Earth memory and carbon dioxide.
The recent passing of the 400ppm level of CO2 will persist in the memory of the Earth for millennia. The chemical changes as well as the climate effects are slow to dissipate even if there is no further anthropogenic perturbation of the system.
But the Earth can remember bigger and older changes. The Earth remembers by writing the events into the geology of the surface. Small changes only write a small memory that may get overwritten by subsequent events. But the big changes leave traces that persists for many millions of years.
Coal forms from peat and similar buried organic material that fails to decay. There is quite a lot of coal from geological eras more recent than the Carboniferous, but there is more coal found from this period than any other. Despite the fact it is the most distant time when coal could form and has had the most chance to be eroded, forgotten and destroyed by the passage of time.
But the reason that the first coal is also the most abundant is because it was the first.
This picture shows a sedimentary ocean bed layer with glacial deposits below a thick band of calcium carbonate.
The Earth can even remember how the rising CO2 first caused acidic oceans and weathering.
Kang-Jun Huang et al. Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation, Proceedings of the National Academy of Sciences (2016). DOI: 10.1073/pnas.1607712113
“Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition. “
Multi-cellular biology was just getting going. Communal algae were inventing plants. Algal mats that surpassed their bacterial stromatolite predecessors. The invasion of the land was the spur. To grow on land required water retention, UV protection and structural strength. Soft flat green mats could be replaced by something with a waterproof structural material that enabled it to distributive the water, chemicals and nutrients throughout the plant and enable it to rise above its lowly cousins, literally putting them in the shade. Early vascular systems used cellulose. Tough but not indestructible and a future food for the organisms that could find a way to digest it.
On land the energetics are against limestone formation, air coral, unfortunately precluded that route. Instead several families of plants developed a near indestructible hydrocarbon polymer. It’s strength came from the long chain polymers and the chaotic branching structure they adopted. This was not regular biochemical with many identical bond sites that could be dismantled with one targeted enzyme. This was a super strong material with a nano structure that might have been designed to be maximally resistant to chemical attack.
And so it proved. Lignin plants took over the world. If they had evolved air coral the effect may not have been as bad. For every C atom locked into a CACO3 skeleton another is released as CO2. But lignin just eats up CO2. In a geologically short time the CO2 levels dropped. Temperatures fell and most of the planet iced over.This is the common graph presented for past CO2 – Temperatures on the Earth. Sometimes it is presented as though some of the shifts refute the CO2-temperature linkage, as if the fact there are other factors negates the clear role of CO2.
It took several million years for a few bacteria to develop peroxidase enzymes powerful enough to digest wood. During that time a massive amount of CO2 was sequestered into the long term mineral store of the bedrock. Never since has so much carbon been removed from the atmosphere. Only slowly has erosion and subduction returned a proportion of it back to the atmosphere.
Although it is tempting to speculate that the PETM may have been triggered by widespread burning of coal deposits as tectonic and erosive forces combined to expose and ignite large deposits at the same time.