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So, as per my Oxygen levels and explosions question, there is a planet with substantially lower oxygen (16%) and a planet with higher than average oxygen (25%) that are fairly earthlike in temperature, locked in a binary orbit around a sun-like star, far enough away that it is possible to travel from planet to planet with 1960 era tech. That was in the past, and it is now near future/2150 tech, and a rebellion of succession from low-planet. However, their tech level is scattered and thus is their weapons, causing lower oxygen to be relevant.

In my original question, I stated that the plants were able to re-absorb oxygen and use it somehow to make energy. Alex-P pointed out that it is not likely for that to be possible.

In short, I need a low-oxygen planet with high plant life and low animals. What is a feasible or semi-feasible reason why the planet has such an oxygen level?

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Oxygen production and forest levels need not corelate!

Net oxygen production by forests on Earth is basically zero. Forests have microbes (and animals) which consume almost all the oxygen the forest creates.

(In case it needs to be said This is not a reason to cut the rainforests down)

So answers to your question:

  • The ocean is responsible for most of the oxygen we're breathing now. So your lower oxygen planet has lower algae concentrations in its ocean - for example.
  • The lithosphere (rocks) are also responsible for oxygen levels. Some ferrous carbonate rocks (siderite mineral), Sodium/Calcium/Potasium/Magnesium ascorbate, or sodium hydrogen carbonate rocks (nahcolite mineral) exposed to the atmosphere.

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Larger Oxygen Sinks, like dissolved iron in oceans

In earth's history, the amount of oxygen in the atmosphere has not been constant, in fact there was a mass extinction event that corresponded to the drastic increase in oxygen (which was toxic to many of the existing life forms at the time). During this period there were a few sources of oxygen, one of which was photosynthesis, and there were also consumers or sinks of oxygen. Eventually the sources and the sinks balanced out.

One of the most significant sinks was dissolved iron in the ocean, when the oxygen dissolved in the water it would react to oxidize the iron, which would then become insoluble, and sink to the ocean floor. If these deposits were a lot larger, the atmosphere's oxygen concentration would be lower.

There are more oxygen sinks like volcanic gasses and methane, any one of these could also work by making them much larger compared to earth.

Just by having larger sinks and decreasing oxygen concentration (and perhaps increasing CO2) would have the effect of increased plant life and decreased animal life, so that part is easily solved by the other.

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Plantlife may be the source of your planet's oxygen, but their production is only very loosely coupled to the total oxygen levels in the air.

For example, if all plantlife on Earth were to die, immediately, including all ocean plankton etc. Zero remaining oxygen producers. . . We would all die of old age before we ran out of oxygen to breathe. Assuming an infinite food supply, our great-great-great-great-great-great-great grandchildren (200 years) would notice the oxygen being about 5% lower than it is today.(this assumes we also keep all animal, insect and microbial oxygen consumers going, somehow.. if Only humans at present population?... about 250 thousand years))

The balance level is dictated (over very long timespans) by the rate at which oxygen is removed from the system, vs rate oxygen is added. Look to thinks like oxides of calcium and magnesium settling to the seabed, forming limestone deposits.

We are talking long periods in geological terms! Millions to hundreds of millions of years.

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