Back then, plant life would have depended heavily on water
to reproduce and most probably wouldn't have appeared in regions that were dry
for part or all of the year. The wildfires discussed in the study would have
burned through very short vegetation, plus the occasional knee- or waist-high
plant.
The landscape would have been dominated not by trees but by
the ancient fungus Prototaxites, the researchers say. Not too much is known
about the fungus, but it's thought to have been able to grow as high as nine
meters (or almost 30 feet) tall.
"It looks now as though our evidence of fire coincides
closely with our evidence of the earliest land plant macrofossils," says
paleobotanist Ian Glasspool from Colby College in Maine.
"So as soon as there's fuel, at least in the form of
plant macrofossils, there is wildfire pretty much instantly."
To exist, wildfires need fuel (plants), an ignition source
(which here would have been lightning strikes), and enough oxygen to burn.
That the fires were able to propagate and leave charcoal
deposits suggests that Earth's atmospheric oxygen levels were at least 16
percent, the researchers say.
Today, that level is at 21 percent, but it has varied
dramatically during the course of Earth's history. Based on their analysis, the
team thinks the atmospheric oxygen levels 430 million years ago may have been
21 percent or even higher.
That's all very useful information for paleontologists. The
thinking is that increased plant life and photosynthesis would have contributed
more to the oxygen cycle around the time of these wildfires, and knowing the
details of that oxygen cycle across time gives scientists a better idea of how
life may have evolved.
"The Silurian landscape had to have enough vegetation
across it to have wildfires propagated and to leave a record of that
wildfire," says paleontologist Robert Gastaldo, also from Colby College.
"At points in time that we're sampling windows of,
there was enough biomass around to be able to provide us with a record of
wildfire that we can identify and use to pinpoint the vegetation and process in
time."
The landscape that is now Europe looked a lot different
hundreds of millions of years ago, and the two sites that the researchers used
for their analysis would have been on the ancient Avalonia and Baltica
continents at the time these wildfires were raging.
Wildfires then, as now, would have contributed significantly
to the cycles of carbon and phosphorus, too, and to the movement of sediment on
the Earth's surface. It's a complex combination of processes that takes a lot
of unpacking.
This discovery certainly helps in that unpacking – smashing
the previous record for the oldest wildfire on record by 10 million years – and
it also highlights the importance that research into wildfires could have in
charting the history of Earth.
"Wildfire has been an integral component in
Earth-system processes for a long time and its role in those processes has
almost certainly been underemphasized," says Glasspool.
The research has been published in Geology.