Geoscience Reference
In-Depth Information
boundary at Gubbio, representing some 300,000 years of sedimen-
tation. The authors ruled out bioturbation by citing their own work,
referring to the claim made by Officer and Drake in 1985 that bio-
turbation affects only 5 cm of rock on the average, far less than the
4-m spread observed.
In the spring of 1988, Robert Rocchia and an international group
of pro- and anti-impactors returned to Gubbio to remeasure the
magnetic stratigraphy and the iridium distribution.
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1
They could not
reproduce the high iridium readings above and below the boundary
reported by Crocket et al. In 1990, Walter Alvarez, Asaro, and Ales-
sandro Montanari measured a detailed iridium profile across 57 m of
the K-T section at Gubbio, which represents about 10 million years
of sedimentation.
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2
They found an iridium anomaly of 3,000 ppt
exactly in the K-T boundary clay, with small molehills of 20 ppt-
80 ppt on either side, fading away to the background level of about
12 ppt. Their results essentially matched those of Robert Rocchia and
his colleagues.
When responsible authorities come to different conclusions over
what is essentially an analytical matter (how much of an element is
present in a set of samples), the best procedure is to have the sam-
ples analyzed in several independent laboratories, in what is called a
blind test, with none of the laboratories knowing the exact deriva-
tion of the individual samples. Robert Ginsburg of the University of
Miami supervised the collection and distribution of samples from
Gubbio. When the results were returned, only a single iridium peak
had been found, though it did retain its adjacent shoulders.
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3
This debate reveals the difficulty of saying whether the vertical
spread of a chemical signature such as iridium's indicates that the
element was deposited with that distribution, as Officer and Drake
argued, or whether instead it was deposited in a sharp peak that was
later degraded by secondary processes, as the Alvarez team would
have argued had they believed the data. To rephrase the question: Is
a spread-out iridium hill, as opposed to a sharp peak, a primary or a
secondary feature? We certainly know of processes that can degrade
a sharp peak into a hill: reworking, bioturbation, erosion and deposi-
tion, and chemical solution and reprecipitation. Since 1980 when the
Alvarez theory appeared, processes have also been discovered that
can remove the iridium naturally present in minute amounts in a sec-
tion of rock and concentrate it at a particular geologic level, thus
turning a broad distribution into a peak. However, these processes do
not appear to be able to produce the high iridium levels found at the
K-T boundary. Thus it seems safe to conclude that the sharp peaks
found at Gubbio and the Raton Basin, for example, are highly likely
