4 ms·

If by air you mean (di)oxygen, it kinda does come from the plants, but prehistoric ones rather than modern ones. E.g. see [1] "The growth of these forests remo

If by air you mean (di)oxygen, it kinda does come from the plants, but prehistoric ones rather than modern ones. E.g. see [1] "The growth of these forests removed huge amounts of carbon dioxide from the atmosphere, leading to a surplus of oxygen." [1] https://www.nationalgeographic.com/science/article/carboniferous https://www.nationalgeographic.com/science/article/carbonife...

pfdietz 5y ago
The timescale for O2 to be stripped from the atmosphere (by reaction with reduced chemicals exposed by erosion, weathering, and emitted from volcanoes) is only about 10 million years, so the current composition of the atmosphere is due to recent conditions, not ancient conditions. There is an open question of why atmospheric O2 has been relatively stable (varied by a factor of several) in the Phanerozoic. This may just be anthropic selection: if it ever fell too far, higher life would have been wiped out, and we wouldn't be here. This has implications for the existence of extraterrestrial intelligence: it might be that most life-bearing worlds suffocate themselves before ETs can evolve.
- bnegreve 5y ago
  Hum, I'd like to read more about this. I find hard to believe that the current atmospheric conditions are completely unrelated to the rise of oxygen attributed to photosynthesis, and that they can only be explained by anthropic selection (which is always a possible explanation anyway).
  - pfdietz 5y ago
    Oxygen has cycled many times through the crust and back out again since the GOE (or even since the Neoproterozoic Oxygenation Event) so I don't see why you think any sort of "memory" of those ancient events would remain in the system.
    - bnegreve 5y ago
      Ok, I'm not saying that the exact same molecules of O2 from the GOE were preserved until today, rather that the presence of O2 in the atmosphere is related to the emergence of plants and photosynthesis. This is what I meant by "O2 comes from prehistoric plants". For the record, the "memory" if you want to call it like this, would be the (unoxidized) carbon buried underground and/or segregated in the plants body.
      - pfdietz 5y ago
        There is unoxidized carbon buried underground, but the bulk of the Earth has vastly more reducing capacity than relatively piddling amounts of buried organic carbon. Fully oxidized, the Earth could consume orders of magnitude more oxygen than exists in the atmosphere. Simply oxidizing all the minerals in the crust down to a couple of hundred meters would use all the atmosphere's O2, and then there's thousands of kilometers of mantle loaded with ferrous iron and sulfides, and then the iron core.
        bnegreve 5y ago
        I see, it's actually a bit scary. But if oxidation doesn't happen fast enough (no matter why) the O2 surplus can still be explained somewhat indirectly by the emergence of photosynthesis billions years ago. Or maybe by something else. Anyway, I finally got your point, interesting.
        pfdietz 5y ago
        The atmosphere is like a leaky boat. Oxygen leaks out when rocks are eroded and weathered and when volcanoes release reduced gases, and burial of photosynthesized material pumps it back in. The troubling part is that there's no obvious reason why these two processes have to be balanced over periods much longer than the time constant of the system.
        emmelaich 5y ago

pfdietz 5y ago

The timescale for O2 to be stripped from the atmosphere (by reaction with reduced chemicals exposed by erosion, weathering, and emitted from volcanoes) is only about 10 million years, so the current composition of the atmosphere is due to recent conditions, not ancient conditions. There is an open question of why atmospheric O2 has been relatively stable (varied by a factor of several) in the Phanerozoic. This may just be anthropic selection: if it ever fell too far, higher life would have been wiped out, and we wouldn't be here. This has implications for the existence of extraterrestrial intelligence: it might be that most life-bearing worlds suffocate themselves before ETs can evolve.

bnegreve 5y ago
Hum, I'd like to read more about this. I find hard to believe that the current atmospheric conditions are completely unrelated to the rise of oxygen attributed to photosynthesis, and that they can only be explained by anthropic selection (which is always a possible explanation anyway).
- pfdietz 5y ago
  Oxygen has cycled many times through the crust and back out again since the GOE (or even since the Neoproterozoic Oxygenation Event) so I don't see why you think any sort of "memory" of those ancient events would remain in the system.
  - bnegreve 5y ago
    Ok, I'm not saying that the exact same molecules of O2 from the GOE were preserved until today, rather that the presence of O2 in the atmosphere is related to the emergence of plants and photosynthesis. This is what I meant by "O2 comes from prehistoric plants". For the record, the "memory" if you want to call it like this, would be the (unoxidized) carbon buried underground and/or segregated in the plants body.
    - pfdietz 5y ago
      There is unoxidized carbon buried underground, but the bulk of the Earth has vastly more reducing capacity than relatively piddling amounts of buried organic carbon. Fully oxidized, the Earth could consume orders of magnitude more oxygen than exists in the atmosphere. Simply oxidizing all the minerals in the crust down to a couple of hundred meters would use all the atmosphere's O2, and then there's thousands of kilometers of mantle loaded with ferrous iron and sulfides, and then the iron core.
      - bnegreve 5y ago
        I see, it's actually a bit scary. But if oxidation doesn't happen fast enough (no matter why) the O2 surplus can still be explained somewhat indirectly by the emergence of photosynthesis billions years ago. Or maybe by something else. Anyway, I finally got your point, interesting.
        pfdietz 5y ago
        The atmosphere is like a leaky boat. Oxygen leaks out when rocks are eroded and weathered and when volcanoes release reduced gases, and burial of photosynthesized material pumps it back in. The troubling part is that there's no obvious reason why these two processes have to be balanced over periods much longer than the time constant of the system.
        emmelaich 5y ago