At some point about 12,800 years ago, a tenth of the Earth's surface was suddenly enveloped in roaring fire. The firestorm rivaled the hurricane that wiped out the dinosaurs.
Most likely, the storm was caused by a comet fragment measuring about 100 kilometers across. When a dust cloud covered Earth, it started a mini-ice age that kept the planet cold for another thousand years, just like when Earth emerged from a glacier-covered 100,000-year period. After the fire is extinguished, life begins again.
"The hypothesis is that a large comet fragmented and its chunks impacted Earth, causing this catastrophe," said Adrian Melott of the University of Kansas, who co-authored a 2018 study detailing this catastrophic event.
"A number of different chemical signatures, carbon dioxide, nitrate, ammonia and others, all seem to indicate that 10% of the Earth's surface, or about 10 million square kilometers, has been burned," he said.
To peer back into the burning flames and shockwaves of this massive event, a large number of geochemical and isotopic markers were measured from more than 170 sites worldwide, involving a team of 24 scientists.
Part of the analysis carried out was on patterns of pollen levels, which showed that pine forests were suddenly burning to be replaced by poplars, a species that specializes in covering barren soils. This event is like what might happen when the planet is hit by a series of massive fireballs.
In fact, parts of comets that disintegrate in space will likely still be floating around our Solar System 13,000 years later.
High concentrations of platinum, which is often found in asteroids and comets, and high levels of dust were also noted in the samples analyzed by the researchers, in addition to increased concentrations of combustion aerosols. We will be able to see if a lot of biomass is burned i.e. ammonium, nitrate, etc.
When plants die, food sources become scarce, and previously retreating glaciers begin to move again, the research team notes. Human culture had to adapt to harsher conditions, with populations declining as a result.
"Calculations show that the impact will deplete the ozone layer, leading to an increase in skin cancer and other negative health effects," Melott said.
The team hypothesized that the widespread impact of cometary fragments, and subsequent firestorms, was responsible for the extra bit of cooling known as the Younger Dryas period. These relatively brief changes in the planet's temperature are sometimes caused by changes in ocean currents.
However, comet collisions are not really a new idea, although this latest research is very in-depth to try and find the evidence. Scientists have debated whether comet impacts triggered the Younger Dryas event for several years now.
Not everyone agrees that the data suggests a comet strike, but this comprehensive work offers more support for the hypothesis, as does an ancient engraving found in Turkey in 2017. This engraving depicts the devastating impact of an interstellar object.
"The impact hypothesis is still a hypothesis, but this study provides a large amount of evidence, which we think can only be explained by a large cosmic impact," Melott said.