What could lead a large ocean on a planet's surface to recede completely only to reappear later on in a cyclical process?

Question

I don't just mean tides or large-scale floods and droughts but a premise where one or more large bodies of water (or even every such body if that makes it easier) disappear, only to reappear one or two millenia later.

My intuition tells me this would logically happen either through a form of cyclical geological porousness that leads water to sink under the planet's surface or some kind of evaporation mechanic, but that assumes the water has to go somewhere. Perhaps large volcanoes that then act as sinkholes? Wormholes that suck out the water? Saturnine water rings?

Answer 1

Ice ages combined with a Mediterranean type basin

Your planet has the following features:

• a more extreme climate than Earth, perhaps because it has very high obliquity or perhaps because it is an eyeball planet. Because of this it has very large ice sheets at the poles (or dark side) and a region at the equator (or sunlit side) that's so hot that ocean water readily evaporates, leaving salty desert or a body of salty water like the Dead Sea only much bigger. It might have less water than Earth, to facilitate these features.

• a strong seasonal cycle that lasts several thousand years. (I'm unsure of the orbital mechanics needed to achieve that; it could be worth an extra question if there isn't one already.)

• a particular quirk of topography, described below.

Your civilisation lives in and around a basin in the temperate zone, somewhat similar to the Mediterranean basin on Earth. The basin has an opening where the water can flow out into the hot region. But this opening is far to the North (or South or towards the dark side, wherever it's cold), so the water has to flow North and then turn around and flow South again, until it eventually reaches the hot region. The basin floor slopes downward slightly towards the North, and the opening is deep enough that the entire basin can drain out through it.

But because of the seasonal cycles, this opening is blocked with ice most of the time. As the planet slowly starts to get warmer, the ice sheets start to melt, and this fills up the basin. (So this will be a freshwater ocean.) In the hottest years of the cycle, so much ice melts that the opening strait becomes unblocked, and at that point the entire basin suddenly drains. Once that has happened the water will flow to the hot region and start to evaporate. The evaporation explains why the water can flow downhill instead of just meeting another ocean. The evaporated water will ultimately fall as snow on the ice sheets, replenishing them.

After this, the basin will remain dry for most of the thousands-of-years cycle, until the planet has cooled and heated up again and the melting ice refills the basin.

If desirable, the cycle could be shorter, say around 100 years or so, but the ice doesn't usually melt enough to drain the basin. This means that the oceans will fill up much quicker, since the ice will be melting again only 100 years after the draining, but it also means the draining will be much less predictable. A hundred year cycle could be achieved by having your planet orbit a brighter star than our Sun, at a larger distance.

Answer 2

Bridge of the gods landslides.

the bridge of the gods geologic formation in Oregon was a series of land slides that crossed the columbia river damming it for years, a lake would build up behind it. The dam created a natural bridge. that lake would eventually breach the natural dam again cutting off the the ability to travel. The lake was hundreds of feet deep, so a serious barrier. This even got incorporated into the local mythology. But best for your story this happened over and over again, several times in human history. the last one around 600 years ago.

Because of the local geology these huge landslides are common.

https://www.youtube.com/watch?app=desktop&v=Y-w65F_b91U

Answer 3

Oceans are interconnected, and form an equipotential surface for the planet's gravity field plus centripetal force, in order for the entire ocean to recede it's required to somehow move a part of the planet against gravity (up or down), then back. This takes too much energy to even bother with a realistic explanation. However, there is one force that's strong enough to perform this:

Incomplete black hole tidal locking

If there is a black hole somewhere nearby your planet, and your planet being decently close to a black hole to experience tidal disruption of several kilometers, while also being not totally tidally locked to face one side towards it, then your oceans might be able to recede then get back with a period of half a day. So you make the day last several millennia, relative to the black hole, while the warmth and light would come from another star located elsewhere (at a distance far greater than the planet's, making this binary system a separated one). Carefully selecting orbital parameters could net you a planet close enough to a black hole to have tides of several kilometers, lasting millennia because of leftover rotation relative to the black hole, yet day and night lasting about normal because of the distant star that's heating the planet from afar.

Answer 4

You could appeal to oscillations in the orbital eccentricity of your planet, due to gravitational perturbations from a giant planet or binary star. This could cause an extreme version of Earth's Milankovitch cycles.

See here for more details on extreme Milankovitch cycles: https://planetplanet.net/2014/10/08/real-life-sci-fi-worlds-3-the-oscillating-earth/

See here for more on Earth's orbital oscillations and "standard" Milankovitch cycles: https://planetplanet.net/2022/08/22/billion-year-evolution-of-the-solar-system-climate-forcing-and-orbital-chaos/

Answer 5

Ice ages are an example of this. Just with solid water instead of liquid water. Over the last couple million years, huge bodies of solid water formed in North America and Eurasia, dissipated, and reformed dozens of times.

Answer 6

Have a bit more tectonically active planet.

Take a body of water such as the Gulf of Mexico or the Mediterranean and have the crust rise and fall. A more active crust can raise or sink large chunks of the crust. For a small example, see how the Campi Flegrei caldera is rising and sinking over hundreds of years.

There are two ways to remove an ocean. One is to block off the water inlet such as the Strait of Gibraltar and it dries up. The other is to raise the land under the ocean so that it recedes hundreds of miles. For example, much of North America west of the Appalachian Mountains was under sea level for a long time. The evidence is both the limestone layers and the beach front sandstone layers. It is only recently (geologically speaking) that it became high mountains. The mantle under North America has pushed it up.

By having a more active planet, these changes can happen far faster than on Earth.

Answer 7

To make it possible through a geological cycle, you may use astronomical nutation. This is a change of the planet's axial tilt over a millenia. The axial tilt may change its orientation relative to the sun. On Earth we have polar ice caps right at the poles. The accumulation of polar ice caps can change the sea level only if the body of ice is located on land. Ice forming as it floats on the water will not change the water level until it is large enough to touch the sea floor. Put an ice cube in a cup of water and watch the cube melt. You will notice that the water level does not change. Ice forming on land (like on Antarctica) takes water from the ocean and the ocean level will go down. As the ice cap over the continent melts and water flows into the sea, the ocean level rises.

As the axial tilt changes, North and South poles can either be over a deep ocean or a continent. If the antipodal points (North and South) are both the same (either land or sea) then the cycle variation is amplified. If the antipodes are opposite (one land and one sea) the effects mostly cancel each other and the cycle variation is small.

Answer 8

If the core of your planet had cooled, there would no longer be a mechanism for creating mountains and valleys. Over the millennia, the taller parts would be worn down and the deeper parts would be filled in. Mars, for instance, would currently only have oceans about 300 meters deep.

At that point, it would be fairly easy to have a cycle of ice ages that dry out a basin and refill it. That, however, happens over a period of 10k years, which might be a bit long for your concept of "history."

Answer 9

Lake Megachad

Quoting from Wikipedia on conditions about 10,000 years ago:

This enlarged Lake Chad reached dimensions of 1,000 by 600 kilometres (620 mi × 370 mi) in north–south and east–west direction respectively, covering the Bodélé Depression and perhaps as much as 8% of the present-day Sahara desert.

Current day Lake Chad is significantly smaller than that. So to reiterate, real-world history supports massive inland seas forming and dissipating on timescales of 10,000 years.

Cyclically.

It looks like every ~20,000 years, the Sahara enters a ~6,000 year "wet" period, where monsoon rains basically transform it from desert to savanna. This is likely based on small, cyclical changes in Earth's orbit that change the amount of sunlight hitting the ocean, which impacts monsoon winds.

Thus, the major requirement for this cycle to produce large inland seas for your story would be: there's a endorheic basin for the monsoon rains to fill. As long as there is a depression with no outlet, the rains will do the work of creating your inland sea, and then the later heat will remove it.

Answer 10

A slow moving, massive object almost in orbit (small black hole), creating a massive tide at the poles, freezing the whole water into a huge mount everest sized mountain of ice, then once it departs (or the planet is detached from its orbit around it), the ice slowly melts and the oceans return.

Answer 11

There is lots of room in the atmosphere to hold all the water. The planet's orbit around the sun is elliptical, just enough to warm up the water into the atmosphere, then cool it back down into the oceans.

Or make the planet orbit a binary system (two Suns, one slightly larger), so it still can have normal seasons.