Asteroid impact

In the late 1970s, geologist Walter Alvarez and a team of co-workers (Figure 15.1) were studying K/T marine outcrops now exposed on land near a town called Gubbio, in Italy. They were struck by the fact that the lower half of the Gubbio outcrop is composed of a rock made up entirely of thin beds of the microscopically sized shells of Cretaceous marine organisms. The upper half of the exposure was almost exclusively of thin beds of the microscopic shells of Tertiary marine organisms. Between the two was a thin (2-3 cm) layer of clay, the K/T boundary.

Analyses showed that the clay layer contained unusually high concentrations of iridium, a rare, platinum group metal.2 Instead of the expected amount at the Earth's surface, about 0.3 parts per billion (ppb), the iridium content was a whopping 10 ppb at Gubbio. So the iridium anomaly, as it came to be called, contained about 30 times as much iridium as Alvarez and his co-workers had expected to find (Figure 15.2).

Iridium is normally found at the Earth's surface in very low concentrations, but it is found in higher concentrations in the core of the Earth and from extraterrestrial sources; that is, from outer space. Given that, the Alvarez group determined that the source of the iridium had to be extraterrestrial. The deal was sealed when they found iridium anomalies at two other K/T sites, one in Denmark and the other in New Zealand. With stunning intuition, they concluded that at 65.5 Ma, an asteroid had to have smacked into Earth, delivering the iridium and, coincidently, causing the K/T mass extinction. Luis Alvarez, Nobel Prize-winning physicist, and a member of the team, described the relationship between an asteroid impact and the iridium layer in this way:

When the asteroid hit, it threw up a great cloud of dust that quickly encircled the globe. It is now seen worldwide, typically as a clay layer a few centimeters thick

2. It is a common misconception that iridium metal is toxic and deadly. In fact, like its chemical relatives gold and platinum, it is quite unreactive. For those with significant disposable incomes, boutique fountain pens and watches made with iridium are available.

Figure 15.1. The team of University of California (Berkeley) scientists who first successfully proposed the theory of an asteroid impact at the K/T boundary. Left to right: geochemists Helen V. Michel and Frank Asaro, geologist Walter Alvarez, and physicist Luis Alvarez.

in which we see a relatively high concentration of the element iridium - this element is very abundant in meteorites, and presumably in asteroids, but is very rare on earth. The evidence that we have is largely from chemical analyses of the material in this clay layer. In fact, meteoric that is, extra terrestrial iridium content is more than that of crustal material by nearly a factor of 104. So, if something does hit the earth from the outside, you can detect it because of this great enhancement. Iridium is depleted in the earth's crust relative to normal solar system material because when the earth heated up [during its formation] and the molten iron sank to form the core, it "scrubbed out" [i.e., removed] the platinum group elements in an alloying process and took them "downstairs" [to the core].

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Because the three sites are distributed around the globe, Alvarez and co-workers calculated that the asteroid had to have been about 10 km (about 6 miles) in diameter to spread an iridium dust layer globally.

In the intervening years, a tremendous amount of work has been done to explore the possibility of an asteroid impact at 65.5 Ma. Most importantly, the number of K/T sites with anomalous concentrations of iridium at the boundary has reached well over 100 (Figure 15.3). Moreover, the iridium anomaly was discovered on land (Figure 15.4) as well as in ocean sediments, affirming that it is a global phenomenon.

Shocked quartz and microtektites also came to be recognized as part of the fingerprint left by the asteroid. "Shocked quartz" is the name given to quartz that has been placed under such pressure that its molecular structure becomes deformed (Figure 15.5). It is now recognized that the kind of pressure that can cause such deformation could only be generated by impacts; indeed, shocked quartz is now known from many different impact sites, and has come to be recognized as a diagnostic criterion for meteor impacts.

Microtektites are small, droplet-shaped blobs of silica-rich glass. They represent mat erial thrown up into the atmosphere in a molten state due to the tremendous energy released when a meteor strikes the Earth. Quick cooling occurs while they're still airborne and then they plummet down on Earth as a rain of solid, glassy blobs.

Height above or below K/T (cm)

2468 Iridium abundance (ppb)

The "smoking gun". As early as 1981, a bowl-shaped structure 180 km in diameter, buried under many meters of more recent sediments

Figure 15.2. The iridium (Ir) anomaly at Gubbio, Italy. The amount of Ir increases dramatically at the clay layer to 9 parts per billion (ppb), and then decreases gradually above it, returning to a background count of about 1 ppb. On the right are numbers representing the thickness of the rock outcrop; on the left the time intervals (in millions of years (Myr)) and rock types are identified. Note that the vertical scale is linear close to the K/T boundary, but logarithmic away from the boundary, to show results well above and well below the boundary.

Figure 15.3. More than one hundred and three known iridium anomalies around the world.
Figure 15.4. The iridium-bearing clay layer in Montana; one of the first localities on land where anomalous concentrations of iridium were discovered.

was reported from the Yucatán Peninsula of Mexico, in the region near the town of Chicxulub (translated approximately as "devil's tail;" Figure 15.6). Ten years later, drill cores taken through the structure revealed shocked quartz: Chicxulub was a buried impact structure.

At about the same time, an approximately 1 m thick sequence of glass was discovered in Haiti, suggesting that the source of the glass had to be somewhere, relatively nearby. Its chemical composition was shown to be the same as the composition of the rocks that make up the Chicxulub structure.

The pieces really started falling into place. Several years earlier (1988) evidence of a tsunami in K/T deposits in the Gulf Coast region of Texas had been reported. The Chicxulub site was well situated to produce the tidal wave deposits recognized in the sedimentary record. Finally, the Chicxulub structure was dated at 65.5 Ma, the time of the K/T boundary.

Figure 15.5. Shocked quartz from the terrestrial K/T boundary in eastern Montana. The etched angled lines across the face of a grain of quartz represent a failure of the crystal lattice along known crystallographic directions within the mineral. Grain is 70 pm across (1 pm = io-6 m).

Further study of Chicxulub below the surface of the Earth, using sophisticated geophysical techniques, showed a bullseye pattern with a circular peak and large concentric rings around it, representing topography preserved in buried rocks below the surface (Figure 15.7). Interestingly enough, the northwest part of the outermost ring is broken through. The distinctive ring pattern suggests that a large asteroid, 10-15 km in diameter3 approached Earth from the southeast at a low angle of about 30°. The distribution of iridium, shocked quartz and microtektites across the Western Interior of North America (north and west of the crater) reinforce the idea of a low-angle, directional impact (Figure 15.8).

What did the asteroid do to Earth when it struck? Numerous scenarios were initially proposed, most of them inspired by post-nuclear apocalyptic visions. Of these, only a few remain current:

• Blockage of sunlight. It was initially hypothesized that sunlight would have been blocked from the Earth for about three to four months. This would have caused a cessation of photosynthesis and a short-term temperature decrease (now called an impact winter).

• Infra-red radiation pulse. It has been theorized that tremendous amounts of energy in the form of infra-red radiation and heat must have been released immediately upon impact. The initial global heat release at ground zero might have been 50 to 150 times as much as the energy of the sun as it normally strikes Earth. One group of scientists likened this radiation at the Earth's surface to an oven left on the broil.

• Global wildfires. With so much instantaneous heat production, fires might have broken out spontaneously around the globe. Soot-rich horizons from five K/T sites in Europe and New Zealand have been identified, in which the amount of the element carbon (the soot) was enriched between 100 and 10,000 times over background. The soot has been attributed to wildfires, perhaps the result of the infra-red heat pulse.

3. Recall that previous estimates of size were based upon the global distribution of the ejecta; this estimate was based upon the morphology of the impact site.

Figure 15.5. Shocked quartz from the terrestrial K/T boundary in eastern Montana. The etched angled lines across the face of a grain of quartz represent a failure of the crystal lattice along known crystallographic directions within the mineral. Grain is 70 pm across (1 pm = io-6 m).

All of these catastrophic effects are short term, which means that they affected the globe for days, months, or at most a few years. In a longer-term sense, that is on eological time scales (104-106 years), climates were little affected by the asteroid impact. What we know of climates in the latest Cretaceous suggests that they did not differ significantly from those in the early Tertiary.

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