Lecture 10: Oxygen Regulation

Question 1

What experiment did Andy Watson conduct in 1978?

Answer 1

He wanted to determine the impact of different oxygen concentrations in the air and moisture contents of the paper he was burning on how likely that paper was to burn

Question 2

From Watson’s 1978 model, what was the minimum temperature that dry paper needed to combust?

Answer 2

18 Celsius

Question 3

What experiment did Belcher and McElwain (2008) conduct?

Answer 3

They determined how sensitive different materials were to combustion depending on different variables

Question 4

What is charcoal and therefore what does it serve to suggest?

Answer 4

Burnt wood - therefore a fire must have occurred in order for it to form

Question 5

When does charcoal deposit record extend back to? What were they found in? What is the significance?

Answer 5

~400Ma in sedimentary rock record. Oxygen concentration of the atmosphere must have stayed above the level it was then in order for it to keep forming through fires

Question 6

From the graphs produced by Belcher & McElwain (2008), combined with the knowledge that fires happened at least 400mya, what are we therefore able to say about the oxygen concentration of the atmosphere?

Answer 6

That it must have been a minimum of 15% as that is the minimum concentration at which fires start to happen. It is possible that the concentration was 17% but this is less likely.

Question 7

We know how oxygen is mostly produced and so what happens if the oxygen concentration of the atmosphere increases too much?

Answer 7

There will be too many fires probably which will burn vegetation and it will not be able to regenerate in time to keep producing oxygen and so eventually the oxygen will be all used up in fires and it will start to diminish.

Question 8

What would oxygen concentration be if it was to prevent the regeneration of forests? and why?

Answer 8

25-30% - because in this range the fires would have occurred very frequently

Question 9

Combining the knowledge that we know oxygen concentration has to be above 15% for fires and therefore charcoal to form with the maximum concentration it can be which is 30% then what can we say about the oxygen concentration of the earth around 400Ma? What does this suggest?

Answer 9

It needs to be between 15-30%. This suggests that there must be an oxygen regulator preventing it from going outside of this range

Question 10

What was known to have occurred around 350Ma? What does this mean?

Answer 10

Forests - this means that our estimation of the oxygen concentration needs to be a suitable level in order for them to exist (not too high so they do not get burnt but at a minimum so they can form in the first place)

Question 11

What happens to the phosphorus concentration of oceans as the oxygen concentration of atmosphere decreases?

Answer 11

Oxygen reductions mean it becomes more anoxic which means it starts to expand. This expansion leads to increased weathering of phosphorus from marine sediments which leads to slight increase of concentration in oceans.

Question 12

What effect does the increased concentration of phosphorus have on the role the ocean plays in regulating oxygen concentration of atmosphere?

Answer 12

The increased phosphorus increases biosphere productivity in oceans which means more organic carbon is buried in to the ocean floor when organisms decompose. This prevents it from interacting with atmosphere and so the oxygen concentration of the atmosphere starts to replenish

Question 13

Describe the feedback within the process of how oceans regulate oxygen concentration of the atmosphere?

Answer 13

Negative - reduced atmospheric oxygen eventually leads to its replenishment much later on

Question 14

What is the significance of the ocean regulatory process on oxygen concentration?

Answer 14

It shows how oxygen concentrations can be regulated without land plants being involved

Question 15

How does the land regulate oxygen atmospheric concentration?

Answer 15

Plants increase silicate rock weathering which means the access to nutrients they have increases meaning their productivity increases and there is more organic carbon buried following decomposition. Over time, this helps future plants with their productivity meaning that the photosynthesis process becomes more effective and more oxygen is produced

Question 16

What factor can plants speed up the weathering process?

Answer 16

~10

Question 17

What statistics demonstrates how efficient the land is at regulating oxygen concentrations of the atmosphere?

Answer 17

They only take up to 30% of the land surface but produce as much oxygen as the oceans as well as transporting the same amount underground

Question 18

What would happen to the oxygen concentration of the atmosphere if the long-term oxygen balance solely relied upon the ocean-based regulatory process? What needs to exist?

Answer 18

The oxygen concentration would rise sky-high to dangerous concentrations. We need a negative feedback loop whereby increased oxygen concentration leads to a reduction in its production by land-based plants.

Question 19

What is the land-based feedback that is so crucial for limiting the atmospheric oxygen concentration and preventing it from reaching sky-high amount?

Answer 19

If oxygen concentration becomes too high then rubisco plants can taken in its molecules, this leads to inefficiency in the photosynthesis process because it promotes photorespiration. This inefficiency in photosynthesis means that less oxygen is produced.

Question 20

What do rubisco normally do in the plant and why are they able to take in oxygen molecules if the concentration gets too high?

Answer 20

They normally fix CO2 for photosynthesis, but if oxygen concentration gets too high then they outweigh CO2 and because they are a similar sized molecule, the rubisco are able to take them in to the plant

Question 21

What is the other land-based negative feedback that limits oxygen production?

Answer 21

If the oxygen concentration gets too high then it can increase the frequency of fires which burns vegetation and obviously prevents it from producing oxygen.

Question 22

What is the other way that fires lead to the regulation of oxygen?

Answer 22

They produce phosphorus-rich ash that is very soluble and able to be washed to the ocean. If phosphorus is buried in the ocean it does not bury as much carbon at the same time compared to if phosphorus was buried on land. This means there is more carbon present in the atmosphere to react with oxygen to from CO2 meaning in turn that there is a reduction in the amount of single oxygen molecules.

Question 23

How much carbon is buried when phosphorus is in the oceans and on the land?

Answer 23

Oceans = Carbon molecules for every phosphorus is ~100
Land = ~1000

Question 24

What was Tim Lenton’s study in 2001?

Answer 24

He inputted the different oxygen regulating feedback in to the model created by Berner & Cranfield (1989) to see how oxygen concentrations would have changed over time

Question 25

What was the conclusion of Lenton’s 2001 study?

Answer 25

That oxygen concentrations must have ranged between 15 and 25%

Question 26

What event happened between 400-500ma and why?

Answer 26

there was a massive rise in oxygen concentration - because this is when plants started to colonise the land and therefore produce oxygen in abundant amounts

Question 27

Does Tim Lenton think that even the smallest plants at ~400Ma could have caused oxygen concentrations to increase to a degree that charcoal could form? What evidence is there for this?

Answer 27

Yes. The presence of charcoal dated back to the same time

Question 28

What is thought to have assisted to the formation of charcoal ~400ma and why?

Answer 28

the continents were generally decreasing in altitude which would have allowed sediments to accumulate in wet swampy conditions that are ideal for their decomposition in to charcoal.

Question 29

What do Scott & Glasspool estimate the oxygen concentration of the atmosphere could have risen to following the early forms of charcoal?

Answer 29

~35%

Question 30

How would have the estimated increase of oxygen concentrations to 35% have affected insects and why?

Answer 30

there are many which are oxygen diffusers which would have promoted their growth

Question 31

What evidence is there for dramatic insect growth following the massive 35% oxygen concentration?

Answer 31

There was the discovery of a fossil dragonfly with a wingspan of ~70cm.

Question 32

What was the problem with the theory consisting of how insects could have grown to such a massive scale following a rise in oxygen concentration?

Answer 32

Oxygen diffusion insects scale linearly with oxygen concentration. An increase of the oxygen concentration was not proportional to the amount the dragonfly increased

Question 33

What evidence is there for oxygen concentration levels during the Mesozoic era?

Answer 33

Oxygen concentration cannot fall below 15% for charcoal which was found and dated back to the Mesozoic era. Therefore oxygen levels must have been similar to what they are today. They must have been slightly higher in order to promote the increased growth of massive insects and dinosaurs, but have since then decreased to allow modern sized animals to form.

Question 34

What idea did Falkowksi et al. (2005) propose?

Answer 34

Rising CO2 levels triggered the evolution of placental mammals after the fall of the dinosaurs.

Question 35

How was Falkowski et al’s. 2005 theory proved false?

Answer 35

We know that Oxygen was higher than modern day, indicated by presence of charcoal volumes. Of which at these oxygen levels it would not be suitable for the sort of life that they proposed in the paper

Question 36

What level of oxygen concentration does the brain start to loose a lot of functionality? How do we know this?

Answer 36

15% - experiments at high altitude and biosphere 2.

Question 37

Because we know oxygen concentration has not fallen below 15% since ~400Ma (because of charcoal record) what conclusion do we reach?

Answer 37

Oxygen concentration has been at a level since ~400Ma that would not limit the evolution of life and so an insufficient level of oxygen cannot be blamed for the limited evolution of intelligent life (humans). There must have been another factor limiting their evolution