# 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). 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. Apr 13 '15 at 12:02
• thnks for help. I edited the question. 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

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.
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.