PASADENA, Calif. — Though the dinosaurs fared poorly in the comet or meteor
impact that destroyed two-thirds of all living species 65 illion years ago,
new evidence shows that various other forms of life rebounded from the
catastrophe in a remarkably short period of time.

In the March 9 issue of the journal Science, a team of geochemists reports
that life was indeed virtually wiped out for a period of time, but then
reappeared just as abruptly only 10,000 years after the initial collision.
Further, the evidence shows that the extinctions 65 million years ago,
which mark the geologic time known as the Cretaceous-Tertiary (K-T

boundary, were most likely caused by a single catastrophic impact.

“There’s been a longstanding debate whether the mass extinctions at the K-T
boundary were caused by a single impact or maybe a swarm of millions of
comets,” says lead author Sujoy Mukhopadhyay, a graduate student at Caltech.
“In addition, figuring out the duration of the extinction event and how
long it took life to recover has been a difficult problem.”

To address both questions, Mukhopadhyay and his colleagues measured the
amount of cosmic dust in the sediments of an ancient sea bed which is now
exposed on land about 100 miles north of Rome. In particular they focused
on a two-centimeter-thick clay deposit that previously had been dated to
about 65 million years ago. The base of this clay deposit corresponds to
the date of the extinction event.

The clay deposit lies above a layer of limestone sediments, which are
essentially the skeletons of microscopic sea life that settled at the
bottom of the ancient sea. The limestone deposit also contains a certain
percentage of clay particles, which result from erosion on the continents.
Finally, mixed in the sediments is extraterrestrial dust that landed in
Earth’s oceans and then settled out. This dust carries a high concentration
of helium-3 (3He), a rare isotope of helium that is depleted on Earth but
highly enriched in cosmic matter.

The lower limestone layer abruptly ends at roughly 65 million years, since
the organisms in the ocean were suddenly wiped out by the impact event.
Thus, the layer immediately above the limestone contains nothing but the
clay deposits and extraterrestrial dust that continued to settle at the
bottom of the ancient sea. Immediately above the two-centimeter clay
deposit is another layer of limestone deposits from microorganisms of the
sea that eventually rebounded after the catastrophe.

In this study, the researchers measured the amount of 3He in the sediments
to learn about the K-T extinction. They reasoned that a gigantic impact
would not change the amount of 3He in the clay deposit. This is because
large impacting bodies are mostly vaporized upon impact and release all
their helium into the atmosphere. Because helium is a light element, it
is not bound to Earth and tends to drift away into space. Therefore, even
if a huge amount were brought to Earth by a large impact, the 3He would
soon disappear and not show up in the sedimentary layers.

In contrast, 3He brought to Earth by extraterrestrial dust tends to stay
trapped in the dust and not be lost to space, says Kenneth Farley,
professor of geochemistry at Caltech and coauthor of the paper. So 3He
found in the limestone and the clay deposits came from space in the form
of dust.

Based on the 3He record obtained from the limestones, the researchers
eliminated the possibility that a string of comets had caused the K-T
extinctions. Comets are inherently dusty, so a string of them hitting
Earth would have brought along a huge amount of new dust, thereby
increasing the amount of 3He in the lower limestone deposit.

But the Italian sediment showed a steady concentration of 3He until the
time of the impact, eliminating the possibility of a comet swarm. In fact,
the researchers found no evidence for periodic comet showers, which have
been suggested as the cause of mass extinction events on Earth.

Mukhopadhyay and his colleagues reason that because the “rain-rate” of the
extraterrestrial dust from space did not change across the K-T boundary,
the 3He concentration in the clay is proportional to the total depositional
time of the clay. “It’s been difficult to measure the time it took for this
two-centimeter clay layer to be deposited,” says Farley.

The researchers conclude that the two-centimeter clay layer was deposited
in approximately 10,000 years. Then, very quickly, the tiny creatures that
create limestone deposits reemerged and again began leaving their corpses
on the ocean bed. The implication is that life can get started again very
quickly, Farley says.

Thus the study answers two major questions about the event that led to the
extinction of the dinosaurs, says Mukhopadhyay. In addition to Mukhopadhyay
and Farley, the paper is also authored by Alessandro Montanari of the
Geological Observatory in Apiro, Italy.

Related Links

* Dr. Ken Farley, Associate Professor of Geochemistry

* The Division of Geological and Planetary Sciences at Caltech

* Science