Preemptive Strike

If the Alvarez theory is correct, the change from Cretaceous time to Tertiary time happened instantaneously, everywhere around the globe, in which case the K-T boundary clay can represent but a mere eyeblink of geologic time—a few hundred or a few thousand years at most. Furthermore, the boundary would have to be the same age everywhere. Officer and Drake claimed to have evidence that, on the contrary, the age of the boundary differs by hundreds of thousands of years at different locales. If the K-T boundary has one age at one site and quite a different age at another, it obviously was not created instantaneously and the impact theory is falsified. On the other hand, without impact as its cause, the K-T boundary is likely to have somewhat different ages at different locations around the globe. This surprising fact has its roots in the way geology began.

As their understanding of the earth developed, early geologists began to recognize that they could define distinctive rock units that differed both from the older rocks below and the younger ones above. More than a century ago, geologists began to give names to these characteristic units that they could trace over wide distances: Cambrian, Ordovician, Devonian, and so on. In this way the standard geologic column—the ideal sequence if all rock units were pre sent, none having been removed by erosion—was constructed. It is the basis for the subdivisions shown in Figure 2. Since in a given geographical area only a limited portion of the geologic column is exposed, its fundamental units were sometimes situated in entirely different countries, leaving no way to correlate them precisely. Near the end of Cretaceous time, different types of rocks were being deposited in different environments: a limestone on the undersea shelf near one continent; a sandstone on a beach halfway around the globe; a shale in a swamp on another continent. In the absence of a worldwide, short, terminal event, these processes would not have ended at exactly the same time, and thus the K-T boundary would have a slightly different age at different places around the world.

Compare, for example, the K-T boundary at Gubbio with that at Hell Creek, Montana, source of Tyrannosaurus rex and the best-studied dinosaur fossils in the world. As shown in Figure 3, the K-T boundary at Gubbio is easy to spot—you can place your finger right on it. On the other hand, at Hell Creek the boundary is exceedingly difficult to locate, or even to define (it is described in the literature as "above the highest dinosaur fossil and just below the level of the lowest coal bed," neither of which occurs at Gubbio). Let us suppose, however, that we could find a boundary at Hell Creek that we believe demarcates the K-T. How could we determine whether it is of exactly the same age as the K-T boundary at Gubbio? We cannot do so by comparing fossils, because those at Gubbio are marine microfossils (foraminifera) whereas the rocks at Hell Creek were formed in freshwater and contain dinosaur and mammal bones, but no forams. One way to determine rock ages precisely is through the use of a pair of elements in which one, the parent, decays radioac-tively into the other, the daughter, as when uranium decays into lead. If one knows how much uranium is present in a sample, and how much lead, and one knows how fast uranium decays into lead (the half-life), one can calculate how long the process of decay has been going on in that sample, and thus derive the age of the rock. But none of the rocks from Gubbio has enough of the parent element or occurs in close association with the volcanic rocks that are best suited for parent-daughter (radiometric) dating. Even if the parent-daughter methods could be used, however, they are insufficiently precise for exact correlation.

Very well, if we cannot show that two rocks from the same section of the geologic column on different continents are exactly the same age, can we do the opposite and show that their ages differ measurably? Not by using forams on one continent and dinosaurs on another, nor by using radioactive parent-daughter ages, where the same lack of precision remains a limitation. But there is one possibility: the magnetic reversal time scale that led Walter Alvarez to Gubbio in the first place. It would be ironic if that same scale could be used to falsify the Alvarez theory, but that is exactly what Officer and Drake claimed to have done. Here is the basis for their approach.

As briefly described in Chapter 1, over the past few decades, geo-physicists have established that the earth's magnetic field has repeatedly reversed its polarity.3 The north magnetic pole has acted alternately in the way we define a north magnetic pole as acting, then as a south magnetic pole, then as a north pole again, and so on, over and over, throughout hundreds of million of years. These reversals have affected the entire magnetic field, all around the globe. During a period of reversed magnetism, a compass needle, which seeks a north magnetic pole, would instead point toward magnetic south. It is not known why the earth's magnetic field reverses, though supercomputer modeling is beginning to shed light on the mystery. But remember that we discover facts and invent theories. We have discovered that the earth's magnetic field has reversed itself hundreds of times, on the average about every 500,000 years; so far we have not been inventive enough to figure out why.

Magnetized rocks of different ages around the world have been dated using one of the radioactive parent-daughter pairs, and thus we know, within the precision of those methods, when each reversal occurred. The K-T section of the magnetic reversal time scale is shown in Figure 11. The major intervals are represented by shaded and light bands, called chrons, numbered and designated R for reversed and N for normal. Ordinarily, the magnetic time scale suffers from the same lack of precision as the other methods and cannot be used to show that two rocks have precisely the same age. For example, if all we know is that two rocks belong to Chron 29R, we have not pinned their ages down to better than ±750,000 years, the duration of that chron. But suppose on the other hand that we can establish that one rock unit belongs to 29R while the other belongs to 28R. We can then be certain that the two are not of the same age and that their ages must differ by at least 800,000 years, the duration of intermediate Chron 29N. (The absence of 29N in the region under study means either that rocks from that age were never deposited, or that they were subsequently removed by erosion.) Thus, paleomagnetism may be able to show that although two rock layers date to the same general part of the geologic time scale, they do not have identical ages. This was the opening that Officer and Drake hoped to exploit.

figure i i The magnetic reversal time scale around K-T time. Dark bands represent periods of normal magnetization (like today); light bands show reversed magnetization.[ After Berggren et al.4]

In their 1983 paper in Science, Officer and Drake reviewed the literature on magnetic reversals near the K-T boundary, focusing their attention on six deep-sea drilling sites and nine continental sites, including Gubbio and Hell Creek. Most fell into Chron 29R, providing no support for their thesis, but three did not, giving Officer and Drake a foothold. A sample that came from a deep-sea core, and another that came from the San Juan Basin in Colorado, appeared to belong to 29N; the third sample, from Hell Creek, they assigned to 28R. If these interpretations were correct, the K-T boundary was not the same age everywhere and therefore could not have been produced by an instantaneous event. The Alvarez theory would have been preemptively falsified, the battle would have been over before it began, and geologists could return to business as usual.

Officer and Drake's argument received a quick rebuttal from the Alvarezes, who accused the pair of breaching scientific etiquette by failing to cite any of the papers presented at the 1981 Snowbird I conference, even though first the abstracts and then the entire volume of papers from the conference had been published and even though Drake himself had not only attended the meeting but, months before the Science paper appeared, had published a critique of some of the papers presented there.5'6 Why was this a serious error? Because scholarship is cumulative, with each generation standing on the shoulders of those who have gone before. To fail to cite relevant papers that one knows about is a grievous error (to overlook one you should have known about is bad enough): It cheats the authors of those papers of their rightful recognition; it misleads readers who are not expert in the subject at hand; and it avoids contradictory evidence, thereby falsely fortifying your own position. Most scientists would say there are only two reasons for failure to cite a relevant paper: ignorance or dishonesty. Neither seemed to be an explanation in this case, first because Officer and Drake clearly knew of the Snowbird I conference and report, and second because any attempt to cheat in discussing such a controversial matter in the most widely read scientific journal would have been instantly apparent. Not only is the failure of Officer and Drake to cite the Snowbird I report a mystery, so is why their oversight was not corrected during the peer-review process.

The Alvarez team accused Officer and Drake of making another scientific error: ignoring explicit warnings from the authors of the original papers on paleomagnetism that their data might be unreliable. Subsequent writers ought not give more credence to data than those who report them in the first place. Officer and Drake began their description of the deep-sea core, which appeared to place the K-T boundary in Chron 29N, by quoting the original paper, where it was described as "the most complete biostratigraphic record of the Cretaceous-Tertiary transition."7 However, the original authors also said that this particular core had been disturbed during drilling and affected by stirring of muds by burrowing organisms (an effect known as bioturbation), both of which would have upset the magnetic patterns, making correct interpretation difficult to impossible.8 By failing to cite published, negative evidence, Officer and Drake appeared to be unfairly favoring their own position. They also failed to cite detailed magnetic studies from the San Juan Basin that had indicated at first that a particular magnetic reversal was present there that had been found nowhere else, which was so unlikely as to call into question the interpretation of the magnetic reversal data.9 After further work led to the correction of the discrepancy, the K-T boundary at that site was found to fall in Chron 29R, which Officer and Drake neglected to reveal. This left the Montana magnetic result as the only one remaining from their original set of 15 in which the K-T boundary appeared to reside somewhere other than in Chron 29R. Officer and Drake claimed that "The magnetic stratigraphy observations cover only a short interval of geologic time, but on the basis of faunal correlations with the San Juan Basin, we infer that the reversed interval is 28R."10 In this case they did refer to the original paper, but failed to note that the original authors had also warned that "The magnetic zones recorded in these terrestrial sections in Alberta, Montana, and New Mexico cannot be securely correlated with the magnetic polarity time scale."''

Officer and Drake had offered evidence from three geologic sections that they claimed showed that the K-T event took place at different times around the world and therefore could not have been the result of an instantaneous global catastrophe, which if true would falsify the Alvarez theory (argument la). When the original papers that they cited were reviewed in detail, however, their claimed evidence was found either to be nonexistent or to be in serious doubt. (Later work confirmed that Officer and Drake were indeed wrong: Wherever it has been studied, the K-T boundary falls firmly within Chron 29R.)

In a '984 paper, Walter and Luis Alvarez, Asaro, and Michel had the last word: "A review by scientists who have not been active in the field might have been valuable if it had been balanced, but unfortunately, Officer and Drake use a double standard, in which they apply keen scrutiny to evidence favoring the impact theory— as, of course, they should—but uncritically accept any results, no matter how flawed, that contradict it. They fail to mention most of the data that support the theory. Instead, they fix their attention on a few cases that can be made to look like contradictions."'2

The title of Officer and Drake's '983 paper in Science, "The Cretaceous-Tertiary Transition," conveyed the message of their argument lb, showing that the change from the one geologic period to the other had not been sudden, much less instantaneous. In their view, there had been instead a finite "transition," a gradual shift, consistent with uniformitarianism. Under the impact scenario, the time taken by the transition "should be zero," they argued, and if the fossils on one side of the boundary had been gradually replaced by those on the other, rather than disappearing suddenly right at the K-T boundary, the event could not have been instantaneous. But as we will see, the fossil record can be hard to read. Fossil-bearing sediments, after being deposited initially, can be stirred by waves and redeposited, upsetting the original stratigraphy in a process called reworking. Burrowing organisms carry material from one stratigraphic level to another, mixing up the sedimentary and fossil record and making once sharp peaks appear gradual (bioturbation). Officer and Drake did acknowledge this difficulty, writing, "Bioturbation is an important process affecting marine sedimentary sequences and can blur or obscure transition events," but never again in their paper did they refer to the process as having any actual effect.'3 Thus they appear to have paid only lip service to bioturbation. Because of the indeterminacy introduced by reworking and bioturbation, pro-impactors regarded the Officer-Drake argument from fossils as completely unproved and no threat to the new theory.

Officer and Drake ended their abstract with this plea: "It seems more likely that an explanation for the changes during the K-T transition will come from continued examination of the great variety of terrestrial events that took place at that time, including extensive volcanism, major regression of the sea from the land, geochemical changes, and paleoclimatic and paleoceanographic changes."14 The Alvarezes, Asaro, and Michel responded that "Officer and Drake's article seems to be a plea for a return to the time before the iridium anomaly was discovered, when almost any speculation on the K-T extinction was acceptable. This idea is pleasantly nostalgic, but there is by now a large amount of detailed astronomical, geological, pale-ontological, chemical, and physical information which supports the impact theory. Much interesting work remains to be done in order to understand the evolutionary consequences of the impact on different biologic groups, but the time for unbridled speculation is past."15

Thus both counterarguments concerning the timing of the K-T boundary and extinction failed: Officer and Drake were unable to falsify the Alvarez theory preemptively by placing the K-T boundary in different magnetic chrons (argument la). Due to the nature of the fossil record, they could not show convincingly that changes in fossil abundances had not taken place in zero time—reworking and bioturbation made it difficult if not impossible to say how long the transition took (argument lb).

Beyond the disagreements that proponents of the Alvarez theory had with Officer and Drake over scientific facts and interpretations, the failure of the pair to follow standard scientific procedures aroused suspicion of their motives and their modus operandi. In their papers in Science, Officer and Drake at times failed to cite relevant papers and contradictory evidence, ignored cautions of primary authors about the reliability of data, used techniques that were more common to debating tournaments than scientific literature, and appeared to pay only lip service to facts that complicated their arguments. The impression created by their methods sowed seeds of distrust that were to bear a bitter fruit.

Officer and Drake soon turned to the second part of their attack: to show that the evidence of impact presented by its supporters—the iridium spike, the shocked minerals, the microtektite-like spherules— were not markers of an extraterrestrial event after all.

Was this article helpful?

0 0

Post a comment