In many works of science fiction, our favorite characters travel back in time to an age when
alien like creatures roamed the planet such as dinosaurs, often times in landscapes which
look completely alien.
While these stories are fun to watch or read, there is a notable problem with a few of them.
Simply put, many people have the expectation that if somehow they ever ended up several
hundred million years in the past, the atmosphere would be about the same, and breathing would
not be an issue.
However, the amount of oxygen in the atmosphere as indicated by rock samples and fossils has
not always been the same.
While the concentration of oxygen in the atmosphere is 21% percent today, if you were to travel
back in time to 184 million years ago, the amount of oxygen in the atmosphere was slightly
below 19.5%.
For context, this is below the minimum threshold OSHA considers a safe environment to work
in, meaning after prolonged exposure you would suffer continual adverse health effects.
Yet, if you went back even further, there was a period of time during the so called
"age of fishes" when the amount of oxygen in the atmosphere was so low that a type of
natural disaster became impossible.
Between slightly before 380 million years ago and 363 million years ago, the amount
of oxygen in the atmosphere was so abysmally low that wildfires no longer became possible.
How do we know this?
Throughout the history of the planet, charcoal has been a consistent product in the fossil
record and rock units, being caused by the heating or burning of organic material to
a temperature of several hundred degrees celsius in an oxygen poor environment.
For example, in the world of volcanology finding a piece of charcoal buried by a lava flow
is useful in constraining the age of the adjacent formerly molten rock.
While charcoal has been consistently present throughout the geologic record ever since
widespread plant life colonized various landmasses, there was a notable absense of any form of
charcoal found between 380 and 363 million years ago.
Because this largely occurred during the Frasnian period, this phenomena is referred to as the
"Frasnian charcoal gap".
While models of the amount of oxygen in the atmosphere at the time greatly vary, it is
somewhat accepted that oxygen was at a notable low point during the period.
Oxygen concentrations were estimated to be 13% 380 million years ago and 16% 363 million
years ago.
The figure of 16%, as stated by the national parks service is the minimum amount of oxygen
required for a fire to begin.
Otherwise, a fire would not ignite.
Fire needs three items to ignite; fuel, oxygen, and heat.
While many fires are caused by humans today, in the distant past they were largely caused
by lightning strikes.
The vegetation acted as fuel, while in the case of the charcoal hap the low oxygen prevented
the reaction from occurring.
Of course, this lack of charcoal in the geologic record does not mean a complete lack of any
fires occurred.
Some incredibly small traces of charcoal have been found during the period, likely being
attributed to point sources for where a fire began in very specific circumstances but did
not spread or last long.
Although scientists are not exactly sure why there was a lack of oxygen in the atmosphere,
there are a few potential causes and a notable overlap with a somewhat brief unusually warm
period.
While average ocean temperatures today are 61 degrees fahrenheit, during the charcoal
gap they were around 85 degrees fahrenheit.
In the geologic record, warm periods are often correlated with a decline in average atmospheric
concentration of oxygen.
Another example occurred during and slightly after the Permian Triassic extinction event.
This could be correlated with an increased rate of oxidation around the planet, where
rock units combined with oxygen, taking the gas out of the atmosphere.
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