A little less than 13,000 years ago, as the world was starting to leave the last ice age, it experienced a sudden sharp cooling, leading to the extinction of many large mammals. The theory of an asteroid impact has sparked one of those particularly heated scientific debates. Two new papers published this month tip the scales a little in the theory’s favor.
The Younger Dryas period, which started 12,800 years ago and ran for 1,400 years, got its name from the spread of subarctic flowers to lower latitudes. It's one of the most dramatic events humanity has experienced making scientists very interested in how and why it happened.
Since 2006 evidence has been proposed it was caused by an asteroid, albeit one much smaller than the dino-killer. This Younger Dryas Impact Hypothesis (YDIH) has been criticized primarily because we would expect such a recent event to have left a highly visible impact crater, but key pieces of evidence in its favor have also been heavily disputed. Earlier this month a paper in Paleontologia Africana addressed one problem with the theory: the lack of supporting evidence outside North America. Now Scientific Reports has published several more lines of evidence.
The inspiration behind the YDIH comes from high levels of platinum found at several North American sites whose formation coincided with the sharp chill. Like iridium, platinum is much more common in asteroids than the Earth’s crust.
In the Paleontologia Africana paper, Professor Francis Thackeray of the University of Witswatersrand reported a peat deposit near Pretoria also has a platinum spike at the time the Younger Dryas began. "Our evidence is entirely consistent with the Younger Dryas Impact Hypothesis," Thackeray said in a statement. Species of buffalo, wildebeest, and zebra all disappeared from Africa around the Younger Dryas period, although their demise has not been as precisely linked to the cooling as their counterparts on other continents.
Besides adding White Pond, South Carolina, to the places with suspiciously timed platinum spikes the sediment core described in Scientific Reports also has an unusual amount of soot deposited at the same point, followed by a decrease in fungi associated with herbivore dung.
Something set local forests ablaze, and then halved the large herbivore population.
“We speculate that the impact contributed to the extinction, but it wasn't the only cause. Overhunting by humans almost certainly contributed, too, as did climate change,” Dr Christopher Moore of the University of Southern Carolina said in a statement.
YDIH supporters have addressed the key objection, the lack of an impact crater, in two ways. The first is by proposing a crater under Greenland's Hiawatha Glacier was responsible. At 31 kilometers (19 miles) wide it is certainly large enough, but the timing of its creation is very uncertain, ranging from 2.6 million to 11,700 years ago, a gap into which the Younger Dryas only just sneaks in.
The second proposal is that the asteroid was actually a “rubble pile” of smaller objects, none of which were large enough to create a major crater, but collectively sufficient to both cool the planet and distribute platinum widely. The finding of platinum in South Africa, like a recent one in South America, adds to the credibility of this version of the theory.
The YDIH’s main competing theory proposes an enormous flood of fresh water left North America and shut down the Gulf Stream. The water is thought to have been glacier runoff, released suddenly when a dam broke. Without the Gulf Stream’s tropical waters, temperatures in Northern Europe plunged enough for ice sheets to return, reflecting more light back into space and therefore cooling the whole planet, rather than just one region.