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For my current project, I think of making one of the planets have oceans that are on average 10km deep with some regions even being as deep as 40km.

The planet is roughly Earth-sized, maybe a little larger or denser to keep gravity the same, but has slightly less landmasses than Earth, with around 80% of the planet being covered in water.

What effects would this have on climate or oceanic life?

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    $\begingroup$ An ocean 3 times as deep or deeper than Challenger's Deep can pulverize human bone. Boneless creatures are more viable, and will be specialized for the deep. $\endgroup$
    – Zautech
    Jan 24 at 18:01
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    $\begingroup$ This is a very broad question. Are you asking how it would affect currents? How it would affect wind prevailing westerlies? How it would affect the viability of land-based plant life? Whether or not it would make a difference to creatures living in the top 50 meters? What creatures might exist at the bottom? VTC:Needs More Focus. Please be very specific about what help you need. If you need to ask multiple questions, please do. I'll retract once you've edited your post. (*Continued*) $\endgroup$
    – JBH
    Jan 24 at 18:11
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    $\begingroup$ Yeah, sorry for the broad question. I'm still very early in developing the planet, so I guess my question intended as more of a "What should I account for when writing about life on this world to avoid any egregious plot holes". I was primarily going to work backwards from whatever issues people thought could potentially arise and flesh out the actual geographies and cultures with that in mind. $\endgroup$
    – Doguskang
    Jan 24 at 18:58
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    $\begingroup$ At these depths It may be likely pressure isn't the worst problem. deep ocean pressures do affect basic chemical reactions within an organisms biochemistry. Enzymes and proteins begin reacting differently at depth. Even the physical shape of a water molecule changes. I would like to know at what point would the chemistry allowing life as we know it to function, would stop? $\endgroup$
    – Gillgamesh
    Jan 24 at 19:20
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    $\begingroup$ Also Doguskang, you never specified if the ocean would reach 40km depths in normal areas or in trenches. Can you please confirm? $\endgroup$ Jan 24 at 20:42

2 Answers 2

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First of all, with oceans at 40 km deep, you would run into the Mohorovičić discontinuity which is the location/zone that that the mantle begins on Earth. This boundary on Earth is around 10-15 km under the ocean and on average the ocean is 3-5km and the deepest point on Earth is around 10km. You said that your planet is about Earth sized, which means that the Mohorovičić discontinuity would be around the same location. (I am assuming that the sea level is the same) Your oceans would literally run straight into the mantle. This is the major problem with very deep oceans. The way to fix this is to make your planet larger but this would have major consequences on the gravity of your planet.

The increased pressure in that deep of an ocean might mean that organisms would be more adapted to the pressure zones. These organisms might form additional climate zones on your planet like the ocean zones on our planet.(epipelagic, mesopelagic, bathypelagic, abyssopelagic, and the hadal zone) Boneless creatures would be most viable as such pressures would pulverize almost all types of bone. However at more mild depths similar to those on earth, I imagine that life would be relatively similar.

Additionally, the oceans on earth act as heat sinks, absorbing and storing solar radiation. With deeper oceans, the heat absorption capacity would increase, potentially leading to more stable and moderate temperatures on your planet. For land animals this might mean that they are more sensitive to temperature change due to more tightly regulated temperature.

For the affects on land, saltwater intrusions into your continents might occur but this largely depends on the type of rock your continents are. These intrusions might cause larger amounts of saltwater in your bodies of water. In addition the intrusions might form large quantities of subsoil saltwater brine under the surface of your continents.

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – HDE 226868
    Jan 24 at 21:40
  • $\begingroup$ Do we know enough about planet structure to conclude that any roughly earth like planet has a Moho discontinuity around 10-15 km from the surface? Maybe other planets can have it much deeper or also much shallower? $\endgroup$
    – quarague
    Jan 25 at 8:46
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Not sure on the overall effects, but at the depths you are discussing the bottom of your ocean could be solid ice. The pressure at depth of 40km of pure water is around ~400MPa, depending on mineral contents it would be a few percent higher in an ocean. On earth at depth the ocean is pretty close to 0 degrees C. This puts you right around the solid phase boundary of some exotic ice phases. The exotic ice phases are also denser than water so these ices (Ice III or V most likely) would sink instead of float, and would form a solid bottom to your ocean. So unless there is geothermal activity, past a certain depth it wouldn't be an ocean anymore, but rather an undersea glacier.

water phase diagram

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