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favors a mantle [volcanic] rather than meteoritic origin for these ele-
ments. These results are in accord with the scenario of a series of
intense eruptive volcanic events occurring during a relatively short
geologic time interval and not with the scenario of a single large aster-
oid impact event."
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Officer and Drake continued to press the claim that the spread
of iridium values was too great to be explained by bioturbation, cit-
ing evidence of iridium hills rather than spikes from other localities.
They again used Wezel's report of high iridium in samples far from
the K-T boundary at Gubbio, but they failed to cite the point that
the Alvarezes, Michel, and Asaro had made that at least some of
Wezel's anomalous iridium levels were due to contamination. Curi-
ously, Officer and Drake did not attempt at all to rebut the charges
made in 1984 by the Alvarez team, merely lumping them together
with several others under the catchall of "a variety of responses."
One coming late to the debate would never have known that the
"variety" included many substantive criticisms and an accusation of
outright error.
Jan Smit and UCLA's Frank Kyte responded that the Officer and
Drake bioturbation model "is inaccurately applied and inadequately
explains possible sedimentary effects for any given section."
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Smit
and Kyte describe what once must have been sharp microtektite lay-
ers that are now dispersed over an average of nearly 60 cm, showing
that bioturbation and reworking can affect far more than a few cen-
timeters and that a stretched-out iridium signature need not falsify
the impact theory. Since it was not even certain which mineral
phases contained the iridium, it is not hard to think of ways of
broadening a once-sharp peak. (1) Before the original sediments
that contained the iridium hardened into rock, they might have
been stirred by waves and then redeposited, which would have
smeared out any originally sharp peaks (reworking). (2) Sediments
rich in iridium derived from the impact cloud might have been
washed off the continents and into the ocean basins where they
would mix with other sediments being deposited there, a process
that could have taken hundreds or thousands of years and spread
iridium over a vertical distance. (3) Iridium might have been dis-
solved chemically from its original level in the boundary clay and
been reprecipitated up or down the section.
In 1988, my MIT graduate school colleague Jim Crocket, of Mc-
Master University in Hamilton, Ontario, an expert in measuring
the concentrations of platinum-group metals, reported a new set of
iridium results.
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With Officer and others, Crocket presented high
iridium values that spread for 2 m above and 2 m below the K-T
