Chunks of coal buried in ancient rocks unearthed in Wales and Poland push back the earliest evidence of wildfires to around 430 million years ago. In addition to breaking the previous record by about 10 million years, the findings help determine how much oxygen was in Earth’s atmosphere at the time.
The ancient atmosphere must have contained at least 16 percent oxygen, researchers report June 13 in geology. That conclusion is based on modern laboratory tests that show how much oxygen is needed for a wildfire to take hold and spread.
While oxygen makes up 21 percent of our air today, over the past 600 million years, oxygen levels in Earth’s atmosphere have fluctuated between 13 percent and 30 percent (Serial Number: 12/13/05). Long-term models that simulate past oxygen concentrations are based on processes such as coal swamp burial, mountain building, erosion, and the chemical changes associated with them. But those models, some of which predict lower oxygen levels of up to 10 percent for this time period, provide broad glimpses of trends and may not capture brief spikes and dips, say Ian Glasspool and Robert Gastaldo, both paleobotanists at Colby College in Waterville, Maine.
Charcoal, a remnant of a forest fire, is physical evidence that provides at least a minimum threshold for oxygen concentrations. That’s because oxygen is one of the three ingredients needed to create a forest fire. The second, the ignition, came from lightning in the ancient world, says Glasspool. The third, fuel, came from flourishing plants and fungi 430 million years ago, during the Silurian Period. The predominant vegetation was low-growing plants barely a couple of centimeters tall. Scattered among this tiny ground cover were occasional knee- and waist-high plants and prototaxis mushrooms that rose up to nine meters high. Before this time, most plants were single-celled and lived in the seas.
Once the plants left the ocean and began to thrive, wildfires followed. “Almost as soon as we have evidence of plants on land, we have evidence of wildfires,” says Glasspool.
That evidence includes small bits of partially charred plants, including charcoal identified by its microstructure, as well as conglomerates of charcoal and associated minerals embedded within fossilized chunks of prototaxis fungi. Those samples came from rocks of known ages that formed from sediments dumped near the shoreline of ancient landmasses. This wildfire debris was transported offshore in streams or rivers before it settled, accumulated and preserved, the researchers suggest.
The discovery adds to previous evidence, including analyzes of pockets of fluid trapped in halite minerals formed during the Silurian, which suggests that atmospheric oxygen during that time approached or even exceeded the 21 percent concentration seen today. , points out the pair.
“The team has good evidence for charring,” says Lee Kump, a Penn State biogeochemist who was not involved in the new study. Although his evidence points to higher oxygen levels than some models suggest at the time, it’s possible that oxygen was a substantial component of the atmosphere even before the Silurian, he says.
“We can’t rule out that oxygen levels weren’t higher even further back,” says Kump. “It could be that the plants at that time were not capable of leaving a carbon record.”