Modelling the regulation of atmospheric oxygen

The Geocarb Model

The Planetary Energy Balance Equation

 

Frequently Asked Questions	Links to Answers
What is meant by negative feedback?	Go to answer
From the course so far, are there any examples of positive feedback?	Go to answer
Look at Figure 7.2. Is Holland’s model empirical or process-based?	Go to answer
Why is O2 input the key factor regulating atmospheric oxygen in Holland’s model?	Go to answer
Equilibrium between O2 input and output in Holland’s model is achieved at an atmospheric O2 level of 21%. Is there any significance to the fact that 21% happens to be the present-day level?	Go to answer
In apparent contradiction to Holland’s model, why is it thought that at 2.1 Ga phosphorous was not the limiting factor in regulating atmospheric O2 levels?	Go to answer
During the oxygen high at 300 Ma, when O2 concentration reached about 35% (ref. The Berner & Canfield model), how might Holland’s negative feedback mechanism based on phosphorous have worked?	Go to answer

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What the Geocarb Model estimates
Through the Phanaerozoic, the model tries to plot the changing levels of atmospheric CO2 by estimating the changing rates of sources & sinks of CO2.
Sources	Sinks
Tectonic outgassing	Silicate weathering (factoring in solar luminosity and the spread of land plants)
Corg erosion & weathering	Corg burial in the Permo-Carboniferous Continental uplift over the last 150 million years

  

What the GEOCARB Model measures
What	How
Volcanism	using hot spot frame of reference, & estimates of sea level
Weathering rates	using 87Sr/86Sr ratios
Carbon burial	using d13C
Weathering feedbacks	using estimates of effect of the appearance of land plants

 

How the GEOCARB Model's results are verified

Sensitivity analysis. To test the importance of different controls over CO2. The results of each analysis are compared to those produced by the model with all the processes factored in.

Comparison to independent evidence, including: Paleosols Stomatal index Palaeoclimate indicators Energy Balance model results d13C in organic and carbonate sediments

 

Features of the Model

Process-based. The processes controlling the level of atmospheric CO2 are integral to the model.

Divides the Phanaerozoic into 1-million year intervals.

Disequilibria are adjusted within each 1-million year interval.

 

Assumptions made by the Model

Simple equations can be used to express the changes over time in the factors controlling the carbon cycle.

Evidence of changing rates of the processes controlling CO2 sources & sinks can be related to CO2 levels estimated by other means.

Sources & sinks of CO2 balance overall in each 1-million year interval.

 

Processes controlling atmospheric CO2
Process	Comments
Volcanism, emitting CO2	Related to the rate of sea-floor spreading/subduction. Estimated using hot spots & sea level estimates.
Global weathering rates	Of silicate, organic and carbonate sediments. Silicate weathering rates are estimated using 87Sr/86Sr ratios. Organic carbon weathering increases atmospheric CO2. Carbonate weathering has no effect on atmospheric CO2 on short timescales. Silicate weathering reduces atmospheric CO2.
Weathering feedbacks	Emergence of vascular plants. Has increased weathering rates over the Phanaerozoic (est. 7x over the rate for bare ground, but poorly constrained).
Increasing solar luminosity	Estimated using the solar luminosity equation (AEL, Eq. 10.3, p. 122).
Carbon burial	Including organic and carbonate carbon sedimentation & burial rates. Estimated by measuring abundance of Corg & Ccarb in sedimentary rocks (AEL, Fig. 11.10, p. 151), and by using d13C.
Shift of carbonate precipitation from shallow seas to deep oceans	Evolution of carbonate secreting plankton in the Cretaceous ("deep water carbonate factory"). Speeds up the rate of return of carbon to the atmosphere (because carbonate sediments can be subducted).

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Some Questions to Enhance Understanding
On Atmosphere, Earth and Life, p. 119, what is meant by "a perfect radiating black body"?	A black-body source, also called an "ideal thermal source", is "a source of radiation with the property that it would absorb completely electromagnetic radiation of any wavelength." So, a black body absorbs all the radiation that strikes it (is black!) and re-radiates it again at a different wavelength.
Is there such a thing in Nature as "a perfect radiating black body"?	No.  Even a carbonaceous chondrite has an albedo – no rocky body is a perfect black body!
What is meant by the expression Te (Earth’s effective temperature)?	Te is the temperature at the surface of a black body in equilibrium. So Te for the Earth is the temperature the surface would have in the absence of an atmosphere.
What is the point of knowing Te for the Earth at any time in the geological past (i.e. why use the planetary energy balance equation)?	Knowing Te gives us a basis for comparison with values of Ts, actual surface temperature, estimated by other means. This in turn allows us to measure the greenhouse effect (Ts-Te). Knowing the scale of the greenhouse effect enables us to estimate the minimum level of atmospheric CO2.
How is the level of atmospheric CO2 estimated from knowing the magnitude of the greenhouse effect?	By using GCMs - general circulation models. S269 does not go into specifics!
CO2 is now just a trace gas in the atmosphere, having been removed progressively and stored in long-term sinks (such as carbonate rocks) in response to the increase in solar luminosity (S) over geological time. Will all the CO2 eventually be removed from the atmosphere?	No. Up to the present the rate of removal has exceeded the rate of return through weathering and volcanic processes. But eventually a thermodynamic equilibrium will be reached when the concentration in the atmosphere has fallen to the level where the rate of return is sufficient to maintain it at that level.
What, then, will eventually happen to global mean surface temperature (GMST)?	The Earth will lose the ability to respond through negative feedback to increasing solar luminosity. A runaway greenhouse effect will commence, with water vapour as the principal greenhouse gas as the oceans vaporize. A "steam Earth" would be similar to Venus, but not exactly the same, since Venus’s atmosphere is dominated by CO2.
In Kasting’s Energy Balance model, why are the Huronian and Late Precambrian ice ages important?	They constrain GMST (at 5-20° C) at specific times in the Proterozoic. This gives us a check on our GCM calculations for other times in the Cryptozoic, to see if they are reasonable (ref. Fig. 10.8).

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Frequently Asked Questions	Answers
What is meant by negative feedback?	Any mechanism whereby a change in one direction of a variable in a system causes further changes in other linked variables which reverse the direction of the original change. Return to question
From the course so far, are there any examples of positive feedback?	(1) in the Archaean, the reinforcing effect of increasing atmospheric oxygen, rising ozone levels in the stratosphere, better protection from UV radiation for plant life, and hence more oxygen; (2) the runaway greenhouse effect on Venus; (3) there are several others. Return to question
Look at Figure 7.2. Is Holland’s model empirical or process-based?	Empirical, because it does not explain why the O2 output and input curves have the shape that they do. It merely models the observed reality and uses it to make predictions about the effect of changes in O2 input. Return to question
Why is O2 input the key factor regulating atmospheric oxygen in Holland’s model?	There are only two variables in the model – O2 input and output. Since O2 output is nearly unchanging above 0.1% atmospheric O2, the other variable must be the key to any observed changes. Return to question
Equilibrium between O2 input and output in Holland’s model is achieved at an atmospheric O2 level of 21%. Is there any significance to the fact that 21% happens to be the present-day level?	Yes and No! The negative feedback based on phosphorous proposed by Holland should act to push deviations either way in O2 levels back towards equilibrium. Berner & Canfield’s results, Fig. 6.1, do show O2 levels fluctuating back and forth throughout the Phanerozoic. However, Fig. 6.1 also shows that O2 levels have only transiently been at 21% three times in the Phanerozoic before the present. The negative feedback mechanism cannot fine-tune the O2 level over geological timescales. Return to question
In apparent contradiction to Holland’s model, why is it thought that at 2.1 Ga phosphorous was not the limiting factor in regulating atmospheric O2 levels?	Inorganic sinks for O2 predominated. At about 2 Ga this restraint on the rise of atmospheric O2 levels was lifted and the phosphorous-based negative feedback mechanism came into play for the first time. Return to question
During the oxygen high at 300 Ma, when O2 concentration reached about 35% (ref. The Berner & Canfield model), how might Holland’s negative feedback mechanism based on phosphorous have worked?	High O2 levels Ù  High level of dissolved O2 in the oceans Ù  Increased oxidation of phosphorous to apatite & ferric phosphates Ù  Reduced C:P ratio (less P for biomass production) Ù   Reduced burial of organic carbon Ù  Reduced level of atmospheric O2. Return to question

 

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