Geoscience Reference
In-Depth Information
methods yielded an age of 73.8 million years. And not only must
we know the age of a candidate crater, we must know the age of
the extinction boundary with which it might be correlated. But the
ages of many boundaries, and even their positions, have yet to be
pinned down.
When we consider all these uncertainties, the accidental finding
of the iridium spike at Gubbio, and the diligent search that led to
the discovery of the iridium-rich layer amid the lava flows and inter-
trappean sediments of the Deccan, appear all the more remarkable.
Even if impact has occurred at another geologic boundary, we could
easily miss it. If after a diligent search, however, no evidence of im-
pact has turned up, practical geologists, with limited time and re-
sources, would move on to fields with more chance of results. While
the absence of evidence may not be evidence of absence, it is dis-
couraging. Being human, scientists tend to go where positive evidence
and rewards can be found.
T
HE
B
IG
F
IVE
The K-T mass extinction was one of five in which more than
70 percent of species died. If there is anything to the notion that
impact has caused other mass extinctions, it is here, among the
other four, that we should first look. Table 4 summarizes the ages
of the Big Five plus the Eocene-Oligocene and Jurassic-Cretaceous
extinction boundaries, and the evidence of impact that has so far
been found associated with each. The three right-most columns give
age and size information for craters that happen to have the same
approximate age as the boundary. The table implicitly asks for each
of these extinctions: Is there any evidence of impact, and is there a
large crater of the same age?
THE LATE DEVONIAN
The earth guards its secrets. Each of the Big Five extinctions, when
examined in detail, turns out to be complicated and different. Take,
for example, the late Devonian extinction. The iridium there, not
high to begin with, appears to be strongly associated with the re-
mains of the bacterium
Frutexites.
Some experts believe that
Frutex-
ites
was able to extract and concentrate iridium from seawater, indi-
cating, they say, that iridium is not a reliable marker of impact after
all. Pro-impactors respond that
Frutexites
was able to concentrate
iridium just because an impact inserted excess iridium into the
oceans in the first place.
