Your diagram leaves out minerals that get trapped in sediments and then swallowed in subduction zones. A prime example of the former is the massive salt deposits under the Mediterranean seafloor. They were form when tectonic movements closed the Strait of Gibraltar and the Mediterranean dried up. By the time it refilled, much of the salt was covered with non-salty sediments, so it could not be dissolved again. Because of plate tectonics, this pattern has been played out frequently in the Earth’s long history. And consequently, the ocean’s salinity has varied.



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