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I am concerned that the centrifugal force will draw out all the dissolved gases from the ocean in a colossal O'Neill Cylinder in the same way blood centrifuges do in lab. Fishes and most marine life would die from hypoxemia and laying gas pipes stretching thousands of miles may not be a cost effective solution.

First thing first, is this a baseless concern, and the effect would not be sufficient to harm the ecosystem, or would the threat come in another form which would be much more severe?

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    $\begingroup$ It seems to me that in a typical size O'Neil cylinder bodies of water would be ponds and lakes, not ocean sized. Exactly how large do you want those O'Neil cylinders to be and how structurally strong would they be? Your reference to "oceans" makes it seem possible that that you desire cylinders too large to hold together with present day materials. $\endgroup$ – M. A. Golding Jul 21 '20 at 16:18
  • $\begingroup$ O'Neil cylinder is only 8 km in diameter - not enough to support an ocean. Maybe you mean "O'Neil cylinder" in more general sense (like McKendree cylinder) to suit your scale? $\endgroup$ – Alexander Jul 21 '20 at 16:34
  • $\begingroup$ Assuming that the outer level is 1g, and that the bottom of the "ocean" is 1.5g, then no. Centrifuges produce a much higher gravitational gradient. $\endgroup$ – NomadMaker Jul 22 '20 at 0:30
  • $\begingroup$ most of your oxygen is going to come from marine algae just like earth. $\endgroup$ – John Jul 22 '20 at 12:44
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Centrifuges that separate blood spin at a high RPM simulating between 500 and 2000 times Earth gravity. So with your O'Neill Cylinders spinning at ~1G you will still have this problem (when it comes to important minerals settling), but WAY slower.

On Earth, collating of the Ocean is prevented by tidal forces caused by the sun and moon. These forces help keep water churning in a way that it both absorbs and distributes atmospheric oxygen and kicks up minerals for plankton to consume.

The issue with O'Neill Cylinders comes into play when they are tidally locked to thier star and not in orbit of a planet. In this case, there is no significant forces causing still bodies of water to churn and you would still get the hypoxia die off and mineral deprivation you are worrying about, just much slower than you would in a centrifuge, and for slightly different reasons. That said, this can easily be solved for through a variety of means including something as simple as using a staggered re-spin cycle

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  • $\begingroup$ Or have fans and pumps working underewater for artificial currents and waves (+1). $\endgroup$ – The Square-Cube Law Jul 21 '20 at 13:26
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    $\begingroup$ @Renan Yes, that was my first thought too but artificial currents at that scale are enormously expensive. If you need fish that badly, and you don't have any planets you could use, then from an engineering perspective if makes much more sense to just farm them in smaller more manageable aquarium tanks than naturalistic oceans. $\endgroup$ – Nosajimiki Jul 21 '20 at 13:42
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    $\begingroup$ I think if you have the resources to build an O'Neill cylinder, artificial currents would be cheap by comparison. $\endgroup$ – The Square-Cube Law Jul 21 '20 at 13:43
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    $\begingroup$ I'd think the cylinder's orbit around the sun would induce tides on the cylinder's oceans. $\endgroup$ – Daron Jul 21 '20 at 14:25
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    $\begingroup$ @Nosajimiki That's not how cost amortization works. It's just an accounting trick. In reality, you still need to pay the entire cost up-front. The tax office may let you spread the cost of building the cylinder over 10000 years, but unless you actually take 10000 years to build it, you still need to buy all the materials first. $\endgroup$ – user253751 Jul 21 '20 at 21:32
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Yes you are being paranoid. Are you worried about gravity doing the same thing on Earth? The O'Neil cylinder shouldn't be spinning fast enough to produce a force any greater since it is just trying to simulate 1G after all.

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  • $\begingroup$ Having a moon rather helps earth's oceans in this respect. $\endgroup$ – Russell McMahon Jul 22 '20 at 12:04
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The issue here is similar to a large fish tank. The fish tank is basically still water and will eventually be deprived of oxygen throughout the tank. This is due to the surface conditions more than the pull of gravity. In small tanks, this is alleviated with small bubbles that forced from an air pump. In larger tanks, you will usually see some type of waterfall that drops into the water and drags new oxygen into the tank water with the plummeting water using a water pump. The speed this happens depends on surface area to depth ratio, wind, sea life, and atmospheric oxygen levels (and likely many others).

You could have waterfalls in parks for people where the now highly oxygenated water would flow into the sea or recycle the air by bubbling it through the water.

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