Why Is There So Much Coal From One Period? Trees Without Rot

The landscapes that produced these deposits would be unrecognisable, since almost nothing growing in them has close living relatives of comparable size. The dominant trees were giant relatives of modern clubmosses, reaching more than thirty metres with trunks covered in diamond-patterned scars where leaves had fallen, and they were structurally unlike modern trees, with relatively little wood and a great deal of thick bark providing the support. Giant horsetails formed thickets. Tree ferns and early seed plants filled other niches. The ground was waterlogged, the climate tropical and wet, and the whole system sat at low elevation on subsiding coastal plains, which is the setting that allowed material to accumulate rather than being washed away.

The sequence requires several conditions in succession:

• •Dead plant material must accumulate faster than it decays, which needs waterlogging
• •Standing water excludes oxygen, which slows the organisms that break tissue down
• •The resulting peat must be buried by sediment before it is eroded or oxidised
• •Continued subsidence allows the sequence to repeat, stacking many layers
• •Burial pressure and heat drive out water and volatile compounds over long periods
• •Progressive alteration yields lignite, then bituminous coal, then anthracite

One influential explanation holds that the fungi capable of breaking down lignin, the rigid compound in wood, had not yet evolved, so dead trunks simply did not rot and accumulated in enormous quantities until those fungi appeared and ended the process. Genetic work dating the origin of the relevant fungal enzymes was taken to support this. The explanation has been challenged substantially since, on the grounds that the dominant plants were not especially rich in lignin, that decay-resistant bark rather than wood makes up much of the deposit, and that the timing of the fungal origin is uncertain enough to be compatible with several stories. The current view gives more weight to the tectonic and climatic setting, namely vast subsiding wet tropical basins in the right place at the right time.

The deposits are an archive as well as a fuel, and the fossils in and around them are unusually informative. Compressed plant material preserves leaf outlines, bark patterns and reproductive structures in enormous quantity, which is why the flora of the period is better known than that of many later ones. Occasional preserved lumps within a seam retain three-dimensional cellular detail, allowing internal anatomy to be sectioned and studied directly. Tree stumps are found rooted in place beneath seams, showing forests standing where they grew rather than washed together. Animal remains occur in the associated sediments, including early reptiles, amphibians and the giant arthropods, and one famous locality preserved small animals inside hollow fossil stumps they had fallen into.

Removing that much carbon from circulation had planetary consequences that are readable in the rocks. Oxygen levels rose to the highest in Earth's history, well above the present figure, because burying organic carbon without letting it oxidise leaves the oxygen produced in making it. That elevated oxygen is the leading explanation for the giant arthropods of the period, including dragonfly relatives with wingspans approaching seventy centimetres, since insect breathing through tubes works better in richer air. Carbon dioxide fell as the same carbon was withdrawn, and the planet cooled into a prolonged glaciation with ice sheets across the southern continents. The coal being burned now is that buried carbon returning to the atmosphere, several hundred times faster than it was removed.

Waterlogged tropical basins on subsiding coastal plains let dead plant material pile up faster than it decayed, and repeated burial stacked the layers that became coal. The trees were giant clubmoss relatives supported by thick bark rather than wood. The claim that lignin-digesting fungi had not yet evolved is now contested, with more weight given to the tectonic setting. Burying that carbon raised oxygen, cooled the planet and produced giant insects.