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From what I read they've been at 3.5% salt equilibrium for millions of years. How is this possible when rivers are constantly flowing dissolving salt into them?

Groveish
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Because salt gets buried over time. Most of the salt we mine is old seas / oceans that dried up and then got buried. Subduction zones carry salt-laden and water-laden sediments very deep underground. The water tends to escape by volcanism. The salt does not.

David Hammen
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  • So the ocean gets less deep over time? Because the ocean floor is getting higher with the salt buildup and formation of new surface – Groveish Jul 13 '22 at 22:41
  • @Groveish Google "subduction zone". The continents are in general much older than are the oceans. The oldest continental crust is over 3 billion years old. The oldest ocean crust is less than 200 million years old. – David Hammen Jul 14 '22 at 04:51