Rex Dalton, “Blast in

 Passage 1 is adapted from Rex Dalton, “Blast in the Past?”

©2007 by Nature Publishing Group. Passage 2 is adapted

from Michael Balter, “What Caused a 1300-Year Deep

Freeze?” ©2014 by American Association for the

Advancement of Science. Clovis hunters are widely

regarded as among the first people to inhabit North

America.

Passage 1

At the 2007 American Geophysical Union’s

meeting in Acapulco, Mexico, some two dozen

scientists presented multiple studies arguing that a

comet or asteroid exploded above or on the northern

ice cap almost 13,000 years ago—showering debris

across the North American continent and causing

temperatures to plunge for the next millennium.

The team argues that its idea explains multiple

observations: not only the climate cooling and the

disappearance of the Clovis hunters, but also the

near-simultaneous extinction of the continent’s large

mammals.

Not all will be convinced. Several leading

hypotheses already explain each of these three events.

A change in ocean circulation is generally thought to

have brought about the onset of the millennium-long

cooling, which is known as the Younger Dryas. This

cooling might, in turn, have caused the Clovis

hunters to disappear. And, if they had not previously

been killed by disease or hunted to extinction, the big

prehistoric beasts may also have been doomed by this

change in climate.

The new evidence comes in the form of

geochemical analysis of sedimentary layers at 25

archaeological sites across North America—9 of

them Clovis. Certain features of the layers, say the

team, suggest that they contain debris formed by an

extraterrestrial impact. These include spherules of

glass and carbon, and amounts of the element

iridium said to be too high to have originated on

Earth. In addition, the rocks contain black layers of

carbonized material, which the team says are the

remains of wildfires that swept across the continent

after the impact.

Passage 2

Proponents of the Younger Dryas impact

hypothesis have claimed various kinds of evidence

for the hypothesis, including deposits of the element

iridium (rare on Earth but abundant in meteorites),

microscopic diamonds (called nanodiamonds), and

magnetic particles in deposits at sites supposedly

dated to about 12,800 years ago. These claims were

sharply contested by some specialists in the relevant

fields, however, who either did not detect such

evidence or argued that the deposits had other causes

than a cosmic impact. For example, some say that

nanodiamonds are common in ordinary geological

formations, and that magnetic particles could come

from ordinary fires.

Now comes what some researchers consider the

strongest attack yet on the Younger Dryas impact

hypothesis. In a paper published recently in the

Proceedings of the National Academy of Sciences, a

team led by David Meltzer, an archaeologist at

Southern Methodist University, Dallas, in Texas,

looks at the dating of 29 different sites in the

Americas, Europe, and the Middle East in which

impact advocates have reported evidence for a

cosmic collision. They include sites in which

sophisticated stone projectiles called Clovis points,

used by some of the earliest Americans to hunt

mammals beginning about 13,000 years ago, have

been found. The team argues that when the quality

and accuracy of the dating—which was based on

radiocarbon and other techniques—is examined

closely, only three of the 29 sites actually fall within

the time frame of the Younger Dryas onset, about

12,800 years ago; the rest were probably either earlier

or later by hundreds (and in one case, thousands) of

years.

“The supposed Younger Dryas impact fails on

both theoretical and empirical grounds,” says

Meltzer, who adds that the popular appeal of the

hypothesis is probably due to the way that it provides

“simple explanations for complex problems.” Thus,

“giant chunks of space debris clobbering the planet

and wiping out life on Earth has undeniably broad

appeal,” Meltzer says, whereas “no one in Hollywood

makes movies” about more nuanced explanations,

such as Clovis points disappearing because early

Americans turned to other forms of stone tool

technology as the large mammals they were hunting

went extinct as a result of the changing climate or

hunting pressure.

But impact proponents appear unmoved by the

new study. “We still stand fully behind the [impact

hypothesis], which is based on more than a

confluence of dates,” says Richard Firestone, a

nuclear chemist at the Lawrence Berkeley National

Laboratory in California. “Radiocarbon dating is a

perilous process,” he contends, adding that the

presence of Clovis artifacts and mammoth bones just

under the claimed iridium, nanodiamond, and

magnetic sphere deposits is a more reliable indicator

that an extraterrestrial event was responsible for their

disappearance.