Tectonics and Climate Identified as Primary Drivers of Carboniferous Coal Accumulation Peak

The Carboniferous period, spanning approximately 359 to 299 million years ago, is renowned for its vast coal deposits. A long-standing popular theory, recently highlighted by Sam Pullara in a tweet, suggests this abundance stemmed from a 60-million-year gap where newly evolved trees, rich in indigestible lignin, piled up due to a lack of decomposing organisms. However, recent scientific research challenges this "evolutionary lag" hypothesis, presenting a more complex geological and climatic explanation for the era's prolific coal formation.

The tweet succinctly summarizes the "lignin lag" idea:

"one of my favorite things I learned relatively recently is that coal was formed between 360 million and 300 million years ago because trees appeared 360 million years ago and nothing could digest them for the next 60 million years so they just kept piling up and carbonizing." This hypothesis posits that the evolution of lignin, a tough plant polymer, preceded the development of efficient lignin-degrading fungi, leading to massive accumulations of undecayed plant matter. This undigested biomass was then buried and transformed into coal.

Studies, including a 2016 paper in Proceedings of the National Academy of Sciences by Nelsen et al., refute the lignin lag as the primary cause. They argue that lignin was often of secondary importance in Carboniferous floras, with many coal deposits dominated by low-lignin plants like lycopsids. Furthermore, evidence suggests that various organisms, including fungi and bacteria, possessed lignin-degrading capabilities earlier than previously thought, making a 60-million-year "lag" unlikely to be the sole driver.

Instead, scientists now emphasize a unique confluence of geological and climatic conditions during the Carboniferous. The assembly of the supercontinent Pangea created extensive foreland and cratonic basins, providing "accommodation spaces" for vast amounts of organic debris. These basins were situated in the humid, equatorial zone, fostering highly productive, wet tropical environments conducive to massive plant growth.

In these waterlogged, anoxic (oxygen-depleted) swamp environments, dead plant matter was largely protected from decay by decomposers, which require oxygen to thrive. This rapid burial of organic carbon prevented its return to the atmosphere as carbon dioxide. The significant sequestration of carbon contributed to unusually high atmospheric oxygen levels during the period, reaching 25-30%.

While the Carboniferous period was indeed a peak for coal production, accounting for a significant portion of global reserves, it is not the exclusive source, with substantial deposits also forming in later geological eras. The formation of coal, therefore, is understood as a complex interplay of high plant productivity, specific environmental conditions that inhibit decay, and geological processes that provide sufficient burial space over millions of years.