Oxygen, mass extinctions, what?

[Naraoia]

This came in the 11th September ScienceDaily newsletter:

Oxygen theory of mass extinction questioned by new research findings

“By performing experimental burns using pine wood, moss, matches, paper and a candle at 20°C in varying ranges of oxygen concentrations and comparing these results to the occurrences of fossil charcoal throughout the Mesozoic (250-65 million years ago), we were able to identify that prolonged periods of low oxygen are unlikely to have occurred,” says Dr Claire Belcher from the School of Biology and Environmental Science, University College Dublin, the lead author of the report.


“Low oxygen atmospheres, less than 12%, are considered to be the primary driver of at least two of the ‘big five’ mass-extinction events,” explains Dr Belcher. “But our research findings question that hypothesis and highlight the need for more detailed studies of fossil charcoal across these mass extinction events.”

I have a few wee questions/problems wrt this:

(1) How does ambient temperature influence the results? Was 20°C a wise choice? I can't recall all the climate trends of the Mesozoic off hand but I'm quite sure many modern summer nights at my latitude are warmer than that.

(2) Where are the studies that talk about these oxygen levels <12%? I'm sure they exist if they are referred to by the authors but I can't seem to find a graph that shows such a low oxygen level for any point in the Phanerozoic.

(3) I'm not sure which extinction the second of the "at least two" is (assuming one is the Permo-Triassic affair), but if it's the Late Devonian (the Devonian has an oxygen dip in the graph I'm looking at) then that has frank all to do with Mesozoic charcoals.

EDIT: it was in the blurb, which I didn't read at first, and it's the Late Triassic extinction. Please kindly forget point (3).

(I was also wondering why paper, matches and candles? Moss and wood are kinda realistic, as would be ferns, random dry leaves/needles but I have no idea what insights a candle can give about wildfires )

In summary... what am I to think of these findings?

 

[juvenissun]

(2) Where are the studies that talk about these oxygen levels <12%? I'm sure they exist if they are referred to by the authors but I can't seem to find a graph that shows such a low oxygen level for any point in the Phanerozoic.

Robert A. Berner (2006) GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2. Geochimica et Cosmochimica Acta 70: 5653–5664

Sorry that not many comment I can make on this subject. Obviously the issue involves more than just burning plants. The concentrations of carbonates and sulfates should also be considered. The periods of significantly low oxygen (15% or so) include 400 Ma and 200 Ma. If you don't have an access to this article, please let me know how could I help.

Another article addressed a whole lot of details about the oxygen cycle. But it does not answer your question directly.

 

[ChordatesLegacy]

Not certain about this article, I am sure there is far more to it than can be explained in 500 words.

From my understanding of mass extinction events, the causes of such event seem to be multiple, i.e. mass extinctions seem to occur during times of several stress events, not one such as low oxygen levels.

 

[Naraoia]

Not certain about this article, I am sure there is far more to it than can be explained in 500 words.

From my understanding of mass extinction events, the causes of such event seem to be multiple, i.e. mass extinctions seem to occur during times of several stress events, not one such as low oxygen levels.

I'm inclined to agree about multiple causes... it's just how well this study (which doesn't seem to be out yet ) challenges the role of the oxygen factor.

 

[Naraoia]

Robert A. Berner (2006) GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2. Geochimica et Cosmochimica Acta 70: 5653&#8211;5664

Sorry that not many comment I can make on this subject. Obviously the issue involves more than just burning plants. The concentrations of carbonates and sulfates should also be considered. The periods of significantly low oxygen (15% or so) include 400 Ma and 200 Ma. If you don't have an access to this article, please let me know how could I help.

Another article addressed a whole lot of details about the oxygen cycle. But it does not answer your question directly.

Thanks! I have access to the article (gotta love university libraries ).

I quickly checked some of the O[sub]2[/sub] vs time graphs - from what little I understand it seems that you can get quite different numbers based on the assumptions and the data you put in, though the rises and drops seem to be mostly in the same places... Maybe I'll try to digest the actual text to see if they tell me which assumptions are the most reasonable and which data are the best. Though I'm afraid I don't understand most of the stuff behind these models.

---

On a slightly different note... this occurred to me a moment ago. The real question is not whether atmospheric oxygen ever fell below the minimum for wildfires in the Mesozoic. Isn't the more important question whether a relatively rapid drop from a higher level to something like 12% would be able to cause a mass extinction?

 

[juvenissun]

On a slightly different note... this occurred to me a moment ago. The real question is not whether atmospheric oxygen ever fell below the minimum for wildfires in the Mesozoic. Isn't the more important question whether a relatively rapid drop from a higher level to something like 12% would be able to cause a mass extinction?

I think you are talking about a much shorter time scale than the one suggested by the article. I don't know, but I guess a global fire could significantly reduce the oxygen level (and increase the CO2 level?) very quickly.

 

[Naraoia]

I think you are talking about a much shorter time scale than the one suggested by the article.

Maybe. I don't know how quickly oxygen level would have to drop to cause a mass extinction. Too quickly for most oxygen-breathing creatures to adapt, but I don't know how quick that would be.

I don't know, but I guess a global fire could significantly reduce the oxygen level (and increase the CO2 level?) very quickly.

Global fire? Sounds a bit... unlikely.

 

[juvenissun]

Maybe. I don't know how quickly oxygen level would have to drop to cause a mass extinction. Too quickly for most oxygen-breathing creatures to adapt, but I don't know how quick that would be. Global fire? Sounds a bit... unlikely.

Yes, it is possible. Imagine a big comet/meteorite hit, plus volcanic eruptions. People suggested that was what happened in Permian extinction.

 

[Naraoia]

Yes, it is possible. Imagine a big comet/meteorite hit, plus volcanic eruptions. People suggested that was what happened in Permian extinction.

You are right, thanks for reminding. I can imagine these things causing massive, if not global, fires.

 

[ChordatesLegacy]

The one thing that occurs to me is this:

If you have a mass extinction event, particularly if it effects the photosynthesising flora (Plant life) you would expect a fall in O2 levels, for the simple reason that the atmospheric O2 levels are maintained by life, thus if primary producing decreases, the atmospheric O2 levels would follow.

Therefore lower O2 level could be an effect of mass extinctions, not the cause.

 

[Naraoia]

The one thing that occurs to me is this:

If you have a mass extinction event, particularly if it effects the photosynthesising flora (Plant life) you would expect a fall in O2 levels, for the simple reason that the atmospheric O2 levels are maintained by life, thus if primary producing decreases, the atmospheric O2 levels would follow.

Therefore lower O2 level could be an effect of mass extinctions, not the cause.

Good point. (Too bad I don't know anything about the mass extinctions of algae, plants & co. or the timing of these relative to the animal extinctions.)

Although, if animals start to die off, wouldn't the demand for atmospheric oxygen also decrease? OTOH, oxygen is reactive, so it would keep disappearing even if there's nobody to breathe it in. Plus, if species start to disappear that doesn't necessarily mean there are less individual oxygen consumers.

Ah, the amazing complexity of nature

 

[ChordatesLegacy]

Good point. (Too bad I don't know anything about the mass extinctions of algae, plants & co. or the timing of these relative to the animal extinctions.)

Although, if animals start to die off, wouldn't the demand for atmospheric oxygen also decrease? OTOH, oxygen is reactive, so it would keep disappearing even if there's nobody to breathe it in. Plus, if species start to disappear that doesn't necessarily mean there are less individual oxygen consumers.

Ah, the amazing complexity of nature

There is no real consensus of the causes of mass extinctions, causes range from super nova explosions, large igneous provinces, ocean anoxia, viruses and bacteria, meteorite impacts and probably a lot more. However, there does seem to be evidence that there needs to be more than one stress factor i.e. the KT boundary is associated with both the Mexican meteorite impact and the Deccan traps volcanism, whereas the PT Permian-Triassic mass extinction event occurs at the same time as the Siberian Trap volcanism, mass ocean anoxia and run away global warming.

So I think its far to say, a lot more research is needed and I believe being done as we speak.

Getting back to the point at hand, the larger mass extinction events did seem to affect the planktonic flora and fauna badly, if this is the case, oxygen production would have been affected badly, thus no replenishment of atmospheric oxygen. Even if the fauna was greatly reduces and thus the use of oxygen by said creatures reduced, the oxygen levels would still drop due to oxygen being very reactive, i.e. it reacts with weathered produces to produce insoluble oxides, which removes oxygen from the atmosphere.