Out with a Bang - Four Revolutions in the Earth Sciences - page 188

the identical result within the error. Using the argon analyses only, the authors put
the age difference between the impact and the K-T boundary at 5 ± 27 thousand
years. To the state of the art of modern science, the Chicxulub impact and the K-T
extinction happened at the exact same instant of geologic time.
Renne and colleagues noted that climate instability in the late Cretaceous could
have made the dinosaurs and other species more susceptible to the effects of the
impact. Perhaps so, but an event with the energy of 100 million megatons of TNT
might have needed no help from erupting volcanoes. Now to argue that volcanism
caused the extinction of the dinosaurs is like claiming that tobacco was the real
cause of death of a lifelong smoker who died in a head-on automobile collision.
The Deccan trap eruptions took place over hundreds of thousands of years, yet
the K-T extinction occurred at single instant, the exact instant that an asteroid
crashed to create the Chicxulub crater. If this is not cause and effect, it will have to
do until positive evidence for some other process comes along.
The Report of My Death Was an Exaggeration
The Alvarezes no doubt hoped they had discovered the first of several extinctions
caused by impact. 18 After all, besides the K-T there are four other great mass
extinctions and countless smaller ones. Now that scientists knew what to look
for—iridium spikes, glassy spherules, impact breccias, melt rocks, and shocked
minerals—surely they would quickly find other examples of extinctions caused by
impact. But by the thirtieth anniversary of the Alvarez theory, they had not done
so. Nevertheless, two extinctions, one relatively small and recent, the other ancient
and even larger than the K-T, were receiving new attention.
Just shy of 13,000 years ago, the world was warming as it emerged from the
last Ice Age. Then global temperature suddenly dropped and stayed low for 1,300
years. In North America the Clovis people, known for their distinctive fluted ar-
rowheads and spear points, disappeared, as did the bison, horse, and mammoth on
whom they used those weapons. An alpine-tundra wildflower called Dryas oco-
petala thrived in the cooler temperatures, lending the name Younger Dryas to the
period. The YD, as it is known, thus presented three mysteries at once: the temper-
ature drop, the disappearance of the Clovis culture, and the large mammal extinc-
tion. Some scientists believed that a change in Atlantic Ocean circulation might
haveledtothecooling,whichinturncouldhavecausedboththeClovispeopleand
the large mammals to vanish. Another school of thought held that Clovis hunters
had already driven the large animals near to extinction in advance of the cooling,