Abby Tabor
At the end of the last ice age, as the Earth was warming, the climate suddenly entered a period of rapid cooling. One explanation proposed for this abrupt change is the catastrophic impact of a comet or meteorite, nearly 13,000 years ago. The controversial idea has never been independently confirmed, calling some of the data into question, nor has it been definitively rejected. Numerous groups continue to explore the evidence in the geological record. This week, a team whose members include the authors of the original paper has revealed the most convincing data yet: melted glass that could only have formed in the high temperatures of an impact. Will this be the evidence that persuades a skeptical community?
Almost 13,000 years ago, the Earth was waking. The ice age was ending, glaciers were receding and temperatures warming. Then, suddenly, the world was plunged back into the cold. Temperatures fell rapidly, prompting changes in vegetation: where once there were forests, the tundra of colder climates took over. In North America, losses were great. This period witnessed the extinction of the continent’s megafauna, like mammoths and the giant ground sloth, as well as the disappearance of the Clovis culture, the earliest Native Americans.
What set off this rapid climate change is still unknown. One hypothesis suggests that a giant lake of glacial meltwater in the region of modern Canada burst through its shores, spilling into the Arctic and North Atlantic Ocean. This huge influx of fresh water would have disturbed the currents that carry warmer seawater north, toward the pole, where it sinks and heads back south. Without this watery “conveyor belt” bringing warmth, air temperatures would have fallen, leading to the cooling that lasted over 1,000 years.
What on Earth could have provoked such a disaster? Perhaps something decidedly extraterrestrial. In 2007, a team led by Richard Firestone of Lawrence Berkeley National Laboratory (California) proposed that the trigger was a cosmic impact – a comet or meteorite that exploded in the air over the Laurentide ice sheet, which covered much of North America during the ice age. The event may have set off coast-to-coast wildfires, reducing the continent to a charred shadow of its former self. Large herbivores would have struggled to feed; human hunters would have suffered the consequences.
Such a monumental event would have left a mark on the planet. That hypothesized trace is something researchers have been exploring since the idea was first presented. A carbon-rich layer of earth, known as the Younger Dryas (YD) boundary, marks the beginning of the cold period in question, and its components have been the subject of much investigation. Analyses suggest that charcoal in the layer may have come from wildfires, possibly ignited by an impact. Magnetic iron microspherules found in the YD boundary, but not in significant quantities above or below it, are a marker of an extraterrestrial impact. They contain elevated levels of iridium, suggesting a cosmic, rather than terrestrial, origin for the material, and have been melted by the high temperatures that an impact could provide. Nanodiamonds, an important area of inquiry, are thought to have condensed out of the cloud of dust produced by the high-energy impact. And, now, a potentially game-changing piece of evidence has just been revealed: melted glass that could only have formed at the high temperatures of a cosmic impact.
Ice Age Cosmic Impact: A Controversial Hypothesis
Deciphering the clues of a 13,000-year-old event is not a straightforward task and the search has been subject to controversy. A good deal of the original data has, at one point or another, been called irreproducible, misinterpreted or possibly attributable to terrestrial processes. A 2011 article, “The Younger Dryas Impact Hypothesis: A Requiem” traces the history of this research and purports to put the question, finally, to rest. However, Ted Bunch, co-author of the impact hypothesis and former Chief of Exobiology at NASA Ames Research Center, observes that this scientific memorial service “presented little original evidence. Rather, they criticized non-peer reviewed documents. Their arguments were answered rather well”, he adds, by the 2012 paper from Israde-Alcántara I, et al., providing evidence from central Mexico in support of the proposed Younger Dryas cosmic impact.
Among the most recent studies unable to find support for the impact hypothesis, is a Dutch group’s April 2012 publication looking at wildfire evidence and nanodiamonds in the Younger Dryas (YD) boundary layer at a site in the Netherlands. If the proposed cosmic body broke apart as it tore through the atmosphere, there may have been multiple impacts around the world. Carbon-14 dating of charcoal here suggested that the original wood burned some 200 years after the onset of the cold period. These forest fires, therefore, would not be related to a climate-changing cosmic impact, explains lead author Annelies van Hoesel, of Utrecht University.
Their nanodiamond analysis also suggests a later origin for these particles, based on their position in the layer and their physical association with glass-like carbon, a product of wildfire. van Hoesel concedes that the nanodiamonds could have formed earlier and only later become stuck to the other carbon particles, although she considers it unlikely. She speculates that perhaps an as-yet-unknown process exists for the formation of nanodiamonds at the relatively low temperature of a wildfire. This has been observed in experimental set-ups; if the phenomenon could be found in nature, it might offer an alternative to the cosmic explanation.
Richard Firestone, one of the originators of the impact hypothesis, is not in the least deterred by the Dutch study. He claims their carbon-14 dates are “unacceptably precise”; if the error were larger, as he feels it should be with this technique, both groups’ dates would coincide. According to van Hoesel, though, the error is in line with other studies published in the field. The disagreement shows the importance of careful analysis of the numbers. Firestone also contends that, had the two groups used the same system of radiocarbon date calibration, the dates assigned to the impact and the Dutch wildfires would have been nearly identical. “In my opinion the van Hoesel paper supports our research. They find nanodiamonds at or near the date of the YD impact. There is no other accepted theory for the formation of nanodiamonds found in a narrow layer around the world. No nanodiamonds are found significantly above or below the YD layer. van Hoesel misinterprets the distribution of nanodiamonds near the YD which is affected by [stirring up] of the sediments by wind and rain. Typically nanodiamonds tend to rise in the sediment appearing a bit younger than they are." Firestone finds that “most of our critics first confirm our work and then try to ascribe it to nebulous other causes.”
New data to fuel the debate
Beyond simply refuting the inconclusive data of others, Firestone et al. are back with novel, intriguing findings of their own. In an open access article published yesterday in Proceedings of the National Academy of Sciences, the team presents data on scoria-like objects (SLOs), particles of melted silica glass, formed at very high temperatures (1,700 to 2,200 °C, or about 3,000 to 4,000 °F). “The heat of the impact melts the ground sediment and the explosive shock wave carries it away,” Richard Firestone explains.
Lightning, the only other source of such high temperatures, produces similar objects called fulgarites. But there is “no mechanism for distributing fulgarites over the wide area that we find SLOs.” To eliminate further the possibility of a terrestrial explanation, the group showed that the SLOs are chemically different from volcanic glass (scoria), but comparable to glass found at “known high-temperature events, including crater-forming impacts (Meteor Crater, AZ), cosmic airbursts (the Tunguska Siberia event in 1908), and the Trinity nuclear detonation of 1945.”
The new evidence seems quite compelling, but it will have to withstand examination by the scientific community before the hypothesis of a cosmic impact at the start of the Younger Dryas can be accepted. Furthermore, as Annelies van Hoesel observes, there are really two questions to answer: Was there an impact, and did it have anything to do with the climate change observed? What was once viewed as a climatic aberration, this sudden return to the cold, may not have been so unique. There is evidence to suggest that the Younger Dryas may have been just a recurring stage, typical of the end of a glacial cycle. The events – causes, consequences, or coincidences – occurring at this distant point in time remain unclear. This new data, and the debate it is sure to provoke, may help elucidate what happened all those 13,000 years ago.
To find out more:
"A Paleo Perspective on Abrupt Climate Change: The Younger Dryas", NOAA Paleoclimatology Program https://www.ncdc.noaa.gov/data-access/paleoclimatology-data
“Why the Younger Dryas Matters”, Discovery News, 2010https://www.seeker.com/why-the-younger-dryas-matters-1765043938.html
“What Caused the Younger Dryas Cold Event?”, Geology, 2010https://pubs.geoscienceworld.org/gsa/geology/article/38/4/383/130267/What-Caused-the-Younger-Dryas-Cold-Event
