For hundreds of thousands of years, the woolly mammoth thrived on Earth, measuring its stately stride over the frozen lands.
Then something happened. The earth has changed. And in a remarkably short time the mammoths (Mammuthus primigenius) had disappeared; the last became extinct 4,000 years ago, on remote Wrangel Island in the cold Arctic.
Although humans are believed to have contributed greatly to their eventual decline, it is not clear what factors could have caused the climate change that endangered them. One idea is that Earth was hit by a cosmic event nearly 13,000 years ago, causing the world to become warmer than what was tolerable for mammoths and paving the way for other species to flourish.
This is called the Younger Dryas impact hypothesis (YDIH), and to call it highly controversial might be putting it mildly. Nevertheless, some scientists believe the idea has legs and have started looking for evidence to support it.
One of these is archaeologist Christopher Moore of the University of South Carolina. “Some of our critics have said, ‘Where is the crater?’” Moore says. “Right now we don’t have a crater or craters.”
Nevertheless, Moore and his colleagues believe the evidence can be found if you do more than just a surface survey of Earth. And they also believe they’ve found some of it – in the form of minerals with properties that, they say, can best be explained by a comet impact.
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In their most recent article, they describe several of these lines of evidence in totalthey say, tell a compelling story.
These various pieces of evidence come from layers of sediment dug from sites around the world, all dated using radiocarbon analysis to about 12,800 years ago – the period in which the impact would have occurred.
From approximately 50 locations around the world, including North and South America, Europe, Asia and the Greenland Ice SheetEvidence has emerged that could indicate Earth is meeting a comet.
Ice cores dug from permanently frozen areas of Greenland have uncovered microparticles linked to widespread fires – so-called combustion aerosols that propagate through the atmosphere when matter burns.
Unusually high levels of platinum can be found in samples from other parts of the world, such as Syria, and three widely separated locations in North America. Platinum, Moore explains, is rare in the Earth’s crust but relatively common in comets.
In the same sedimentary layer there is an increased concentration of small, microscopic iron globules, called microspheres. These form when molten material spews through the air, as happens when a meteorite impacts the surface or melts and explodes in the atmosphere.
And finally, researchers report for the first time the presence of grains of shock-fractured quartz in the Younger Dryas boundary layer at a series of well-separated locations in North America. This is quartz that exhibits microscopic fractures as a result of significant impact.
“It’s like putting 75 elephants on a quarter,” says Moore. “It’s a tremendous amount of pressure that’s creating what we’re seeing.”
The bigger picture that could emerge from these puzzle pieces is a comet that hit Earth about 12,800 years ago with an impact that may not have left a crater. If the comet had exploded in the atmosphere, the resulting shock wave could have washed over the surface and produced all the observed elements, similar to the way the Tunguska event created a gigantic ruckus without leaving a deep scar on the planet’s surface to let.
However, it is far from a smoking gun. In an article published last December, a team led by anthropologist Vance Holliday of the University of Arizona noted: “Evidence and arguments supporting the YDIH include flawed methodologies, inappropriate assumptions, questionable conclusions, misrepresentation of facts , misleading information, unsupported claims, irreproducible observations, logical fallacies and selective omission of contrary information.”
So we’ll probably need a lot more data before the scientific establishment is even remotely convinced. Still, other scientists point out that in the past, many scientific theories that were once dismissed or rejected later gained broad consensus, so while it’s important to remain skeptical, it can pay to keep an open mind.
What cannot be denied is that asteroid and comet impacts are absolutely worth investigating in relation to large-scale environmental changes, whether to understand history or to help guide our decisions for tomorrow. These events have changed the course of all life on Earth before, and although the solar system is a lot calmer than it once was, the possibility of another event happening in the future is not zero.
The new article was published in Air eruptions and crater impacts.