Where does Coal come from?
January 2016

Coal and oil are the two great sources of energy in our modern world. Of the two, coal was earlier taken up by the industrial age, and it was coal that powered Western Europe into the Industrial Revolution of 1760-1840.

However, even as the Industrial Revolution was under way, British geologists noted that almost all of the world's coal came from the same geological era of 300 million years ago. Over a period of about 50 million years most of the world's coal deposits were laid down. As a result this period was named the 'Carboniferous Period' (from the Latin 'carbo' = coal and 'fero' = I carry). Almost as soon as geologists discovered this fact and named the period, scientists were asking another question – why did the Carboniferous Period produce so much coal when earlier and later ages did not?

For many years the answer was summed up in the words 'evolutionary lag'. What this means is that in nature almost everything is consumed by something else. However when nature throws up a new mutation (and over tens of millions of years mutations happen a lot), it takes time for something else to evolve to consume or compete with the new mutation.

For geologists time is divided into eons, eras and periods, rather as mathematicians divide numbers into hundreds, tens and units. Thus the Carboniferous Period is a part of the Paleozioc Era. At this time plants, like animals and insects, were less evolved than today. Most plants were varieties of giant ferns, or huge ancestors of plants that survive today as relatively diminutive herbs. The herbs and ferns were pushed aside by a mutation in the plant world which led to the development of more solid, woody trees such as proto-conifers. The evolutionary advantage of these new trees was a substance called lignin, a complex organic polymer which developed within the cell walls of trees and made them rigid. Lignin was also the precursor material which, given heat, compression and time within the Earth turned into coal.

Scientists speculated that because lignin was a new material, there was nothing that consumed it, rather as most plastic in the modern era is not biodegradable (and when it is biodegradable, plastic has to be deliberately made so). As a result, huge amounts of wood lay in ever-growing piles around the world, undecayed and eventually swallowed up by tectonic processes which buried them underground to become the coal seams of the future.

Then, some time around 299 million years ago, evolution came up with Agaricomycetes, a fungus which feeds on lignin. Now, when a tree fell to the ground, busy Agaricomycetes spores got to work and within a decade (less than an eye-blink in geological terms) the tree had been turned to compost instead of coal. The evolutionary lag had closed, and the Carboniferous Period was over.

This is a neat answer which makes intuitive sense. Now it appears that it might also be at least partly wrong. 'Our analysis demonstrates that an evolutionary lag explanation for the creation of ancient coal is inconsistent with geochemistry, sedimentology, paleontology, and biology', claims Matthew Nelson, one of a group of scientists at Stanford University (California, USA)who have produced a new theory to account for the Carboniferous Period.

In a paper recently published in the journal the Proceedings of the National Academy of Sciences, the researchers demonstrated a lot of coal comes from plants which had very little lignin in their cells. In fact entire coalfields are formed from a plant family called lycopsids which are pretty much lignin-free. So if the lignin mutation and evolutionary lag did not produce the deposits of the Carboniferous Period, what did? Pangaea did, say the Stanford researchers.

Pangaea (from Greek 'Pan'=all, 'Gaia'=the earth) was the last of the planet's supercontinents, when all the land mass of the planet was squashed into a single conglomeration of tectonic plates in the Southern Hemisphere. The formation of Pangaea happened to form a unique environment which was ideal for the creation of coal. First of all, Pangaea was - at least originally – mostly warm and wet. This suits plant life which took advantage of the circumstances to flourish wildly. As a result the atmospheric concentration of oxygen (which gets into the atmosphere through being exhaled by plants) was 35% then as opposed to 22% today.

As might be expected, Pangaea was tectonically active – large mountain ranges rose where the plates jostled together, and deep valleys developed between the ranges. These valleys became flourishing wetlands and were rapidly choked up with abundant plant life. Wetlands slow the process of plant decay (which is why we have peat bogs today) so as the plants died they formed a vegetative substratum, and new plants grew on top. However, tectonic action was deepening the valleys at almost the same rate as they were filled up, so massive amounts of vegetation were steadily buried under the earth and compressed by the addition of subsequent layers.

These deep valleys provided 'accommodation space' for vegetable matter. As the Sanford scientists explain - for coal to develop you need a warm wet environment for the plants to flourish and then a deep hole for them to end up in.

'If you want to generate coal, you need a productive environment where you're making lots of plant matter and you also need some way to prevent that plant matter from decaying. Where that happens is in wet environments … you need both a wet tropics and a hole to fill. We have an ever-wet tropics now, but we don't have a hole to fill. There's only a narrow band in time in Earth's history where you had both a wet tropics and widespread holes to fill in the tropics, and that's the Carboniferous.'

In other words coal was produced by geological processes rather than biological evolutionary lag.

Journal Reference:
Matthew P. Nelsen, William A. DiMichele, Shanan E. Peters and C. Kevin Boyce. Delayed fungal evolution did not cause the Paleozoic peak in coal production. PNAS (2016)
www.pnas.org/content/early/2016/01/13/1517943113.abstract?sid=bf858c24-afbf-4e63-9804-053f09828720