Luis Walter Alvarez - Dinosaur Extinction

Dinosaur Extinction

In 1980 Alvarez and his son, geologist Walter Alvarez, along with nuclear chemists Frank Asaro and Helen Michael, "uncovered a calamity that literally shook the Earth and is one of the great discoveries about Earth’s history"

Walter Alvarez was doing geological research in central Italy during the 1970s on the walls of a gorge whose limestone layers included strata both above and below the Cretaceous–Paleogene boundary, also called the K-T boundary, the boundary between the Cretaceous and Paleogene periods corresponding to a time of 65.95 million years ago (Kuiper et al., 2008). Exactly at the boundary is a layer of clay about 1 cm thick. Walter removed a small piece of the rock containing both sections of limestone and the clay layer and later showed it to his father. Walter told his father that the layer marked where the dinosaurs and much else went extinct and that nobody knew why, or what the clay was about - it was a big mystery and he intended to solve it.

Alvarez first sought to determine how long it had taken to lay down the centimeter of clay. Although the timeframe was not easy to deduce considering the events happened 65 million years ago, the slow deposition of iridium on Earth from the cosmos offered a kind of geological "clock" to answer the question. Elements of the platinum group, including iridium, are rare in interplanetary space, but much more abundant than on Earth. The Earth's crust contains very little iridium, but micrometeorites constantly bombard the planet. As they burn up in the atmosphere, they lightly dust the surface with iridium at a constant and known rate. Alvarez reasoned that if the clay layer contained relatively high concentrations of iridium, then the layer had formed over a long time, whereas if it contained very little, then the layer had formed in a short time.

Alvarez had access to the nuclear chemists at the Lawrence Berkeley Laboratory and was able to work with Frank Asaro and Helen Michel. The chemists used a technique known as neutron activation analysis and were astounded to discover that exactly at the clay boundary the iridium content was enormous, but not in the limestone on either side. Whatever had caused the iridium content in the clay, it was far too high to have come from micrometeorites. Carefully checking their work, they next determined if the clay from other locations contained the same level of iridium (the K-Pg clay is well known and is distributed world wide), which it did. In 1980, Alvarez, Alvarez, Asaro, and Michel published the seminal paper proposing an extraterrestrial cause for the Cretaceous-Tertiary extinctions. Within a few years after that publication, more than 100 iridium-containing clay sites were found. The team, knowing of no terrestrial source which could produce and deliver so much iridium, concluded that the source had to be extraterrestrial. In the years following the publication of their article, the clay was also found to contain soot, glassy spherules, shocked quartz crystals, microscopic diamonds, and other rare minerals formed only under conditions of great temperature and pressure.

The team considered a number of possible sources for the iridium anomaly; the passage of Earth through giant nebular clouds, a nearby supernova, and other low probability scenarios. With time, effort, and subsequent experimentation, all of these were eliminated, leaving a direct impact on the earth by a comet or an asteroid as the only hypothesis which could satisfy all of the conditions, the impact hypothesis, a significant challenge to current theory.

Publication of the 1980 paper spurred criticism from the geological community, and an often acrimonious scientific debate ensued. Ten years after this initial proposal, and after Alvarez's death, evidence of a huge impact crater called Chicxulub was found off the coast of Mexico near the town of Chicxulub. This strongly supported the theory. Other researchers later found that the end-Cretaceous extinction event that wiped out the dinosaurs may have occurred over thousands of years, rather than millions of years as had previously been accepted. Others continue to study such theories as increased volcanism, particularly the massive Deccan Traps eruptions that occurred around the same time, and climate change, checking against the fossil record. Significantly, however, on March 4, 2010, a panel of 41 scientists agreed that the Chicxulub asteroid impact triggered the mass extinction.