The sharp iridium peaks, the spinel spikes described in the last chapter, and the thin boundary clay tell us that whatever the origin of the K-T event, it did not last for millions of years, or even for 1 million years. As we have seen, some think it lasted for no more than 100 years! If Deccan volcanism caused the K-T event, the eruption of the traps must have started just prior to K-T time and lasted just beyond it, covering at most a few hundred thousand years. If the Deccan traps do not date exactly to the K-T boundary, or if they erupted over several millions of years, the volcanic alternative would itself be falsified.
Thus the crucial question is this: When did Deccan volcanism begin and how long did it last? One way to answer the question is to use the magnetic reversal time scale. Frenchman Vincent Cour-tillot and colleagues measured the magnetic reversal age patterns of the Deccan basalts and concluded that the eruptions began during Chron 30N, reached a maximum in Chron 29R, and were waning by the time of Chron 29N.12 This means that Deccan volcanism could have lasted for 1 million to 2 million years (see magnetic reversal time scale in Figure 11). Chron 29R is the time period during which the K-T event, whatever caused it, took place.
As noted before, magnetic reversal dating suffers from an inherent lack of precision, generally coming no closer than a few hundred thousand years. The ages that had been reported in the older literature for the Deccan traps were also anything but precise, ranging from 80 million down to 30 million years, a spread far too great to be plausible and now believed to have stemmed from the difficulty of applying the potassium-argon dating method to altered basalts. A newer technique, the argon-argon method, is more precise and has largely supplanted the older method. Its use narrowed the wide range of the older age results, leaving a spread that was small but real, from 67 million years down to 62.5 million years. As more measurements have been made, this spread has held firm.
To determine whether the Deccan eruptions have the same age as the K-T boundary, we must know the exact age of that boundary. So far, I have simply stipulated that the K-T event took place 65 million years ago, but without presenting any evidence. How is it that we date geologic boundaries precisely? As noted earlier, we cannot do so by using fossils, for they provide only relative ages. We cannot use the magnetic reversal scale because not only does it give a range of ages, ultimately it must tie back to radiometric dating using pairs of parent and daughter atoms, the only method to give absolute rather than relative ages. In short, to date the K-T boundary precisely we must find rocks and minerals from that time whose ages can be measured by one (for comparison ideally several) of the radio-metric techniques.
The great interest in the Alvarez theory naturally placed the then-existing estimates of the age of the K-T boundary under close scrutiny and led to a new set of measurements. Beginning in the mid-'980s, several precise analyses were made using different parent-daughter pairs; the results clustered closely around 65.0 million years. Thus we can say with rare assurance that the age of the K-T boundary is 65.0 million years, plus or minus a few hundred thousand. The K-T is surely the best dated of any of the major boundaries of the geologic time scale.
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