The Carbon Conundrum

Dunkley Jones, T. et al. 2010 submitted a review of recent work into the PETM in an attempt to identify key components that contributed to the sudden warming, as well as to try and relate it to our modern world. 

The paper raises a few key issues, one that I will bring up today - the Carbon Conundrum. The amount of CO₂ released into the environment over the PETM is in the range of 4500PgC. That’s 4.5 billion tonnes of CO₂! Scientists know this because they have looked at the isotopic compositions of various sediments across the PETM from all carbon reservoirs - deep marine, shallow sea and terrestrial. All 3 show an sudden and significant divergence from the standard of roughly -4‰ - that is to say that across the PETM, the average weight of carbon in the environment decreased by 0.4%. That is a small number for such a significant change. Look at the graph below and you’ll see that change.

These δ¹³C values, as they are are known, give the ratios between two different types of C - ¹³C, a carbon atom with 13 nucleons and ¹²C, a carbon with 12 nucleons. As you’d expect, ¹²C is lighter. Each source of C in the world has its own signature δ¹³C. Every single plant on the planet makes CO₂ with a δ¹³C of -24‰, and volcanic outgassing produces CO₂ with a δ¹³C of -5‰. By extrapolating the data from other works, Dunkley Jones et al. managed to calculate the amount of CO₂ that you’d need to produce a global δ¹³C of -4‰ and the results were quite something!

By volcanic outgassing at -5‰, the rate of volcanic outgassing would have to increase over 100 times before it could explain the trends in the PETM - which is geologically unfeasible. More surprising still was that even when considering organic C at -24‰, over 75-90% of the total organic reservoir would be required. This clearly isn’t the case, where where is all the missing carbon?

Here is a diagram from the paper that shows the various sources and reservoirs of C and their relative  δ¹³C values. 

There are many conflicting theories over the source of this carbon, but the leading runner is methane hydrates. Methane hydrates are isotopically very ‘light’ in the carbon dioxide they can liberate with a δ¹³C of up to -60‰. The fear today is that as methane is such a potent greenhouse gas, some 10 times more so than CO₂, any release from clathrates with have a detrimental effect on our climate, accelerating global warming. Here is an article that goes further into the details of methane hydrate, and methane itself as a greenhouse gas.