Salt gets into rivers when material that can’t be dissolved is stripped away by erosion. This exposes new water soluble compounds to the water, where they dissolve into the water and are taken to the ocean.

Over millions of years erosion removes innumerable tons of material, essentially mining the subsurface soluble compounds and delivering them to the ocean. Once there, as you mention ,those salts remain in the ocean. On Earth, this process began billions of years ago and has been adding salt to the oceans ever since.

You can observe this happening in many rivers today. The Colorado River is a great one. If you measure is salinity at the headwaters (or heck, probably even the inlet of Lake Powell), and where it enters the Gulf of Mexico, you will observe an incredible increase in salt. There was an international treaty formed around the US delivering river water that is not too salty to grow crops in to Mexico. The US solved that problem by installing a desalination plant on the river!

However without that land based salt mining process, how salty would the oceans be? Lots of good clues in this thread, but I don’t think anyone has offered a definitive answer.

Sure, I get that, but without land for rivers to essentially mine salt from, the equation changes a lot. Underwater erosion is dramatically less destructive than above water erosion.

Earth’s oceans are in a steady state, where all the addition of salt by rivers is balanced by loss of salt in the ocean. If you removed all the rivers from the equation, Earth’s oceans would find a new balance at a point significantly less salty than they are currently. Though I have little idea if that would be something we consider freshwater, or just “less salty” saltwater.

Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist. That’s the opposite of erosion.

Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it’s pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.

Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist.

Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it’s pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.

I feel like I’d rather drink saltwater before I drink water with ammonia dissolved in it.

Sure but once a continental plate is flooded, isn’t it by definition an oceanic plate at that point? A continent only exists if it isn’t flooded.

Thanks for all the detail! Your observation about comets is really pertinent. Saltwater is probably itself a purer form of water than comets. Maybe an ocean planet is actually more like a muddy swamp of nasty dirty water than a lake.

Exactly. If a planet ever had a salty ocean, adding more water probably wouldn’t dilute it in any meaningful way, so it would need to be a planet that never had continents.

I was trying to figure out how much underwater erosion there is but if you compare the sandy and silty bottom of the ocean to like, Utah, it seems like continental erosion is orders of magnitude more significant.

Conversely, we know oceans deposit all sorts of stuff at their bottoms, which makes me think there is a small amount of salt being deposited. Would that cancel out significant underwater erosion?

Similarly, if underwater erosion was a big deal, wouldn’t old lakes (in geological time) be notably saltier than young lakes? But the only salty lakes we have primarily lose all their water through evaporation, basically ultra concentrated river water.

And if you like Travelers, make sure to watch Sarah Conors Chronicle

Check out Travelers. They start out with a lot of agency but stuff gets wrong and it’s more and more desperate.