The K-T Boundary

From the fossil distribution and other geological evidence, it was established that the Gubbio formation spanned parts of both the Cretaceous and Tertiary periods. The designations of these geological time frames stem from early geologists' interpretations of significant intervals in Earth's history, alongside specific features that mark those periods. The history of life is typically divided into three eras: Paleozoic ("ancient life"), Mesozoic ("middle life"), and Cenozoic ("recent life"). The Cretaceous period, known for its chalky deposits, constitutes the final third of the Mesozoic era, while the Tertiary period (now redefined and subdivided into the Paleogene and Neogene) commences at the close of the Cretaceous, approximately 65 million years ago, and concludes at the beginning of the Quaternary period, 2.6 million years ago.

Walter and his colleague Bill Lowrie dedicated several years to studying the Gubbio formation, taking samples from the Tertiary layer and working their way down through the Cretaceous. Initially, they sought to correlate reversals in Earth's magnetic field with the fossil record in order to unravel the timeline of Earth's history. They learned to determine their position within the rock formation by identifying the forams characteristic of certain deposits and by recognizing the boundary between Cretaceous and Tertiary rocks. This boundary consistently aligned with the notable decrease in foram diversity. The rocks beneath this boundary were Cretaceous, those above were Tertiary, and the thin clay layer marked the divide.

About 1,000 kilometers from Gubbio, in Caravaca on Spain's southeastern coast, Dutch geologist Jan Smit observed a similar trend in foram changes near the K-T boundary. Recognizing that the K-T boundary signified the most infamous extinction event — the extinction of the dinosaurs — Smit's insights piqued Walter's interest in the tiny forams and the K-T boundary even further.

Walter was relatively new to the field of academic geology. After obtaining his Ph.D., he worked in the exploration division of a multinational oil company in Libya until Colonel Qaddafi expelled all Americans from the country. Although his research on magnetic reversals had been fruitful, he recognized that the abrupt transition in the Gubbio forams and the K-T extinction presented a far more complex mystery he was determined to unravel.

One of the first questions Walter aimed to address was the duration it took for that thin clay layer to form. To find the answer, he needed assistance. While it is common for children to seek help from their parents with science projects, it was quite unusual for a "child" in their late 30s to do so — yet few had a father like Walter’s.

From A-bombs to Cosmic Rays

Luis Alvarez may have known little about geology or paleontology, but his expertise in physics was unparalleled. He played a pivotal role in the emergence and development of nuclear physics, earning his Ph.D. in physics from the University of Chicago in 1936 and later working at the University of California, Berkeley under Nobel laureate Ernest Lawrence.

Luis's early contributions to physics were interrupted by World War II. Initially, he worked on radar development and systems that ensured safe landings for aircraft in poor visibility, garnering the Collier Trophy for creating the Ground Controlled Approach (GCA) system.

During the war, he was recruited for the Manhattan Project, the secret national initiative to develop atomic weaponry. Alongside his student Lawrence Johnston, he designed detonators for the bombs and later measured their energy release. Luis was among the few to witness the first two atomic detonations, attending both the initial test in New Mexico and the bombing of Hiroshima.

After the war, Luis returned to physics research, innovating large liquid hydrogen bubble chambers for tracking particle behavior, and in 1968, he was awarded the Nobel Prize in Physics for his contributions to particle physics.

This significant achievement could have served as a fitting conclusion to a remarkable career, but when his son Walter joined Berkeley's geology department, they began conversing frequently about scientific matters. Walter presented his father with a polished cross-section of the Gubbio K-T boundary rock, explaining the mystery contained within it. Captivated at the age of nearly 70, Luis began to devise ways to assist Walter in solving it. They brainstormed methods to measure the rates of change around the K-T boundary, requiring some form of atomic timekeeper.

Luis, an expert in radioactivity and decay, first proposed measuring the abundance of beryllium-10 (¹?Be) in the K-T clay, as this isotope is continually generated in the atmosphere by cosmic rays interacting with oxygen. The longer the clay had been present, the greater the concentration of ¹?Be. He connected Walter with a physicist skilled in these measurements, but just as Walter was poised to proceed, he learned that the published half-life of ¹?Be was incorrect; the actual half-life was shorter, leaving an insufficient quantity of ¹?Be to measure after 65 million years.

Fortunately, Luis had another idea.

Space Dust

Luis recalled that meteorites are approximately 10,000 times richer in platinum group elements than Earth's crust. He theorized that the influx of space dust should occur at a consistent average rate, allowing for the calculation of the time it took for rock samples to form based on the quantity of platinum elements present.

While these elements are not plentiful, they are detectable. Walter surmised that if the clay layer had formed over a few thousand years, it would contain noticeable amounts of platinum group material; however, if it had formed more rapidly, it would likely be devoid of these elements.

Luis determined that iridium would be the best element to measure due to its higher detectability. He also had connections to nuclear chemists Frank Asaro and Helen Michel at the Berkeley Radiation Laboratory who could perform the necessary analyses.

Walter provided Asaro with samples from across the Gubbio K-T boundary, but for months, he received no updates. The analytical techniques employed were slow, the equipment malfunctioned, and Asaro had other commitments.

After nine months, Walter received a call from his father. Asaro was ready to share the results. They had anticipated finding iridium levels around 0.1 parts per billion (ppb) in the samples, but Asaro discovered 3 ppb in the clay layer, approximately 30 times higher than expected and exceeding levels found in other rock layers.

Why would that thin layer contain such high concentrations of iridium?

Before delving into speculation, it was crucial to determine whether the elevated iridium levels were unique to the Gubbio rocks or indicative of a broader pattern. Walter sought another exposed K-T boundary site for sampling and found Stevns Klint, located south of Copenhagen, Denmark. Upon visiting the clay bed there, he quickly recognized that "something unpleasant had occurred at the Danish sea bottom" during the clay's deposition. The cliff face comprised primarily white chalk, teeming with various fossils, while the thin K-T clay layer was black, emitted a sulfurous odor, and contained only fish bones. Walter concluded that during the formation of this "fish clay," the sea had become an oxygen-deprived graveyard. He collected samples and sent them to Frank Asaro.

The Danish fish clay exhibited iridium levels 160 times greater than background levels.

Walter encouraged Jan Smit to analyze his clay samples for iridium, and he found a spike in the Spanish clay as well. Similar results emerged from a K-T boundary sample in New Zealand, confirming the global nature of the phenomenon.

Something extremely unusual and catastrophic had transpired at the K-T boundary. The forams, the clay, the iridium, and the dinosaurs were all clues — but clues to what?

It Came from Outer Space

The Alvarez team quickly deduced that the iridium must have originated from an extraterrestrial source. They speculated about a supernova, an exploding star that could shower Earth with elemental debris. This concept had been previously discussed in paleontological and astrophysical circles.

Luis understood that heavy elements are generated during stellar explosions, leading them to believe that if their hypothesis was correct, other elements would also appear in unusual concentrations within the boundary clay. The pivotal isotope to measure was plutonium-244, which has a half-life of 75 million years. This isotope would still be present in the clay layer but would have decayed in ordinary terrestrial rocks. Comprehensive testing revealed no elevated levels of plutonium, initially disappointing everyone, but the investigation continued.

Luis pondered various scenarios that could explain a global extinction. He considered possibilities such as the solar system passing through a gas cloud, a nova event involving the sun, or the iridium coming from Jupiter. None of these theories held up. An astronomy colleague at Berkeley, Chris McKee, proposed that an asteroid might have struck Earth. Initially skeptical, Luis thought such an event would only result in tidal waves, questioning how a massive tidal wave could kill dinosaurs in distant regions like Montana or Mongolia.

His perspective began to shift as he recalled the 1883 volcanic eruption of Krakatoa. He remembered how rock was propelled into the atmosphere, with fine dust particles lingering for years. Luis also recognized from nuclear bomb tests how quickly radioactive materials mix between hemispheres. Perhaps a substantial amount of dust from a significant impact could plunge the Earth into darkness for an extended period, disrupting photosynthesis.

Given the iridium measurements from the clay, the concentration of iridium in chondritic meteorites, and the Earth's surface area, Luis calculated that the asteroid's mass would be about 300 billion metric tons, with a diameter of approximately 10 ± 4 kilometers (km).

While this size might not seem massive compared to Earth's 13,000-km diameter, the energy released upon impact would be staggering. Such an asteroid would enter the atmosphere at around 25 km per second — over 50,000 miles per hour — creating an impact equivalent to 10? megatons of TNT (the largest atomic bomb ever detonated released one megaton, making this event 100 million times more powerful). The resulting impact crater would span about 200 km in diameter and 40 km deep, ejecting immense quantities of material.

The team had formulated their scenario for the extinction of forams and dinosaurs.

Hell on Earth

The asteroid would traverse the atmosphere in approximately one second, heating the air ahead of it to several times the sun's temperature. Upon impact, the asteroid would vaporize, creating a massive fireball that expanded into space, launching rock particles halfway to the moon. Enormous shockwaves would reverberate through the bedrock, then rise to the surface, propelling molten blobs and fragments into the upper atmosphere and beyond. A second fireball would erupt from the pressure on the shocked limestone bedrock, obliterating all life within a radius of hundreds of kilometers from ground zero. Further away, debris and material ejected into space would fall back to Earth at high speeds, heating the air and igniting widespread fires. Tsunamis, landslides, and earthquakes would further devastate landscapes near the impact site.

Elsewhere on the planet, death would come at a slower pace.

The debris and soot in the atmosphere would block sunlight, potentially plunging the Earth into darkness for months, disrupting photosynthesis and halting food chains. Analysis of plant fossils and pollen grains indicates that half or more plant species vanished in some regions. Animals at higher trophic levels would also perish. The K-T boundary signifies not only the end of the dinosaurs but also the extinction of belemnites, ammonites, and marine reptiles, with paleontologists estimating that over half of all species on Earth went extinct. On land, no creature larger than 25 kilograms survived.

This marked the conclusion of the Mesozoic era.

Where is the Hole?

Luis, Walter, Frank Asaro, and Helen Michel successfully synthesized the entire narrative — from the Gubbio forams to the iridium anomaly, the asteroid hypothesis, and the extinction scenario — into a single paper published in the journal Science in June 1980. This work represents a remarkable interdisciplinary synthesis, possibly unparalleled in modern scientific literature. Meanwhile, Jan Smit and Jan Hertogen published their findings based on Spanish rocks in Nature, arriving at a similar conclusion.

Concerns arose, however, regarding the scientific community's readiness to accept the impact hypothesis. Historically, for the past 150 years, modern geology emphasized gradual changes. The field had supplanted biblical accounts of cataclysmic events. The notion of a catastrophic occurrence on Earth was not only unsettling but also deemed unscientific. Until the asteroid impact papers emerged, explanations for the dinosaurs' disappearance typically invoked gradual climatic or food chain changes to which they could not adapt.

Some geologists dismissed the catastrophic theory, while certain paleontologists remained unconvinced by the asteroid concept. Critics pointed out that the highest dinosaur fossils in the record were found 3 meters below the K-T boundary, suggesting that dinosaurs may have already been extinct by the time the asteroid struck. Others countered that dinosaur fossils are scarce, making it unreasonable to expect to find them directly adjacent to the boundary. They argued that the fossil record of forams and other organisms is more revealing, and those species persisted right up to the K-T boundary.

A more significant challenge remained: where was the massive impact crater? This question posed an apparent weakness to both skeptics and proponents of the hypothesis, igniting a search for the impact zone, if it existed.

At that time, only three known craters on Earth were larger than 100 km, none of which dated to the right period. If the asteroid had struck the ocean, which covers over two-thirds of the planet's surface, the searchers might have faced insurmountable odds. The deep ocean remained poorly mapped, with a considerable portion of the pre-Tertiary ocean floor having been subducted into the Earth's depths due to tectonic plate movements.

In the decade following the asteroid theory's proposal, numerous clues and trails were pursued, often leading to dead ends. As failures accumulated, Walter began to suspect that the impact occurred in an ocean.

A promising lead emerged from a riverbed in Texas. The Brazos River flows into the Gulf of Mexico, and the sandy riverbed lies directly at the K-T boundary. A close examination by geologists familiar with tsunami deposit patterns revealed features attributable only to a colossal tsunami, potentially exceeding 100 meters in height. Moreover, debris found in the tsunami's remnants contained tektites — small, glassy rock fragments ejected from the impact crater while molten and cooled as they fell back to Earth.

Many scientists embarked on the quest for the impact site. Among the most dedicated was Alan Hildebrand, a graduate student at the University of Arizona. Hildebrand deduced that the Brazos River tsunami bed offered vital clues regarding the crater's location — possibly in the Gulf of Mexico or the Caribbean. He examined available maps for potential candidate craters and discovered rounded features on maps of the seafloor north of Colombia. He also identified circular-shaped "gravity anomalies," areas where mass concentration varies, along Mexico's Yucatan Peninsula coast.

Hildebrand sought further evidence to confirm he was on the right track. He came across a report of tektites in late Cretaceous rocks from a site in Haiti. Upon visiting the lab that issued the report, he recognized the material as impact tektites. His subsequent visit to Haiti revealed deposits containing large tektites alongside shocked quartz grains — another indicator of impact events. He and his advisor, William Boynton, concluded that the impact site lay within 1,000 km of Haiti.

At a conference where Hildebrand and Boynton presented their findings, they were approached by Carlos Byars, a reporter for the Houston Chronicle. Byars informed Hildebrand that geologists working for the state-owned Mexican oil company PEMEX might have discovered the crater years earlier. Glen Penfield and Antonio Camargo had studied the circular gravity anomalies in the Yucatan, and though PEMEX restricted their data release, they suggested at a 1981 conference — just a year after the Alvarez team's asteroid hypothesis — that the features they mapped could represent the crater. Penfield even corresponded with Walter Alvarez regarding this possibility.

By 1991, Hildebrand, Boynton, Penfield, Camargo, and their colleagues formally proposed that a 180-km-diameter crater located one-half mile below the village of Chicxulub on the Yucatan Peninsula was the long-sought K-T impact crater.

Yet, critical tests remained to confirm whether Chicxulub was indeed the "smoking gun." One pressing issue was determining the age of the rock, a complex endeavor due to the crater being buried. The most effective approach would involve testing core rock samples extracted from wells drilled by PEMEX decades prior. Initially, fears arose that all core samples had been lost in a warehouse fire, but they were eventually located, and the rocks melted by the impact could be dated by several laboratories. The results were astounding: one lab reported an age of 64.98 + 0.05 million years, while another calculated 65.2 + 0.4 million years. The melt rock was precisely the same age as the K-T boundary.

The tektites from Haiti were similarly dated to this time frame, as was a deposit of material ejected from the impact. Detailed chemical analysis revealed that the Chicxulub melt rock contained elevated iridium levels, and both it and the Haitian tektites originated from the same source. Additionally, the Haitian tektites exhibited extremely low water content, and the gas pressure within was nearly zero, indicating that the glass had solidified while in ballistic flight outside Earth's atmosphere.

In just over a decade, what initially appeared to be a radical and, to some, implausible theory gained support from various indirect evidence before ultimately being validated through direct evidence. Geologists subsequently identified ejected materials spread across much of the Yucatan and deposited at over 100 K-T boundary sites worldwide. Our understanding of life's history on Earth has shifted from a steady, gradual process, as envisioned by generations of geologists since Lyell and Darwin.

While the identification of the vast crater marked a significant advancement for the asteroid hypothesis, it was bittersweet for Walter, as Luis Alvarez had passed away in 1988, just prior to the discovery.

One Punch or Two?

The revelation of the K-T asteroid impact prompted extensive investigations into whether other extinction events were also impact-related. It appears that none of the other four major extinctions over the past 500 million years can be attributed to impacts. Nonetheless, there have been numerous sizable asteroid or comet impacts during that time, though none rivaled the K-T strike. Given that most impacts do not result in extinctions and most extinctions are not due to impacts, the question arises: why was the K-T asteroid so devastating?

Some scientists suggest that the location of the impact was crucial. The target rock vaporized during the impact contained significant amounts of gypsum, which released sulfur aerosols that could exacerbate sunlight blockage and generate acid rain, affecting bodies of water and soils. Moreover, the impact would have liberated enough chlorine to destroy today's ozone layer.

However, further evidence has emerged suggesting that a series of massive volcanic eruptions might have weakened Earth's ecosystems before the K-T impact. The Deccan Traps in present-day western India are believed to have released vast quantities of carbon dioxide and sulfur dioxide into the atmosphere in episodic eruptions that began several hundred thousand years before the K-T impact. Consequently, an ongoing debate among scientists has emerged regarding whether the Deccan Traps or the K-T impact served as the primary cause of the mass extinction. Due to the temporal overlap between the K-T impact and the mass extinction, the prevailing consensus holds that the K-T impact was the primary driver of extinction. Recent geological evidence suggests a scenario that may reconcile both viewpoints, indicating that the largest Deccan eruptions occurred very close to the time of the impact. Some researchers propose that the seismic activity from the impact shaking the Earth's mantle may have triggered significant, climate-altering eruptions. In this scenario, the asteroid would represent the initial blow, while volcanic activity would deliver the knockout punch.

References

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Sean B. Carroll is a professor of molecular biology and genetics at the University of Wisconsin-Madison and Vice President for Science Education at the Howard Hughes Medical Institute.

Carroll, Sean B., Into The Jungle: Great Adventures In The Search For Evolution, 1st, ©2004, pp. 113–131. Reprinted by permission of Pearson Education, Inc., New York, New York.