocean floor. The current salinity of the ocean is at a general equilibrium, affected by evaporation, rainfall, river flow, and other factors, but it grew to those levels of saline with time as the settling water absorbed more and more of the available soluble elements.
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