What kicked off the Earth’s rapid cooling 12,800 years ago?
In the space of just a couple of years, average temperatures abruptly dropped, resulting in temperatures as much as 14 degrees Fahrenheit cooler in some regions of the Northern Hemisphere. If a drop like that happened today, it would mean the average temperature of Miami Beach would quickly change to that of current Montreal, Canada. Layers of ice in Greenland show that this cool period in the Northern Hemisphere lasted about 1,400 years.
This climate event, called the Younger Dryas by scientists, marked the beginning of a decline in ice-age megafauna, such as mammoth and mastodon, eventually leading to extinction of more than 35 genera of animals across North America. Although disputed, some research suggests that Younger Dryas environmental changes led to a population decline among the Native Americans known for their distinctive Clovis spear points.
Conventional geologic wisdom blames the Younger Dryas on the failure of glacial ice dams holding back huge lakes in central North America and the sudden, massive blast of freshwater they released into the north Atlantic. This freshwater influx shut down ocean circulation and ended up cooling the climate.
Some geologists, however, subscribe to what is called the impact hypothesis: the idea that a fragmented comet or asteroid collided with the Earth 12,800 years ago and caused this abrupt climate event. Along with disrupting the glacial ice-sheet and shutting down ocean currents, this hypothesis holds that the extraterrestrial impact also triggered an “impact winter” by setting off massive wildfires that blocked sunlight with their smoke.
The evidence is mounting that the cause of the Younger Dryas’ cooling climate came from outer space. My own recent fieldwork at a South Carolina lake that has been around for at least 20,000 years adds to the growing pile of evidence.
Around the globe, scientists analyzing ocean, lake, terrestrial and ice core records have identified large peaks in particles associated with burning, such as charcoal and soot, right at the time the Younger Dryas kicked in. These would be natural results of the cataclysmic wildfires you would expect to see in the wake of Earth taking an extraterrestrial hit. As much as 10% of global forests and grasslands may have burned at this time.
Looking for more clues, researchers have pored through the widely distributed Younger Dryas Boundary stratigraphic layer. That’s a distinctive layer of sediments laid down over a given period of time by processes like large floods or movement of sediment by wind or water. If you imagine the surface of the Earth as like a cake, the Younger Dryas Boundary is the layer that was frosted onto its surface 12,800 years ago, subsequently covered by other layers over the millennia.
In the last few years, scientists have found a variety of exotic impact-related materials in the Younger Dryas Boundary layer all over the globe.
These include high-temperature iron and silica-rich tiny magnetic spheres, nanodiamonds, soot, high-temperature melt-glass, and elevated concentrations of nickel, osmium, iridium and platinum.
While many studies have provided evidence supporting the Younger Dryas impact, others have failed to replicate evidence. Some have suggested that materials such as microspherules and nanodiamonds can be formed by other processes and do not require the impact of a comet or asteroid.