Geoscience Reference
In-Depth Information
I
RIDIUM
H
ILLS
Even though later evidence appeared to offer more support for the
Alvarez theory than the iridium findings, it was the Gubbio iridium
anomaly that sent the Alvarezes down the trail of impact in the first
place. If that particular evidence were weakened or falsified the
entire theory would be in jeopardy. Iridium had already met two
tests: It proved uncommon in the geologic record and, as shown by
the presence of the iridium spike in freshwater rocks from the Raton
Basin, did not come from seawater. Officer and Drake focused on
two other ways the iridium evidence might have resulted from some-
thing other than meteorite impact. Their first claim was that iridium
was not concentrated in a sharp peak, but spread out above and
below the K-T boundary. On a graph of the amount of iridium found
at different depths, instead of a sharp peak, there would be a "hill." If
such a spread of iridium could be shown not to have been produced
by reworking or bioturbation, then the iridium could not have been
emplaced by an instantaneous event such as meteorite impact. Sec-
ond, if it could be shown that normal geologic processes can concen-
trate iridium, the Alvarez theory would not be required to explain
the high iridium concentrations and the way would be open for a
uniformitarian alternative.
In their 1983 paper, the two authors had claimed that in some
deep sea drill cores that capture the K-T boundary, instead of being
concentrated in a spike, iridium is spread over as much as 60 cm
(2 ft). They cited the measurements of F. C. Wezel, who had
reported high iridium at Gubbio from levels well above and well
below the boundary clay.
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6
In a black shale 240 m below the bound-
ary, equivalent to millions of years of sedimentation before the K-T
boundary, Wezel reported an iridium anomaly twice that of the
boundary clay (which, if true, would falsify prediction 2 discussed
in Chapter 4). When the Alvarezes, Michel, and Asaro attempted
to reproduce Wezel's results, however, they could not. They attrib-
uted the discrepancy to contamination in Wezel's laboratory, which
they said is "all too easy in chemical analytical work at the parts-per-
billion level."
1
7
In 1985, Officer and Drake launched a much more broadscale at-
tack on the Alvarez theory. They repeated their earlier claims and
upped the ante: "The geologic record of terminal Cretaceous environ-
mental events indicates that iridium and other associated elements
were not deposited instantaneously but during a time interval span-
ning some 10,000 to 100,000 years. The available geologic evidence
