Two Stanford geologists are disputing the decade-old explanation of the large amount of coal accumulated during the Carboniferous Period.
Associate Professor Kevin Boyce and Postdoctorate Research Fellow Matthew Nelsen collaborated with scientists across the country to release a paper this past month in which they propose a new understanding of coal development.
The previous hypothesis of coal accumulation focused on a temporal lag between the evolution of lignin production in woody plants and the evolution of lignin-degrading fungi to break down this new material. This would have resulted in the non-degraded lignin building up, depositing massive amounts of coal.
“But [this explanation] can’t be true,” Boyce said.
The paper that Boyce and Nelsen collaborated on uses several lines of evidence in order to disprove the old hypothesis.
The most convincing evidence includes a fossil record of lignin-degrading fungi pre-dating the Carboniferous period. New research also reveals that the standard preparation of fossilized plants washes away much of the fossilized fungi and microbes, suggesting the current fungi fossil record to be an underestimate of what was actually present at the time.
The hypothesized 130-million-year evolutionary lag between the plants and the fungi would also have resulted in severe environmental consequences.
“Even if plants were less productive then, there’s probably a good three or so gigatons of lignin being produced per year,” Boyce said. “If you had 80 million years without lignin decay, you’d run out of CO2 in the atmosphere in a couple hundred years… we’d freeze the earth.”
Their paper also includes a graph displaying the accumulation of organic sediments over time. Rather than showing a steady increase over the Carboniferous Period, the graph has many spikes of accumulation, each of which lasts around 500,000 years.
“If that was fungi, what would they do? Evolve, un-evolve, and then evolve back?” Boyce said. “The actual record of accumulation doesn’t really work for it being a biotic cause. It looks much more grounded in abiotic processes.”
The new theory explains coal accumulation using weather and tectonic activity. The Carboniferous Period was not only warm, but also coincided with the separation of Pangaea. The spikes on the graph coincide with basins opening up and providing a place for plant material to be deposited before eroding away.
Boyce and Nelsen looked at the coal accumulation in Denver around 50 million years ago as a model to explain the 300-million-year-old phenomenon of Carboniferous accumulation.
“It was equivalently productive and wet, and a lot of coal formed because the incipient Rocky Mountains were there so there was a place to put all the coal,” Boyce said. “So the local rates of accumulation were similar to what was going on worldwide during the Carboniferous.”
According to Nelson, discontent with the evolutionary lag hypothesis has been around for some time before the publishing of this recent paper.
“I think there were some grumblings in the literature: people going after this but not anywhere near the level of detail that we did,” Nelsen said.
This raises the larger issue: If geologists had seen problems with the hypothesis, why had nothing been done to disprove it earlier?
According to Boyce, the unique collaborative circumstances hadn’t occurred in the past.
“You’re very often at the edge of what you know yourself and it’s very important to have the right collaborators looking at it from other directions,” Boyce said.