Published on:

Did a cosmic impact cause the Younger Dryas cooling?

latest Mind and Matter column
for the Wall Street

Scientists, it’s said, behave more like lawyers than
philosophers. They do not so much test their theories as prosecute
their cases, seeking supportive evidence and ignoring data that do
not fit-a failing known as confirmation bias. They then accuse
their opponents of doing the same thing. This is what makes debates
over nature and nurture, dietary fat and climate change so

But just because the prosecutor is biased in favor of his
case does not mean the defendant is innocent. Sometimes biased
advocates are right. An example of this phenomenon is now being
played out in geology over the controversial idea that a meteorite
or comet hit the earth 12,900 years ago and cooled the

That the climate suddenly cooled then, plunging the
Northern Hemisphere back into an ice age for 1,300 years, is not in
doubt. The episode is known as the Younger Dryas, because in
Scandinavia abundant pollen from a tundra flower called the
mountain avens, Dryas octopetala, reappears in soil from this date,
indicating that the forest had once more given way to tundra. With
the sudden arrival of cooler, drier and less predictable seasons,
early human attempts at agriculture in the Near East ceased, and
people returned to nomadic hunting and gathering.

The cause of this cold lurch was seemingly settled some
time ago when Wallace Broecker, a Columbia University geochemist,
suggested that a North American ice sheet collapsed, flooding the
Atlantic with fresh water, which interrupted the normal circulation
of the Gulf Stream. Then a marine geologist, James Kennett of the
University of California, Santa Barbara, said he had found evidence
of the impact of a large object from space 12,900 years ago, in the
form of carbon spherules in silt.

Dr. Kennett’s argument is that a swarm of meteorites
punched through the atmosphere and caused a vast conflagration,
filling the air with dust and soot. This shut out the sun, causing
decades of continuous winter -sufficient to trigger an advance of
ice sheets that, even when the dust cleared, kept the climate cool
for more than a thousand years, at least in the Northern

Dr. Kennett prosecuted his case with gusto, also
suggesting that the impact had extinguished North American
mammoths, just as an earlier impact had finished off the dinosaurs
(a theory hard to reconcile with the survival of mammoths for
thousands of years longer on islands off Siberia and Alaska, where
hunters could not reach them). He suffered a key setback in recent
years when several groups failed to find the right kinds of
spherules or otherwise duplicate the results of his team’s
work-and, worse, when a spherule sample from Younger Dryas rocks
proved to be only 135 years old.

But spherules, dated to the right period, now have
apparently shown up. Dr. Kennett and colleagues have published
evidence in the prestigious Proceedings of the National Academy of
Sciences that a “black mat” from the sediment of a Mexican lake
dates to 12,900 years ago and shows a sudden peak of magnetic and
carbon spherules, “nanodiamonds” of a kind known as lonsdaleite,
and charcoal: all of it evidence of extreme heat.

Last year Michael Higgins of the University of Quebec
published details of an underwater crater in the Gulf of St.
Lawrence, arguing that it may also date from as little as 12,900
years ago. The crater, three miles across, has the characteristic
central mound of a fresh meteorite impact. Its meteorite was
probably too small to shift the climate, but perhaps it was part of
a swarm.

After the previous debacles, the jury will take much
convincing that the new results can be replicated. But the burden
of proof has shifted a little in Dr. Kennett’s favor. After all,
Dr. Broecker and his followers, too, may be emotionally invested in
his ice-sheet theory: Confirmation bias can affect us