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So, Here's the scenario:

we start with an ocean planet, with minimal dry land. The planet stays this way for at least 400 million years. (Notably, life evolves during this time period.) Then, a magic portal opens at the bottom of the sea, and drains a large portion of the water into space. The portal closes when the land-to-sea ratio of this planet is roughly the same as Earth's.

I'd like to have another magic portal open at this point so that Earth land life can colonize the new land. However, as I learned from my last post on the subject, the land would likely be far too salty for plants to establish themselves. (What would an until-recently waterworld look like?)

This question may also be relevant: (How to dry an ocean planet and turn it into a salty desert?)

so... what natural event could reduce the salt levels on the land such that plant life from earth could establish itself? If that natural event is simply a lot of time, how much time?

Bonus question: after all this is said and done, how would the land on this planet differ from the land on earth? at what point in their technological history would humans, if they came over with the other animals during the stone age, be able to tell that the planet was submerged for most of it's history?

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  • $\begingroup$ One way of doing it: Oblivion $\endgroup$
    – Alexander
    Commented Jan 29, 2021 at 20:00
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    $\begingroup$ Ugh. A large part of Europe (southern Spain, most of Italy, most of Greece and the Balkans, almost all of Hungary, part of Romania, southern Ukraine etc.) was under the sea (the Thetys Ocean, the Parathetys Sea, the Pannonian Sea) until 5 to 10 million years ago. Today those places have lofty mountains, rolling hills, and fertile plains. People realized that they are walking on what was once the bottom of the sea only towards the end of the 19th century. $\endgroup$
    – AlexP
    Commented Jan 29, 2021 at 20:20
  • $\begingroup$ Just follow the links in my comment and see where those sea were, and when they dried out. Or find a map of Europe in the Miocene period. $\endgroup$
    – AlexP
    Commented Jan 29, 2021 at 20:46
  • $\begingroup$ Why does the ocean water have to be salty? It is salty on Earth because the Earth has a lot of salt to dissolve in the first place. Yet there are many very ancient lakes that are not at all salty. That's because they are in areas that are relatively salt-free. So just make your planet salt-free. $\endgroup$
    – Justin Thyme the Second
    Commented Jan 30, 2021 at 22:47

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Rain. Water that evaporates and precipitates is salt free. Most of Florida was under the sea just a few million years ago. Given the geologic time scale you've proposed, you really don't have a problem with this. The land won't be substantially different from some land on Earth. (You won't be winning any awards for your signature fjords though - no glaciers, no U-shaped valleys!)

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    $\begingroup$ I suspect the time needed to rinse the newly exposed surface of dissolved salts may be well outside what OP is hoping for as part of a colonization plan. $\endgroup$
    – rek
    Commented Jan 29, 2021 at 20:15
  • $\begingroup$ @rek not really. The draining of the oceans alone would take thousands of years, if Randal monroe is anything to go by (what-if.xkcd.com/53), and the timescale is already in the millions of years. I can wait a few million more for the salt to wash off. $\endgroup$
    – Globin347
    Commented Jan 29, 2021 at 20:35
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    $\begingroup$ I don't think it should take that long. Exposed rock would be cleansed almost instantly, but I've never read about any long delay even for the reclaimed lowlands of the Netherlands. On the other hand, some desert basins would never lose their salts, and become evaporite deposits just as on Earth. $\endgroup$
    – Mike Serfas
    Commented Jan 29, 2021 at 20:38
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    $\begingroup$ @Globin347 Your question says the portal with the animals etc opens immediately after the water drains. $\endgroup$
    – rek
    Commented Jan 29, 2021 at 20:54
  • $\begingroup$ @MikeSerfas With all respect to the Netherlands, we're talking about the sea floor here and areas the size of continents, and their reclaimed lands are part of a freshwater river system. $\endgroup$
    – rek
    Commented Jan 29, 2021 at 21:00
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Your world ocean will be freshwater.

https://oceanservice.noaa.gov/facts/whysalty.html#:~:text=Salt%20in%20the%20ocean%20comes,acidic%2C%20so%20it%20erodes%20rocks.

Salt in the ocean comes from two sources: runoff from the land and openings in the seafloor.

Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks. This releases ions that are carried away to streams and rivers that eventually feed into the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not removed, so their concentrations increase over time.

On our world there is a lot of land for water to run across, leaching the salts and carrying them to the ocean. In your world, there is just ocean. Rain lands on the water. Erosion does not happen in the still depths. Salts stay locked into their rocks. A little salt might come out of hydrothermal vents but this is trivial compared to what washes off the land.

Your oceans will be freshwater. Truly the lack of salt might be an issue for earth life colonizing this world.

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Your magical portals filter out fish, algae, seaweed, etc., but not salt or other dissolved minerals. Consequently the salt vanishes with the water. This produces lands much less salty than that caused by evaporation.

For residual salt, have it rain.

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  • $\begingroup$ Actually, it doesn't filter out anything. That means lots of fish go flying out into space. $\endgroup$
    – Globin347
    Commented Jan 29, 2021 at 23:40
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    $\begingroup$ Probably taking some salt with them. . . . $\endgroup$
    – Mary
    Commented Jan 29, 2021 at 23:41
  • $\begingroup$ The only salt to leave the planet will be with the water that goes down the drain. Water left in the retreating seabed will still be salty! $\endgroup$
    – elemtilas
    Commented Jan 31, 2021 at 4:15

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