called the "killing mechanism," to tie the iridium enhancement to the precisely coincident mass extinction of most of the life in the sea. That exact coincidence can be seen by any interested person who drives up the Gubbio gorge, equipped with a geologist's hammer and a hand lens. If he strikes a fresh surface of limestone with his hammer, and looks closely at that surface with his hand lens, he will see many small fossil shells called forams, in every field of view. That is true for rocks anywhere below the K-T clay layer. The prominent forams with a diameter of about 1 millimeter stop abruptly at the lower edge of the clay layer, indicating that they underwent an extinction at the very instant of geological time when the clay layer was deposited on the bottom of the sea. That is also true of the ammonites and several other major marine taxa of the Cretaceous period. Walter aroused my interest in the clay layer (Figure 1) when he told me that it was laid down at very nearly the time of the demise of the dinosaurs. (The slanting sediments shown in figure 1 were of course laid down horizontally on the sea floor, but were tilted as they were raised up to form the Apennines.) Walter said that no one knew what made the clay layer, or what, if anything, it might have had to do with the extinction of the dinosaurs, but we will soon see that the answers to both of those questions are now well in hand. But it was clear that the clay layer was causally related to the extinction of the locai forams; if one tried to say that it was just a coincidence that the large forams disappeared within a millimeter of the bottom of the clay layer, it is easy to calculate the probability of such an accidental coincidence. If we remember that these most conspicuous forams are roughly constant in abundance from the clay layer downward for a kilometer, the odds against such a chance coincidence of two unrelated events is just the number of millimeters in one kilometer, or one million to one.
Frank has said that I tried out a new theory on him, once or twice a week, for six weeks, and shot each one down myself, before anyone else had a chance to do so. Finally, I tried the impact of a ten kilometer bolide, which in turn enveloped the earth in an opaque blanket of dust. While the dust fell in some months onto the ground, and into the ocean, it cut out the sun light, thereby stopping photosynthesis, so most of the animal life on earth died of starvation. And of course we now see the fallen dust as the worldwide K-T boundary layer. I'll describe a number of other consequences of the collision, all of which probably contributed to what has been called "the great dying."
We now know that it was extremely cold during the period of darkness,2 so it was just as though all the animals had been transported to present day Antarctica. Emiliani, Kraus and Shoemaker3 soon published a paper showing that after an impact in the ocean, the temperature would first go way down but would then increase as a result of the greenhouse effect, and the heating would be a major contributor to the killing mechanism. Perhaps the most important killing mechanism would derive from the shock-heating of the atmosphere, by the expanding fireball. It would give rise to the production of huge amounts of nitrogen oxides in the atmosphere, leading to highly acidic rain, and surface ocean waters with a pH so low as to dissolve the calcium carbonate shells of marine invertebrates. This important contribution came from a group at MIT (Lewis, Watkins, Hartman and Prinn)."' I stress it here, because our paleontological colleague, Bill Clemens, recently discovered dinosaur fossils in Alaska,5 and properly asked, "If dinosaurs could survive the darkness and cold of an Alaskan winter, why would they be bothered by the darkness and cold of your K-T scenario?" My best guess is that the answer lies in the rains, with their very high acidity—far beyond anything we now know as acid rain, plus the fact that it was warmer in Alaska during late Cretaceous times. Very recently, Dr. Prinn, of the MIT group said,6 "Essentially pure nitric acid would be pouring over about 10% of the global surface in the first few months." Some paleontologists have been saying for the past few years that although the impact probably took place, it was only the straw that broke the camel's back. Pure nitric acid should make a fairly strong straw! I have no way to check the validity of the MIT statement, but it does come after four years of discussions of this scenario, in one of the most prestigious chemistry departments in the world, so I think it should be taken seriously.
I have just discussed the need for a plausible killing mechanism, but first of all, we must convince ourselves that it is reasonable to assume that the earth was hit, 65 million years ago, by a bolide that was 10 kilometers in diameter. (Mt. Everest is 8.8 kilometers high.) I'll soon show how easy it is to calculate the effective diameter of the bolide, but to do so, we need to know that the world-wide K-T clay layer is everywhere enriched in iridium. That was the first prediction of the theory, and as we see in figure 3, iridium enhancements have now been seen at about 75 locations throughout the world, in rock layers 65 million years old. The numbers in the boxes give the surface density of iridium, in billionths of a gram per square centimeter, and if we add up the amount on each square centimeter of the earth's surface, we find a total of about half a million tons of iridium. If we use the meteoritic iridium abundance of 0.5 parts per million, as representative of solar system debris, it is a simple exercise in geometry to calculate the diameter of a spherical bolide that brings in that much iridium, and the answer is close to 10 kilometers, which is the number we listed in our 1980 paper, and that hasn't been improved on. (Walter has suggested that the unevenness in the surface density, from place to place, can be attributed to the well known Rayleigh-Taylor instability of hydrodynamics, and that suggestion can be, but hasn't yet been tested.)
I will now show you that the impact of a 10 kilometer diameter bolide in the last 50
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