# Sources of $\ce{CO_2}$ during Carboniferous period

It is believed that during the Carboniferous period, fungi and bacteria lacked the biochemical means to decompose lignin and release $\ce{CO_2}$ back to the atmosphere. So dying trees carbon became trapped in coal deposits. But whence came all carbon allowing these plants grow?

• "it is known"? Is there any actual evidence? I thought it was more speculation that a) appears logical; and b) is then used to help explain the high amount of preserved organic material (ie. coal). – winwaed Apr 9 '15 at 12:53
• Maybe change 'known' to 'believed' (besides, that's about as good as it gets in geology :), and offer a reference? It's a reasonable question. – kwinkunks Apr 13 '15 at 12:02
• thnks for help. I edited the question. – rnrneverdies Apr 13 '15 at 13:14
• Stellar nucleosynthesis is surely the answer to the question as it stands. It might be better to ask how the carbon cycle worked before the advent of bacterial/fungal degradation of lignin? or to think about how long (probably not too long) it took for life to evolve a way to get the carbon out. – Siv Apr 13 '15 at 19:09

## 2 Answers

The terrestrial vegetation makes ca. 500 PgC (i. e. $500 \times 10^{15}$ grams of Carbon) which is rather small compared to the carbon stored in the ocean of ca. 37 000 PgC, see Fig. 6.1 in Ciais et al. 2013 1.
Even if we assume that the fossil fuels reserves are the remainders of the Carboniferous period, they only add up to no more than ca. 2 000 PgC.

The terrestrial biosphere only makes use of a very small portion of the total available carbon.

1 Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013. Carbon and Other Biogeochemical Cycles. In T. F. Stocker et al. (Eds): Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

• @plannapus: Well spotted, I changed that to "terrestrial" – makra Apr 17 '15 at 10:24

In a nutshell, if we assume that the earth's atmosphere (and water) came largely from comets, then it's simply about asking, what are comets made of - $\ce{H2O}$, $\ce{CO2}$, $\ce{CH4}$, $\ce{NH3}$, other ices, dust. $\ce{CO2}$ is abundant, it arrived pretty much the same way the water did. The earth has trapped/absorbed most of it's $\ce{CO2}$. There used to be a lot more of it in the atmosphere.