Iridium 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 something 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. Second, if it could be shown that normal geologic processes can concentrate 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." In a black shale 240 m below the boundary, 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 attributed the discrepancy to contamination in Wezel's laboratory, which they said is "all too easy in chemical analytical work at the parts-perbillion level."17

In 1985, Officer and Drake launched a much more broadscale attack on the Alvarez theory. They repeated their earlier claims and upped the ante: "The geologic record of terminal Cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10,000 to 100,000 years. The available geologic evidence favors a mantle [volcanic] rather than meteoritic origin for these elements. 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 asteroid impact event."18

Officer and Drake continued to press the claim that the spread of iridium values was too great to be explained by bioturbation, citing 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. Curiously, 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."19 Smit and Kyte describe what once must have been sharp microtektite layers that are now dispersed over an average of nearly 60 cm, showing that bioturbation and reworking can affect far more than a few centimeters 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 dissolved 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.20 With Officer and others, Crocket presented high iridium values that spread for 2 m above and 2 m below the K-T

boundary at Gubbio, representing some 300,000 years of sedimentation. 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.21 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.22 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 samples analyzed in several independent laboratories, in what is called a blind test, with none of the laboratories knowing the exact derivation 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.23

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 deposition, 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 section 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 to be primary and to record an instantaneous and singular event. For this reason, sharp iridium peaks corroborate the Alvarez theory to a far stronger degree than the negative or indeterminate evidence of iridium hills at other locations detracts from the theory. One peak is sufficiently likely to be primary as to be worth several hills.

In '996, A. D. Anbar and his co-workers at Caltech used ultrasensitive techniques to measure the minute amounts of iridium in rivers and the sea.24 They found the K-T boundary clay to contain ',000 times as much iridium as all the world's oceans put together, confirming that the iridium did not precipitate from normal sea-water. They also determined that iridium, once present in the oceans, remains there for some '0,000 to '00,000 years before it is removed by sedimentation, providing yet another way to explain the iridium hills: They could merely be the result of the long residence time of iridium once it had been injected into the oceans by meteorite impact.

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