By Steve Brusatte

Every so often a revolutionary idea captivates the entire scientific community. It has happened with Galileo and Copernicus, and later with Isaac Newton. The 1800\'s saw amazing advances in medicine and new theories on life and prehistory.

Science may have had its crowning glory in the 1900\'s, with such figures as Albert Einstein, Jonas Salk, Edwin Hubbell, James Watson, Francis Crick, and Enrico Fermi grabbing headlines with their monumental discoveries and hypotheses. And, who can forget about the entire mass extinctions debate of the 1980\'s? During this decadelong period, scientists from tens of disciplines, including paleontology, geology, astronomy, astrophysics, chemistry, and ecology, debated the causes and outcomes of mass extinctions. This entire debate was fueled by the pioneering work-and somewhat accidental discovery-of a young geologist by the name of Walter Alvarez. But, before he discovered striking evidence on dinosaur extinction Walter Alvarez was a common young boy-except for one unusual trait: his father was a Nobel-Prize winning physicist! Therefore, science was introduced to Walter daily, but later on in life he decided not to go into the same field as his father. Instead, young Walter chose geology, a science that his mother found particularly interesting, but his father dismissed as \'boring.\'

Walter\'s mother would regularly take her children out on rock collecting trips near the Berkeley, California area, where Luis taught physics at the University of California. These trips, in part, led Walter to enter Princeton University, where he later received his Ph.D. in his selected field of geology.

From Princeton Dr. Alvarez took many postdoctoral positions around the world, including research posts in Italy, Libya, the Netherlands, and South America. In 1977 Walter learned of a job opening in the Department of Geology and Geophysics at the University of California-Berkeley, and quickly applied. He was later chosen for the job, and joined his father in the science department in one of America\'s premier universities.
But, before the Berkeley job was even vacated Dr. Alvarez had begun an intense research project in the Apennine Mountains of Central Italy. By studying the vast and immense limestone deposits there he hoped to unlock the mysteries of Mediterranean plate tectonics. This search led him to the small, hilly town of Gubbio, a Medievalflavored town about ninety miles northeast of Rome. In Gubbio Dr. Alvarez came upon a towering limestone outcrop which contained rocks from both the Cretaceous and Tertiary periods, and surprisingly even included a thin layer of clay from the precise time the dinosaurs became extinct!

Up until Dr. Alvarez\'s trip to Italy most paleontologists were basically dumbfounded about why the dinosaurs happened to become extinct. Some believed that their environments deteriorated, with the climate and food sources changing, and others blamed the competition from the more \'highly advanced\' mammals.

A major turning point came in the late 60\'s when paleontologist Dale Russell published a paper stating that an exploding supernova may have killed the dinosaurs with radiation. While this scenario seems off base, it was the first step in Dr. Alvarez\'s analysis of the K-T extinction. During his study of the Gubbio limestone outcrop Dr. Alvarez was astonished to see forams, miniature microfossils, which construct amazingly detailed outer shells, abruptly disappear at the K-T clay boundary. Up until the boundary they seemed to be diverse and thriving, and they even made a comeback following the K-T extinction. But, at the clay boundary they abruptly disappeared, and with no trace. What caused this, and was it possibly related to the extinction of the dinosaurs?

To help answer this question Dr. Alvarez brought back some of the Gubbio samples to his father, and it was then when the mystery began to show some signs of letting up!

Back at UC-Berkeley Dr. Luis Alvarez, who was in no way an expert in paleontology or geology, suggested that his colleagues Helen Michel and Frank Asaro put the clay samples under chemical analysis.

After an intense testing of the chemical makeup of the boundary layer Michel and Asaro discovered abnormalities in two rare metals: platinum and iridium. This platinum was later traced back to the wedding ring of one of the lab technicians, but there was no explanation for the iridium.
Iridium, or element number 77 on the periodic table, is a metal that is rarely seen on earth\'s crust. Scientists hypothesize that it may be common in the earth\'s core, and it sometimes makes it way into lava during volcanic eruptions, but basically iridium is a rare earthly element.

But, iridium is common throughout our solar system, especially in comets and asteroids. Could this iridium have been a byproduct of the supernova that may have exploded? Further tests showed that this was impossible, but an even simpler explanation was ready to emerge. Even Walter Alvarez himself cannot remember when the idea of a large comet or asteroid striking the earth first came up, but he surely remembers the \'impact\' it had in the paleontological community (no pun intended!).

After the supernova hypothesis was dismissed the Alvarez team asked this question: \"what event could have caused the immense dying out of forams and the dinosaurs while depositing the iridium layer at the same time?\" Somewhere along the line the answer of comet or asteroid came up, and this idea was later published in the monumental paper entitled \"Extraterrestrial cause for the Cretaceous-Tertiary extinction,\" which appeared in volume 208 (1980) of the journal Science.
As expected, this paper quickly caused considerable debate in paleontology and geology circles. But, what was not expected was the debate this paper in general science circles! Over the next decade scientists from the most unrelated fields, from ecologists to astrophysicists, tried to prove and disprove the Alvarez hypothesis.

The major question plaguing the Alvarez team upon the reports of their data was \"was the iridium at the K-T layer an Italian anomaly?\" In other words, was it restricted just to Gubbio, or would it be found in other K-T sites around the world? Several eager geologists and paleontologists rushed to Alvarez\'s call, and soon iridium layers were found in rocks from Spain to Denmark to the United States.

One of the major contributors to the study of these \'other\' K-T sites was Dutch paleontologist Jan Smit (see the Dino Data portrait). Actually, Alvarez considers Smit the \'co-discoverer\' of the comet/asteroid hypothesis, because Smit was about to publish on the iridium layer but suddenly became sick! Another twisted science story! Upon learning that the iridium layer was worldwide the next major step in the Alvarez research was to find the crater. It had been deduced that dinosaurs and forams likely died out suddenly. An iridium layer had been found, pointing to a comet or asteroid. And, this layer was confirmed worldwide. But, without a crater, the socalled \'smoking-gun,\' the theor would have likely been pushed to the back of the odd annals of science, along with the ideas of an earthly homocentric solar system and coldfusion. But, due to over ten years of research, fieldwork, and luck, a crater was later found, all but closing the dinosaur extinction debate.

At the time of the Alvarez announcement and during the few years thereafter many paleontologists and geologists pointed to the Manson Crater in Iowa as being the scar of the K-T event.
But, further analysis showed this crater to be off time-wise, and this theory \'died out\' (another pun). After the research on Manson went sour researchers began to study rocks for clues to whether this impact occurred on land or in the ocean.

If the rocks pointed to an oceanic impact there would be little hope to recover the crater, as 65 million years of earthly changes would have likely covered it up. This possibly would have explained why hundreds of tirelessly searching geologists could not find the crater.
But, what Alvarez and his team discovered was astonishing: the rocks pointed to both an earthly AND oceanic impact! This meant that the impact occurred somewhere along the shores of a sea or ocean.

This narrowed down the search, and in 1988, paleontologist Alan Hildebrand decided that the key to finding the crater lied in the Brazos River of Texas. Among the rock outcrops, which dotted the river, Hildebrand and his cohorts discovered evidence of tsunamis, which were dated to the exact time of the K-T impact, 65 million years ago.

During this time the Brazos beds marked the southern shores of the Gulf of Mexico. Since these shores would have been shallow, Hildebrand deduced that the tsunamis would have had to develop south of Texas, since they generally travel from deep to shallow water.
Due south of the area is the Yucatan Peninsula, an area once inhabited by the Aztecs, which is now a popular Mexican vacation spot.

Could the crater be near the Yucatan? As a matter of fact, in the 1970\'s a Mexican team consisting of Glen Penfield and Antonio Camargo discovered an odd gravity anomaly, which circled part of the Yucatan coast and extended into the ocean.

This anomaly was circular, and about the right size for a comet or asteroid. Could this be the crater? Hildebrand and his advisor Bill Boynton said yes, and published their results. Today this crater, known as Chicxulub, is widely regarded at the K-T smoking gun!

Sadly, the discovery and naming of this crater in 1991 basically closed the books on K-T extinction research. Today papers continue to turn up, both trying to prove and disprove the Alvarez hypothesis, but the glamour of the 80\'s has disappeared. Regardless, the K-T extinction debate will always be remembered as a type of science family reunion, uniting many branches of science all in the name of a common causethe greatest paleontological mystery of them all.

And, this mystery jumped into the spotlight because of a young, ambitious geologist by the name of Walter Alvarez. Like the extinction debate, Alvarez\'s paleontological work, too, has fizzled out, but surely we will always remember him and his father as the grandmasters of dinosaur extinction! **Steve Brusatte, 15 of Ottawa, Illinois, USA, has been researching famous paleontologists in hopes that he can become the first teen to write a book on the subject. The book focuses on the contributions of modern paleontologists (including amateurs, collectors, and artists).

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