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I’m working on a speculative evolution world on a Europa-like moon. It’s about 2/3rd the mass of earth orbiting a rogue Jovian planet about 5x larger than Jupiter. Around 400 million years after multicellular life appeared, the planet found itself in a stable orbit around a star in the habitable zone. The icy surface of the moon melted, releasing trapped gasses in the ice sheet and from the water, causing a worldwide deoxygenation, killing about 70-75% of all life. After this happened, a massive evolutionary rebound occurred to fill all the new niches. Many different clades of flora evolved to float on the surface (plants had already previously evolved to be disconnected from the ground) and evolved to attach to each other to make reproduction easier, over time evolving into a massive super organism the size of South America. Fauna would then evolve to live around it for food, with some evolving to escape onto the surface for a time, over time evolving lungs and living permanently on land.

One problem I’ve realized is that things will fall off the organism, sinking to the bottom of the ocean, which is 40 miles down. Over large spans of time, this will accumulate until it loses most of its resources.

One solution I’ve come up with for this is convection currents bringing sediment from the ocean floor to its surface. Hydrothermal vents heat up the water, which then rises to the top where is then taken in by fish and plants, where it can then be used.

Could convection currents believably draw mineral mass from the ocean bottom 40 miles deep to the surface?

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  • $\begingroup$ I'm not certain why they would need to. Can you clarify what is in the sediment that the super-plant might not be able to get elsewhere? Presumably not inorganic matter like quartz. $\endgroup$ Commented Feb 1 at 6:48
  • $\begingroup$ @Escapeddentalpatient. If something heavier than water sinks to the bottom, chemicals within that object are lost. This would include heavier elements like iron, magnesium, calcium, and many others from animals falling and sinking, and while some will be caught by fish living in the water, some will get to the bottom. Same thing with some chemicals, for example, quartz is a necessary component to some plants, as their photosynthetic organelles evolved from kinetosynthetic organelles, and integrate the quartz previously required for kinetosynthesis into photosynthesis. $\endgroup$ Commented Feb 1 at 7:04
  • $\begingroup$ What are those "convection currents" which are supposed to carry stuff 40 miles up? Here on Earth the temperature of the ocean water is basically constant below 1000 meters depth. $\endgroup$ Commented Feb 1 at 8:18
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    $\begingroup$ What is wrong with a classic necklace of fire tectonic caused ring of volcoes fertilizing the ocean? en.wikipedia.org/wiki/Pumice_raft $\endgroup$ Commented Feb 1 at 14:55
  • $\begingroup$ We could use some details. Is it enough to simply get the minerals to the surface (i.e., we're just mixing things up) or must they be deposited somewhere? How much mass do we need to lift? What minerals are we talking about? E.G., Lithium, atomic mass 3 vs. Osmium, atomic mass 92. It's a big deal to lift Osmium up 40 miles, even as a fine dust, while raw lithium would float on the water, being a lower density than water, so what we're lifting matters. What is the density of your oceans, anyway? $\endgroup$ Commented Feb 1 at 20:03

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Most nutrients get recycled before they reach the ground
The assumption that everything which sinks down the ocean is lost is mostly wrong. Search how the Marine Snow cycle behaves throughout the different sea levels, most of it gets recycled (eaten) before it reaches the ground. Thus the Marine Snow becomes less the deeper you are. Most nutrients stay in the upper layers of the sea.
So the mechanisms to bring nutrients from the bottom of the ocean up to the surface may not need to be as strong as you beleve it needs to be.

On earth this nutrient cycle involves convection currents, but I doubt that this is entirely possible on a Europa-like moon where no landmass reaches the surface.

Frame challenge: The cycle of life should be able to bring nutrients to the surface and keep most of it in the upper levels.
Luckily you already have an existing ecosystem on your moon which evolved without the need for sunlight and probably started on the ground level.

All you need are enough animals which gather nutrients from the ground and enough hunters which hunt in deeper levels and bring the nutrients up a little bit. Sooner or later those hunters will die as well and indirectly also turn into Marine Snow.

With enough animals hunting on deeper levels and enough animals eating the falling Marine Snow, it sounds belevably that animals should be able to bring the nutrients upwards, step by step.

It may take many millions of years until enough nutrients get distributed throughout the entire ocean and finally reach the surface, but I find it reasonable that plants and animals are able to do that, especially when there are good reasons to live in the upper layers.
One simple but good reason to populate yet unpopulated areas is because there is no one else who tries to eat you, thus it is reasonable that sooner or later the entire ocean gets populated (some levels more, some levels less).

Also dead fishes start to ascent after some time due to gases which form during decay, though in an active ecosystem the dead fishes usually get eaten before that can happen.
A mass extinction, as it happened on your moon, could be a good reason for why there were a lot dead fishes which floated up to the surface. This should be a good kickstart for your new generations of surface animals.

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Very fine grained sediment can travel long distances, see for example this paper: MacDonald, Lee & Coe, Drew. (2007). Influence of Headwater Streams on Downstream Reaches in Forested Areas. Forest Sciences. 53. 10.1093/forestscience/53.2.148.

There you can find this chart

chart showing traveled distance vs particle size

where you can see that particles the size of 50 micron can travel up to 100 km with the right conditions.

If you want a more striking evidence, look at this satellite image

satellite image showing muddy water carried by a river extending into the ocean

showing muddy water (in other words, water carrying very fine particles) reaching up to 60 km from the shore.

Long story short, if your convection current is energetic enough, it can carry matter very far.

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    $\begingroup$ The question is about "convection currents", whatever they may be, carrying sediment 40 miles up from the bottom of the ocean to the surface. Here on Earth oceanic ecosystems sometimes rely on upwelling to get nutrients from the deep-ish ocean to the surface, but I've never heard of anything travelling up to the surface from the really deep ocean floor, with the effect being seemingly limited to a maximum of less than 1 (one) mile of vertical distance and most often no deeper than 400 meters (1/4 of a mile). $\endgroup$ Commented Feb 1 at 8:17
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    $\begingroup$ @AlexP that's why I say "if the current is energetic enough". Reportedly bomber crews saw wooden beam carried up at their cruise level by the updraft when they were flying over the firestorms they had caused upon German and Japanese cities $\endgroup$ Commented Feb 1 at 8:22
  • $\begingroup$ @AlexP, There are a few things that prevent that. The first is that we don't have 40 mile deep oceans, of course, but also we have thermoclines that prevent a lot of mixing. What we don't have is a lot of volcanic activity in the deep ocean. Our oceans are shallow enough that most volcanic activity will create an island. With an ocean that's deeper than our tallest mountains, even the tallest continental plate would be submerged. That leaves plenty of opportunity for volcanic upwelling. $\endgroup$ Commented Feb 1 at 22:08
  • $\begingroup$ @RobertRapplean: Earth has a lot of submarine volcanoes; three quarters of the magma output on Earth come from submarine volcanoes. And we have a lot of ocean deeper than 1 mile, and yet no upwelling was ever observed carrying to the surface nutrients from more than 1 mile down, and even that is very exceptional. (The average depth of Earth's oceans is about 3,700 meters or 2.3 miles.) $\endgroup$ Commented Feb 1 at 22:45
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    $\begingroup$ @AlexP, I checked a few extra sources and concede the point. Volcanic heating doesn't make the water rise more than a kilometer, but does spread out a lot after that. The high pressure prevents density-based rising, keeping even super-critical water from expanding the way gasses do. That really surprised me. By the time it hits a kilometer, enough mixing has happened to even out the density. $\endgroup$ Commented Feb 2 at 0:18
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I had a quick look at the literature and it is split on this topic.

The obvious answer is that the water at the surface would be pure. Any ionic material that dissolves in water increases the density. This would mean any salts should sink under equilibrium. The only thing that would float is something that is lighter but does not mix with water The bottom of Earth's oceans are significantly saltier than the surface. This would suggest that the convection currents would have to be pretty strong to push salts up through 40 miles of water.

Nevertheless there are scientists who believe that such material may rise though these sort of depths of water, and want us to look at the geysers of Enceladus and other places to look for traces of them. We haven't been there and had a proper look, so the question seems to be open.

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