Why so salty, bro?

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When the Earth was a stillborn baby (the baby is now 4.5 billion years old), there were a lot of active volcanoes. The emissions from these volcanoes included acidic gases like Hydrogen Chloride (HCl), Hydrogen Bromide (HBr), along with Carbon Dioxide (CO₂) and Water Vapor (H₂O). All of these gases condensed in the atmosphere and came down as acid rain.

Photo by samsommer on Unsplash

This rain collected in massive craters on Earth’s surface, eventually forming the primordial oceans we see today. But it wasn’t just water - this acidic rain aggressively eroded rocks, releasing ions like Sodium (Na), Calcium (Ca²), Potassium (K), and more. These dissolved ions settled in the early oceans, beginning the long process of making the oceans salty.

Fast forward a few billion years: acidic rain continues to fall, though far less intense than before. As rainwater flows over rocks and soil, it dissolves minerals, which then get carried by rivers into the oceans. Here’s the kickerwhen ocean water evaporates, the salts stay behind. 

So with every drop that leaves the ocean as vapor, a little more salt gets left behind. Year after year, this slow but steady process makes the ocean just a bit saltier.

Ocean	Average Salinity (PSU)
Atlantic Ocean	35-37 PSU
Pacific Ocean	33-35 PSU
Indian Ocean	34-36 PSU
Southern Ocean	33-34.5 PSU
Arctic Ocean	30-33 PSU
Global Average	35 PSU

Now, while you can’t really tell how salty someone is just because they didn’t like your Instagram post, scientists can measure the saltiness of the ocean. It’s called salinity, and it’s measured in Practical Salinity Units (PSU). The higher the PSU, the saltier the water.