Lignin and the carboniferous CO2 drop

If one looks at the graph of CO2 over the last 600million years there is a noticeable drop in CO2 levels in the carboniferous – a period when the world experienced it’s first “CO2 desert” with very low levels of CO2.
One possible cause of this massive drop in CO2 and the subsequent deposit of carboniferous rocks is based on the idea that plants developed with a new component in them called Lignin. Lignins are important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.
Lignin is a complex polymer which is indigestible by animals and indeed many bacteria and fungi. Animals can’t digest it because they lack any enzymes that can degrade this complex polymer. But some fungi (such as the Dryad’s saddle) and bacteria can biodegrade lignin using so-called ligninases.
The idea is that some time at the beginning of the carboniferous, plants evolved to produce Lignin. This protected them from fungi, bacteria and animals. Indeed, the theory goes that because they were so well protected even after death, they did not decompose releasing back the CO2 in the atmosphere. Thus plants grew and died but did not decompose so absorbing all available CO2 leaving us in the first “CO2 desert”.
However, it is then postulated that around 100million years later bacteria and/or fungi evolved the ability to produce lignisase to decompose lignin and this then allowed them to release carbon from dead plants back into the atmosphere so raising the CO2 level.

The Evolution of the second CO2 desert (150millions ago to present)

PetterT asked a very good question which is why did we get the second CO2 desert ending up in the low CO2 levels of the present era. I had wondered that myself as I wrote the article, and whilst I had a few hunches it was just speculation. But as I wrote my reply I found that one possibility matches the dates so well that I decided to include it in the article.
A very good question … Fundamentally the level of CO2 is a balance between emissions and absorptions. So there are two possible groups of explanations:

1. Emissions of CO2 are declining – and fundamentally that means geological processes – so that probably means the earth’s volcanic emissions are reducing. That is not impossible, but it doesn’t seem likely that we would suddenly get a steep decline in the last 150 million years**. Another geological process is the decline in erosion and then decomposition of carbon rich rocks.
2. The other scenario is that something has changed in the way CO2 is being absorbed. And here there are two ways this could drag down the level:

2a) More Locked up: That the CO2 is being locked up by some process. If it were geological – it should have occurred relatively steadily over the entire geological record**. So a more likely explanation appears to be that we’ve seen some kind of biological evolution which is locking up carbon in some way. I don’t know of any such process and given the amount of carbon it seems incredible that so much carbon could be locked up somewhere without us finding it and trying to utilise it. So, it would have to be something like peat or mud shales or some recent geological formations rich in carbon. Perhaps something in the deep oceans?

2b) More “suck” from plants: That a group of plants evolved that could “hoover up” the atmosphere even at very low concentrations. In effect “CO2 desert” plants have evolved specially to thrive at very low levels of CO2. So, it may not be that any more CO2 is being locked up, it is just that these plants grow until there is virtually no CO2 present in the atmosphere.

**If geological processes are involved, then it would need to be something that “happened” and then continued. I’m not suggesting it, but perhaps if something resulted from two plates colliding then this “thing” might explain the second CO2 desert.
A strong candidate for this would be any group of plants that came to be dominant in the last 150 million years. And so the strongest candidate are the flowering plants or angiosperms for which the earliest fossil evidence (Archaefructus liaoningensis) dates to just 125 million years ago, but reading this they say their research indicates that the origin of the crown group of extant angiosperms is Early to Middle Jurassic (179-158 Myr), and the origin of eudicots is resolved as Late Jurassic to mid Cretaceous (147-131 Myr).
Angiosperms are so common it’s easier to mention the commons plants that are not in this group: algae, mosses, conifers, liverworts, lichen(+), ferns. So, everything we commonly think of as a “plant” with the exception of conifers and ferns. It’s also worth mentioning that conifers evolved 300million years in the middle of the first CO2 desert – so they are probably adapted to CO2 desert conditions!
(+) I included lichen as it is so common – but it’s a combination of an algae and fungus so as far as CO2 absorption is concerned it fits with the Algae).
This suggests to me that the evolution of lignin, led to the first CO2 desert encouraging the evolution of the ancestors of flowering plants (and conifers). These started gaining a foothold as they were better adapted to the CO2 desert conditions. Then when bioforms started producing ligninases, we get a relative CO2 flood. But the flowering plants (& conifers) went on to out compete almost all other plant forms except in specialised environmental niches and slowly over the next 150 million years they take over and gradually suck the atmosphere dry of CO2.