Why is the ocean salty?

Rain is slightly acidic from dissolved CO₂, so when it falls on land it erodes minerals — particularly sodium and chloride — from rocks and soil. Rivers carry these dissolved salts into the ocean, depositing roughly 3.86 billion tonnes per year. When seawater evaporates, pure water leaves but the salt stays, slowly concentrating over the past 3.8 billion years. Hydrothermal vents on the ocean floor also contribute minerals. Today the ocean averages 3.5% salinity — about 35 grams of salt per liter. The Dead Sea reaches 34.2% salinity, nearly ten times the ocean average.