Extinction of dinosaurs: Mechanisms of destruction, 1 of 3

By the end of the Cretaceous period, 66.038 million years ago, absolutely all groups of non-avian dinosaurs had become extinct. The extinction of dinosaurs was part of the mass extinction that occurred at the Cretaceous-Paleogene boundary. It affected all continents and wiped out many groups of animals. Researchers explain it by various causes and the effects of several mechanisms of destruction.

Mass Extinction at the End of the Cretaceous

The last mass extinction eliminated 17% of all families, 39–47% of all genera, and 68–75% of all species. Most large tetrapods died out, except for some ectothermic species such as turtles and crocodiles. All the pterosaurs disappeared. Many mammals, birds, lizards, insects, and plants also suffered.

18% of terrestrial vertebrate families became extinct. The destruction affected many Mesozoic ecosystems. However, most taxonomic groups of plants and animals at the order level and above survived this period.

Strangelove Ocean

The most profound change in biota at the K/Pg boundary (otherwise K/T) appears in marine sediments. 16% of marine animal families died out. Mosasaurs, plesiosaurs, pterosaurs, and a whole range of other terrestrial and marine animals disappeared. This included invertebrates—ammonites, belemnites, calcareous and hexactinellid sponges, and a vast number of small algae. Bryozoans (phylum Bryozoa) and crinoids experienced severe impacts. This extraordinary event was considered to be the result of a significant drop in primary oceanic productivity, which was termed the “Strangelove Ocean” (Hsü et al., 1982).

However, hydrocarbon molecules studied in marine sediments in Denmark indicate the existence of extensive photosynthesis in the open ocean, which would have been impossible under the “Strangelove Ocean” model. “Primary productivity came back quickly, at least in the environment we were studying,” says Roger Summons. Everything points to a successfully functioning open ocean ecosystem, with numerous algae photosynthesizing at a fairly good rate. Since these deposits formed immediately after the impact, the “Strangelove Ocean” theory, which suggests a huge lifeless sea, is unlikely (Julio Sepúlveda et al., 2009).¹

Extinction Events

Throughout the Phanerozoic (the last 540 million years), there have been five major mass extinctions and about 20 less extensive ones. The most significant are considered the Ordovician-Silurian, Devonian, Permian, and Triassic. The largest is the “Great” Permian-Triassic extinction (that’s when trilobites went extinct). Although the last Cretaceous-Paleogene mass extinction was not the most significant in Earth’s history, it became the most famous due to the extinction of dinosaurs—a very iconic group of animals.

Dinosaurs had already survived a mass extinction at the end of the Triassic (201.3 million years ago), which had little impact on their diversity. They also survived an important extinction event at the end of the Jurassic period (Tithonian) (Weishampel et al., 2004; Barrett et al., 2009; Upchurch et al., 2011). In the sea, this extinction was associated with the disappearance of a number of families and genera of bivalve mollusks and ammonoids (A. Hallam, 1986). The extinction level of marine animal families reached 5%, and genera—30% (Sepkoski, 1995).

Many extinctions (or other changes in the biosphere that did not reach such a level) closely coincide in time with events of trap volcanism, and the Cretaceous-Paleogene also coincides with the fall of a huge asteroid. However, it should be emphasized that there is no regularity caused by external factors and even less so periodicity in mass extinctions.

Main Scenarios of the Mass Extinction

Among the many different hypotheses about the extinction of dinosaurs, there are now two main scenarios explaining the mass extinction (including the extinction of dinosaurs) at the K/T boundary. These are the “gradualist” model, proposed by Leigh Van Valen (1984), and the “catastrophist” extraterrestrial impact model, proposed by Luis Alvarez (1983, 1987). There is a significant amount of evidence of various kinds for both scenarios. In addition to the causes of extinction, these models suggest different durations for the extinction event. Gradualists submit a gradual extinction of dinosaurs over 7 million years, with an acceleration of the pace in the last 0.3 million years (Sloan et al., 1984; Sakamoto et al., 2016). Catastrophists (Alvarez et al., 1980) suggest that the main extinction happened in just 1–10 years and the duration of the event’s consequences up to 5000 years.

Such physical phenomena as active volcanism, marine regression, and asteroid impact may have contributed to the extinction and influenced its pace. Each had its own active factors (specific mechanisms of destruction) that actually led to the mass extinction. These include:

• Dust that obscured the Sun;
• Acid rains;
• Climate change (increase or decrease in temperature)
• Tsunamis;
• Firestorm (global forest fires);
• Earthquakes;

The question is to what extent they influenced the K/T events, and whether they were the only catastrophic events at that time.

Impact Hypothesis for the Extinction of Dinosaurs

Scientists propose various hypotheses to explain the extinction of dinosaurs, such as an epidemic, climate change, competition with mammals, volcanic eruptions, and others. And, of course, the most media-beloved hypothesis is the fall of a giant meteorite—a carbonaceous chondrite with a diameter of about 10 km, which formed the Chicxulub crater (“tick demon” in the Mayan language) with a size of 180 km in Central America, in the Yucatan Peninsula area. However, the latest data obtained from drilling has reduced the actual size of the crater to 150 km (according to the Earth Impact Database, Planetary and Space Science Centre University of New Brunswick Fredericton, 2019).

The crater itself was an accidental discovery. In 1978, the oil company Petroleos Mexicanos (PEMEX) discovered a ring-shaped structure hidden under a layer of sedimentary rocks near the town of Chicxulub on the Yucatan, capturing part of the peninsula and extending into the sea.

The asteroid hypothesis, proposed by Luis Alvarez and his son Walter in 1980, is based on the high concentration of the relatively rare metal iridium in clay layer deposits, presumably corresponding to the Cretaceous-Paleogene boundary. Much of their article about this is devoted to geological and physical evidence of the impact, as well as the physical results of the collision. The discussion of the biological results of the collision occupies only half a page (Alvarez, L. W., et al., 1980).²

In fact, the first to propose the hypothesis of dinosaur extinction as a result of an asteroid fall was Lobenfels in 1956.

Evidence for an Asteroid Impact

In addition to iridium, evidence of the asteroid impact are the findings of “shock quartz”. It transforms into the mineral stishovite under high pressure (but at moderate temperatures). High temperatures lead to the annealing of quartz back into its standard form. Some meteorite craters contain stishovite. Volcanoes cannot create the high pressure that leads to the formation of “shocked quartz.” This proves that the craters were formed as a result of an asteroid fall.

Additional evidence for an event at the K/T boundary includes findings of tektites—droplets of molten Earth’s crust material that instantly cooled in the atmosphere and then fell as impact glass. However, the source of tektites could be both impact events and volcanic eruptions. This does not rule out the possibility of several catastrophic events.

Spherules—carbon formations with inclusions of nanodiamonds—are also often considered evidence of an impact. Spherules occur in very different geological circumstances and in various types of rocks. But most are erupted volcanic and eroded magmatic materials. Their extraterrestrial origin is not highly credible, but it is not ruled out either.

Asteroid Impact Angle

Schultz and D’Hondt (1996) put forward a hypothesis based on the asymmetry of subsurface features of the Chicxulub crater (observed by geophysical methods). They determined that the impact must have occurred at a low angle (~30°) from the southeast. Based on this, they predicted that the greatest destruction would occur in the Northern Hemisphere at relatively low latitudes. Indeed, the nature of the extinctions roughly corresponds to this hypothesis.³

However, the catastrophe does not seem sufficiently horrific. Therefore, to maximize the effect of the asteroid fall, another hypothesis was proposed: the asteroid fell to Earth at a very steep angle—more than 60° to the horizon, moving towards the Earth from the northeast. Modeling showed that this would have been the most dangerous scenario for falls, as a large amount of dust ended up in the Earth’s atmosphere. (published in the journal Nature Communications)

Uncertainties of an Asteroid Impact: Predicting the Unpredictable

Clemens et al. (1981) commented on the proposed asteroid impact as follows: “Analyses of the paleobiological data suggest that such an event is not required to explain the biotic changes during the Cretaceous-Tertiary transition.”

The asteroid hypothesis predicts that the character of extinction at the K/T boundary worldwide should be abrupt and synchronous in both terrestrial and marine realms. Also, the catastrophic hypothesis fundamentally contradicts the facts (e.g., Archibald 1996), which testify to the absence of side effects of the asteroid fall, such as acid rains, etc.

Besides, we do not know what climatic and ecological changes might occur as a result of a collision with a large asteroid or what the patterns of mass extinction will be. This simply lies beyond human experience. Therefore, in the absence of precedents, scientists resort to modeling to suggest the possible consequences of such an impact.

Asteroid Winter or Impact Winter

In 1981, Carl Sagan added popularity to the catastrophic collision hypothesis by linking it with the “nuclear winter” model. He proposed a scenario of “asteroid winter,” based on the dusting of the atmosphere and a sharp and prolonged decrease in global temperature.

It’s important to recognize that “asteroid winter” and “nuclear winter” models have profound differences, and the use of the latter term to describe the impact is not correct. For more details, see the article Nuclear Winter.

Dust

Proponents of the asteroid hypothesis suggested that the substance of the asteroid, which exploded upon falling to Earth, dispersed in the planet’s atmosphere and supposedly obscured the Sun. An “asteroid winter” began. The temperature sharply decreased for many months, and photosynthesis was suspended. The catastrophe allegedly struck the entire planet, leaving no refuge for the dinosaurs.

The initial theory of impact posited that the dust particles generated by the impact event were the culprits behind the halting of photosynthesis post-impact. This, in turn, led to a mass extinction event (Alvarez et al., 1980). Pollock and others calculated that emissions into the atmosphere from the asteroid impact would have caused three months of darkness.

Dr. Cem Berk Senel and co-authors went further and conducted paleoclimate modeling to assess the role of silicate dust in the climate after the impact. They stated that the dust remained in the atmosphere for 15 years, and the Earth’s surface underwent global cooling by a whole 15 degrees Celsius. At the same time, photosynthesis would have ceased for almost two years after the impact.⁴

However, an asteroid winter may have a very short duration—ash and dust may hang in the stratosphere for a relatively short time, and large and medium particles fall out very quickly. A fine suspension may remain for only a few years. This is not enough for complete extinction.

At the terrestrial Tanis site in North Dakota (United States), located approximately 3,000 km north of Chicxulub, the sizes of silicate dust particles are being studied. In doing so, they forget to mention that “silicate dust” is actually clay, a material formed as a result of the chemical weathering of silicate-containing rocks. And the sizes of these clay particles—oh, wonder!—correspond to the sizes of particles of dusty fine-grained clays, which are widely distributed. Worse still, the rate of sulfur and soot deposition from the atmosphere is calculated, even taking into account the wind speed in North Dakota 66 million years ago! However, researchers found no sulfur or soot in this 2-centimeter layer of clay.

Additionally, very close to the Tanis site is the Manson impact structure (Iowa). The crater has a diameter of 35 km and dates back 74 million years. Its formation also resulted in the ejection of dust and tektites. How was its influence on the studied deposits excluded?

The Main Problem with Dust in the Asteroid Impact

The main issue related to the role of dust in the asteroid impact is that Alvarez and some other researchers used what seemed to them to be an easily accessible model of the Krakatoa volcano eruption in 1883. Again, to model the spread of dust from an asteroid, a sample of ash spread from a volcano. Meanwhile, the dust consists of cosmic material and parts of the Earth’s crust. Volcanic ash, on the other hand, is a product of the pulverization of magma rising from the depths of the planet. Not only their composition differs, but also the sizes of the particles are different. Furthermore, researchers are not to take into account the vast amount of volcanic ash ejected over millennia from the Deccan Traps.

Recently, however, the very concept that the impact released large amounts of dust into the atmosphere has become controversial. A closer examination of the debris in the impact layer suggests that the amount of submicron-sized dust particles in the stratosphere was too small to cause the observed environmental changes. The amount, presumably raised as a result of the asteroid fall at the end of the Cretaceous period, was almost a hundred times overstated (Pope, 2002). Most of the emissions have relatively large sizes of 0.1–1 mm in diameter and settle from the atmosphere within a few hours to several days. Thus, the dust could not have obscured the Sun, stopped photosynthesis, and caused the extinction of dinosaurs.

Acid Rain

Proponents of the catastrophic impact theory now believe that sulfur-rich gases produced from evaporites at the impact location, rather than dust, were the primary contributors to the climatic effects observed (Sigurdsson et al. 1992). They were supposed to create acid rain or form stratospheric sulfate aerosols that block sunlight and thus cool the Earth’s atmosphere and hinder photosynthesis (Pierazzo et al., 1998).

It is assumed that the fall of the asteroid caused significant emissions of sulfur dioxide (SO₂) and sulfur trioxide (SO₃) into the atmosphere, followed by acid rains, leading to ocean acidification and the deaths of marine inhabitants. However, previous impacts have not shown any evidence of increased SO₂ concentrations. And where would they come from? From the asteroid itself? Asteroids do not contain significant amounts of sulfur. But in volcanism, SO₂ and carbon dioxide make up the bulk of volcanic gases and can enter the atmosphere in large quantities.

To justify the occurrence of sulfuric acid rains, Pope et al. in 1997 suggested that the asteroid’s impact site must be rich in carbonates and sulfates. It should be, but it is not. There is no basis or evidence for this idea. Everything is just the opposite.

Drilling in the Chicxulub peak ring confirmed that it consists of granite ejected within a few minutes from the depths of the Earth (and not from the usual seabed rock). It is important to note that the cores also showed an almost complete absence of gypsum, which contains sulfates (the presumed source of sulfur gas). There was simply nowhere for acid rains to come from (Eric Hand, 2016).⁵

In the Solar System, there is an example of a planet where the main sulfur-containing gas is sulfur dioxide, and there is a layer of clouds made of sulfuric acid. This is Venus. The effective reflectivity of this layer is so small that the temperature on the planet’s surface is +467 °C. Moreover, the space stations that landed on its surface show very good illumination. On Earth, acid clouds created a “dark night,” while on Venus, they created a “bright day.” In addition, acid clouds contribute to the strongest greenhouse effect, but on Earth, for some reason, they cause an “asteroid winter.” Wonders, indeed!

But even here, proponents of the catastrophe cannot agree among themselves. According to another hypothesis, the impact produced significant volumes of nitrogen oxide (from the combination of atmospheric nitrogen and oxygen), which was enough to poison the animal and plant worlds and form acid rains with nitric acid content. It turns out that the asteroid can produce any chemical substances, depending on the preferences of the proponents of the catastrophe. Sulfuric acid did not fit, so nitric acid appeared.⁶

Since the emission of nitrogen oxides is difficult to explain, we are waiting for the appearance of hydrofluoric acid and all the others in order.

Theories suggesting that increased acidity, caused by asteroids or volcanoes, led to extinction are challenged by the fact that extinction primarily occurred in the oceans. Oceans have a significant buffering capacity of seawater, which helps to neutralize acidity. However, freshwater bodies, where amphibians are highly sensitive to acidity, remained unaffected. Freshwater vertebrates lost only 10% of their biodiversity at the boundary of the eras. This suggests that increased acidity was not the primary cause of extinction.

Back in 1994, D’Hondt and others showed that the asteroid collision at the end of the Cretaceous period could not lead to the formation of enough acid for acid rain to become a significant factor contributing to mass extinction.

Both geologists and paleontologists now dismiss Doomsday scenarios based on acid rain. Calculations of global amounts of acid indicate that the asteroid impact at the end of the Cretaceous could not have produced a significant amount of acid. There would not have been enough to acidify ponds to such an extent that it could affect life in them (e.g., D’Hondt et al. 1994), and even less so to cause a mass extinction.

In addition, considering what we know about the reaction of our modern biota to acid rain, freshwater inhabitants would likely have suffered complete destruction if acid rain had been a significant factor at the K/T boundary. However, freshwater inhabitants actually show the highest survival rates. Weil also confirmed that acid rain cannot explain the extinction of some taxa and the survival of others.

Over the course of one million years of eruptions, the Deccan Traps released up to 10.4 trillion tons of carbon dioxide and 9.3 trillion tons of sulfur into the atmosphere (Alexander A. Cox & C. Brenhin Keller, 2023).⁷ No asteroid could provide such an amount of emissions into the Earth’s atmosphere. But even this amount is not enough to form acid rains capable of leading to mass extinction.

Conclusion: Acid rains were not a significant factor in mass extinctions. We discard this hypothesis.

Firestorm

Global forest fires could have allegedly arisen because of an asteroid impact (Wolbach et al., 1988; Ivany and Salawitch, 1993). Soot and charcoal were found in several places at the K/T boundary, coinciding with iridium deposits. It was claimed that this charcoal and soot could have formed because of the synchronous burning of vegetation, equivalent to half of all modern forests. According to other scenarios, about 25% of terrestrial biomass burned at the end of the Cretaceous period.

The fall of the asteroid caused a global thermal pulse (Goldin & Melosh, 2009). This, allegedly, led to the ignition of large forest fires near the impact site (Wolbach, Lewis & Anders, 1985; Kring, 2007).

As a result of such an apocalyptic global fire, a large part of the terrestrial biomass around the world would have turned to ash. In freshwater environments, those plants and animals that escaped immediate boiling would have experienced an unprecedented influx of organic and inorganic substances. These organisms would have literally drowned in the remnants or suffocated from a sudden lack of oxygen caused by the oxidation of a massive influx of organic substances. Under such conditions, no selectivity for the survival of the surviving organisms is possible.

A group of scientists from the University of London, the University of Exeter, and the University of Edinburgh, led by Claire Belcher, refuted the 2013 hypothesis that a meteorite ignited a firestorm, leading to the extinction of the dinosaurs. Investigating and modeling the impact’s effects on living organisms, the scientists concluded that the thermal impact could only have been brief (the temperature spike lasted only one minute) and not strong enough to cause a global fire. The absence of large-scale forest fires, emissions of soot particles, and carbon monoxide deprives the hypothesis of the subsequent “asteroid winter” of its validity.

The interpretation of the deposits at Stevns Klint, Denmark, as soot rapidly deposited because of global forest fires after an asteroid impact is impossible (Officer and Ekdale 1986), due to the complex stratigraphy and abundance of fossils directly in these deposits. Ichnologists Ekdale and Bromley (1984) described these deposits as laterally discontinuous, complexly layered, and burrowed clay. The formation of these layers required much more time than can be explained by the forest fire hypothesis (Officer and Ekdale 1986).

The occurrence of fires cannot explain the simultaneous extinction of dinosaurs on continents isolated from each other.

Claire Belcher stated: “We assert that the asteroid did not destroy the dinosaurs; we believe that it could not have caused a global firestorm that arose as a result of the impact. Paleontologists will need to look for new keys and evidence to understand the causes of mass extinction.”

Some researchers previously believed that coal layers in the area of the boundary between the late Cretaceous Hell Creek formation and the Paleocene Tullock formation were remnants of forests burned by an asteroid impact. However, they do not coincide with the K/T boundary defined by iridium (Fastovsky 1987) and pollen and may differ “by as much as 5 m” (Johnson 1992). Again, mismatch.

Characteristics of polycyclic aromatic hydrocarbons in the deposits of the Chicxulub crater and at two deep-sea sites indicate the presence of a fossil carbon source that underwent rapid heating. This corresponds to organic matter ejected during the formation of the crater, rather than soot and ash from burned forests. This suggests that local wildfires were present, but had significantly less impact on the global climate and extinction than previously thought.⁸

In high latitudes, the vegetation did not change (Askin et al., 1994; Johnson, 1993); in low latitudes, it gradually changed up to the K/T boundary! (Sweet et al., 1993; Stromberg et al., 1998). Why assume some global fire if it had no effect on the vegetation that the fire should have destroyed in the first place?

For all the forests on the other side of the Earth, in Australia and China, to ignite from a meteorite fall in Central America (Yucatan), the Earth’s atmosphere would have needed catastrophic heating. This could potentially lead to the complete destruction of all life on the planet. Along with the iridium layer at the K/T boundary, there would have been a layer of carbon from all the burned biota on Earth. However, we would not have seen this because life on Earth would have been destroyed.

Tsunami

The tsunami from the impact could have reached more than 300 km inland around the Gulf of Mexico (Matsui et al., 2002). Since the tsunami could only have destroyed the inhabitants of coastal areas, significant exaggerations are used. A computer model developed by scientists at the University of Michigan (lead author of the study, Molly Range) suggests that this tsunami spread across the ocean floor and left geological traces even in New Zealand—12,000 km from the impact site. But not in South America, on whose shores the crater is located! Researchers exaggerated the size of the asteroid from 10 km to 14 km to create a more dramatic impact. The tsunami wave allegedly reached a height of 1.5 km.⁹

Giant waves with a height of 10–100 m in the Western Interior Seaway, caused by earthquakes with a magnitude of 10–11.5, are also described. Where did these waves come from if the Western Interior Seaway itself had disappeared by the end of the Cretaceous due to marine regression? At the end of the Cretaceous, the Western Interior Seaway in Dakota split into northern and southern parts. Its southern part (the Pierre Seaway) retreated to the Gulf of Mexico. In the early Paleocene, its remnants still occupied the Mississippi Embayment areas, reaching modern Memphis.

This event has drawn comparisons to the 9.1-magnitude Tōhoku earthquake, which caused a 1.5-meter-high ripple in Sognefjorden, Norway. Well, yes, one and a half meters and a hundred meters—almost no difference! After this reasoning, discussions about catastrophic climate change and mass extinction no longer seem surprising.

According to other estimates, in the Gulf of Mexico, tsunami waves could have reached around 100 meters. And in the coastal areas of the North Atlantic and South America, the wave height was almost ten times less and reached only about 10 meters.

Experts from the Los Alamos National Laboratory in the USA, including geophysicist Galen Gisler, along with colleagues, considered various scenarios of an asteroid falling into the ocean. They established that the waves formed during the collision are less dangerous than the waves that occur in the case of an underwater earthquake.

Conclusion: The tsunami does not qualify as one of the mechanisms of destruction that caused the extinction of dinosaurs.

Earthquakes

The fall of an asteroid could potentially have caused an earthquake of about 11 on the Richter scale (Ivanov, 2005). Hermann Dario Bermúdez asserts that powerful seismic activity did not subside for weeks, or even months, after the celestial body’s fall.

On Colombia’s Gorgonilla Island, Bermúdez, and his colleagues from Montclair State University discovered spherules and tektites that appeared as a result of the collision. Gorgonilla Island is located in the Pacific Ocean, 3000 kilometers southwest of the impact site. Moreover, at that time, the island did not yet exist, and these deposits accumulated deep on the ocean floor (2 km), only later rising to the surface.

This tektite-rich layer of the Cretaceous-Paleogene boundary turned out to be heavily deformed, which testifies to the powerful tremors that occurred then. And since it was deep at the bottom at the time, the authors conclude that powerful seismic activity continued for weeks or even months after the impact, while the layer of tektites was accumulating.¹⁰

Astonishing conclusion! Volcanic activity, which raised the island from the ocean floor to the surface and radically changed the geological structure of the area, did not affect the Cretaceous-Paleogene layer, but the supposed earthquake did. Moreover, if powerful earthquakes constantly occur during the deposition of the boundary layer, its upper part should be blurry and mixed. However, we observe that the deformation with the K/T layer occurred as a single whole. Therefore, its formation was already completed, and the deformation relates to a later time. The same applies to the cracks and faults in the K/T boundary layer found in Texas, Alabama, and Mississippi.

To this should be added the fact: “Because of their reworked nature, Gorgonilla spherules provide no stratigraphic evidence from which the timing of the impact can be inferred.”¹¹

Overall, tsunamis and earthquakes attract little attention from catastrophists, as they were not global phenomena and could not instantly kill all dinosaurs.

Marine Regression

During maximum marine regression, a number of consequences occur from a significant reduction in the area of the continental shelf. These include the reduction of coastal and marine habitats, fragmentation of the remaining habitats (leading to a reduction in biotic diversity), the formation of land bridges, and the elongation of river systems.

The regression was global in nature, but the record of the last Cretaceous dinosaurs in the western part of North America is the most studied. Overall, the global sea level fluctuated more intensely on short time scales during the Maastrichtian than in the Campanian (Miller et al., 2005).

The Western Interior Seaway, a shallow epicontinental sea, at times connected the Arctic Ocean and the Gulf of Mexico (Lillegraven & Ostresh, 1990). It was extensive for most of the Campanian and Maastrichtian periods. In the late Maastrichtian, it sharply narrowed and split into two parts, likely due to the global low sea level combined with local tectonism (Weimer, 1984; Lillegraven & Ostresh, 1990). This significantly reduced the areas of coastal plains where various communities had previously thrived.

Impacts of Marine Regression on Habitats

According to David Archibald, the loss of coastal habitats due to marine regression sixty million years ago stressed out a large number of vertebrate species.¹²

Some evidence indicates that the maximum regression occurred shortly before the end of the Cretaceous period. Keller and Stinnesbeck (1996) suggest that the maximum global regression occurred approximately 300–100 thousand years before the end of the Cretaceous period. The Western Interior Seaway apparently became land in the early Paleocene, and possibly earlier, at the K/T boundary. At the lithostratigraphic contact of the Hell Creek/Tullock formations, transgression is already beginning—that is, a rise in sea level.

Because of the regression, the shallow seaway began to retreat south and east. Huge herds of hadrosaurs and ceratopsian dinosaurs followed it. But as their habitat on the low coastal plains rapidly diminished, their species disappeared one by one, until the huge herds were reduced to a maximum of two or three remaining species. Dinosaurs, like any large vertebrates, were the first to experience biotic stresses leading to decline and extinction.

Decline of Some Populations

Populations of smaller terrestrial vertebrates also declined, but due to their shorter lifespan and high rate of species turnover, they adapted more quickly to the ecological stresses caused by the loss and fragmentation of coastal plains.

Unlike terrestrial vertebrates, freshwater species may have experienced much less stress, mainly because the size of their habitat was preserved and even increased as the lengthening rivers followed the retreating seas.

All ectothermic (cold-blooded) species of aquatic vertebrates (bony fish, amphibians, turtles, champsosaurs, and crocodiles) survived the events of the end of the Cretaceous well in their freshwater habitats (80%, or 37 surviving taxa out of 46).

The western continents of Laramidia and eastern Appalachia reunited, forming North America. The Bering Land Bridge reappeared near the K/T border. This apparently led to the migration of placental mammals from Asia (Nessov et al., 1998), which in turn contributed to the extinction of marsupials in North America. This, too, is part of the mass extinction.

Climate at the K/T Boundary

The hypothesis of atmospheric darkening due to dust led to another assumption—that it caused a significant decrease in temperature.

Proponents of the asteroid hypothesis claim that after the impact, “continents and oceans cool by as much as 28 °C and 11 °C, respectively” (Bardeen et al., 2017).¹³

This means that negative temperatures prevailed across the entire planet. The temperature below the freezing point lasted from 3 to 16 years, which inevitably should have led to the beginning of glaciation. However, there are no traces of glaciation at the Cretaceous-Paleogene boundary. Therefore, the assumptions about such a sharp and significant drop in temperature are incorrect.

On the continents, the temperature should be below zero for a period ranging from 45 days to six months (Toon et al., 1982). The temperature supposedly remained below zero for approximately twice as long as the period of darkness caused by dust.

Lécuyer stated that after the Cretaceous period, average temperatures in some areas fell by a whole 8 degrees Celsius (Lécuyer et al., 1993). Icelandic volcanologist and geochemist Haraldur Sigurdsson and others concluded that the average global temperature dropped by only 2–3 degrees Celsius from the middle of the Cretaceous period to the boundary of the Cretaceous and Tertiary periods (Sigurdsson et al. 1992). However, there is no unequivocal evidence of a sharp drop in temperature across the boundary.

As we can see, the supposed temperature drop becomes smaller over time and with new research. Assumptions about a sharp and significant decrease in ocean temperature are unrealistic due to the enormous heat capacity of the oceans.

However, all data on the assumed temperatures and the amount of dust and soot ejected are obtained as a result of modeling, which entirely depends on the initial conditions set by the experimenter. There are assumptions, but no facts.

Lack of Cooling at the End of the Cretaceous

Extinction, however, does not coincide with significant climate changes. Notable cooling began much later, at the end of the Eocene. Analysis by Christopher Scotese and colleagues shows that there were no climatic jumps at the boundary of the Cretaceous and Paleogene periods. Not even minor changes are present.

The analysis of climate changes indicates a consistently warm climate at the K/T boundary. There was no significant cooling or warming. However, earlier, at the boundary of the Jurassic and Cretaceous periods, the overall temperature dropped by an average of 10 °C. And nothing happened; the dinosaurs calmly endured it. The carbon dioxide content significantly increased at the boundary of the Jurassic and Cretaceous periods and then gradually decreased throughout the Cretaceous period. At the K/T boundary, there are no changes in its content in the atmosphere. This decrease continued until the middle of the Pliocene and then stopped. Again, there are no catastrophes or possible impacts on the animal world.

The climate of the Campanian and Maastrichtian was generally the same, with relatively low latitudinal temperature gradients. In the Polar Regions, the temperature did not drop below zero (Wolfe & Upchurch, 1987). On a global scale, the Campanian climate was warmer, while the Maastrichtian was more variable (Huber et al., 2002).

Researchers observed a change in paleotemperatures during the Maastrichtian, specifically between the late Campanian and the early Maastrichtian zone of Guembelitria tricarinata. This shift in temperature did not occur at the boundary between the Cretaceous and Tertiary periods. The next deviation of carbon isotope ratios towards negative values was recorded in the earliest Paleocene zone of Globigerina eugubina.

The tropical belts of the planet were much warmer at the beginning of the Cretaceous and colder at the end. At the very end of the Cretaceous (but before the asteroid fell), the temperature dropped almost to the same values that were at the very beginning of this period. However, it was still noticeably higher than, for example, at the end of the Miocene, when no mass extinctions occurred. Already at the beginning of the Paleocene, the temperature significantly rose. Only in the Eocene did it begin its gradual decrease toward modern values. Where in this time interval did the asteroid winter, which killed 75% of all species, fit? It is not here; no research finds evidence of a catastrophic temperature drop. On the contrary, research indicates a temperature increase at the K/T boundary.

Frog Problem

There is also the so-called “frog problem”: these amphibians, known for their sensitivity to climate changes, survived the end of the dinosaur era without losses, while large animals with substantial internal resources all perished.

The comparison of the late Cretaceous fauna of northern Alaska with the fauna of Montana proves that the supposed sudden drop in temperature was not a likely cause of extinction at the K/T boundary. Although the dinosaur fauna of Alaska is similar to that of Montana, it completely lacks amphibians, turtles, lizards, hadrosaurs, and crocodiles. This is despite the fact that the temperature difference was only 2–8 °C (Clemens and Nelm, 1993). This was enough for amphibians and other ectothermic tetrapods to be unable to live in late Cretaceous Alaska. Moreover, a sharp drop in temperature below zero at the K/T boundary should have completely devastated the rich fauna of ectothermic tetrapods in the middle latitudes. However, they thrived. The hypothesis of a sudden temperature drop simply does not match the data on vertebrates at the K/T boundary.

A study conducted in the state of Montana shows that not a single species of amphibian went extinct (Archibald et al., 1990). The asteroid fall does not explain such selective extinction.

Survival in the Tropics

Finally, the tropical climate belts and their inherent flora and fauna did not disappear at this time on the planet. It means that the consequences of the possible fall of asteroids at the end of the Cretaceous were local and insignificant on a global scale. After all, if even a short-term but deep cooling occurred in the tropics, it would be enough to kill a number of animals that had safely passed into the Cenozoic. These are crocodiles, giant turtles, and, especially, coral reefs, for which a temperature drop of just a few degrees is lethal.

Moreover, on the equator in coastal areas, daytime temperatures would have remained positive anyway. Many (if not all) dinosaurs, as has become known in recent decades, were warm-blooded and covered with feathers. Now, most birds (their offspring) tolerate frosts well if there is food. American hummingbirds fly to Russian Chukotka and even to Wrangel Island, where it is very cold. It is therefore incomprehensible why a cooling so small that its traces are still undetectable by instrumental methods could suddenly destroy almost half of the life on Earth. Including the relatives of Cryolophosaurus, who lived in Polar Regions and were accustomed to the cold. However, the extinction does not coincide in time with significant climate changes, and according to modern research, the warm-bloodedness of dinosaurs allowed them to cope with a possible drop in temperature.

The absence of selective extinction of thermophilic species, primarily coral reefs, testifies against cooling.

Gradual Adaptation of Flora

Clemens, Archibald, and others published another refutation of the Alvarez hypothesis. They argued that the fossil record of modern plants shows a gradual, progressive adaptation of flora to colder temperatures as the Cretaceous period ended and the Tertiary period began. The transition of flora and fauna from the Mesozoic to the Cenozoic was gradual, not catastrophic. They rejected the hypothesis of the impact of a supernova or the influx of Arctic seawater into more southern waters, which would have led to a decrease in global temperature.

Glaciations have occurred multiple times in Earth’s history, but they happened because of a gradual decrease in temperatures, not due to a catastrophic drop over a short period of time.

Study of paleotemperatures

In reality, there are many already proven methods for determining paleotemperatures when studying sedimentary deposits. For example, based on the isotopic ratios of ¹³C, changes in clay minerals, oxygen isotopes (¹⁸O and ¹⁶O), isotopic analysis of calcium carbonate CaCO₃ (foraminifera shells, etc.), optical characteristics of microphytofossils, the gaseous component of rocks, planktonic foraminifera using biometric methods, and many others. However, instead of instrumental studies, assumptions and models are used. But there is an explanation for this—because instrumental methods do not show a decrease in temperature at the Cretaceous-Paleogene boundary.

James Barnet, in his 2019 work, investigates changes in temperature and the carbon cycle from 67 to 60 million years ago using samples from a sediment core drilled in the southern part of the Atlantic Ocean at a depth of 4.7 km. The ratios of stable oxygen and carbon isotopes were analyzed, and the temperature was calculated based on oxygen isotope data using a published equation for temperature calibration. Notable warming occurred about 300 thousand years before the K/Pg boundary, which correlates with the dating of the Deccan Traps. The study shows that stable carbon isotope and temperature records clearly demonstrate that no extreme climatic effects like “impact winter” occurred after the asteroid impact.

For determining paleotemperatures over the last 4 thousand years of the Cretaceous and the first 30 thousand years of the Paleogene, fossil peat samples from two sites in Saskatchewan, Canada (paleolatitude ~55° N) were used. These are Ferris Coal at Wood Mountain Creek and Rock Creek West. The distribution of branched tetraether lipids (brGDGTs) was studied, which reflects the thickness of bacterial cell walls. Depending on the temperature, bacteria thicken or thin their cell walls. This allows for the determination of the mean annual air temperature with high resolution in 1,000-year increments.

It turned out that the temperature fluctuated from 16 to 29 °C, reaching a maximum in the first millennia of the Paleogene. The average temperature in the early Paleogene was ~25 °C, maintaining the warmth of the late Cretaceous, after which there was a general cooling to ~20 °C over the next ~30 thousand years. Researchers did not observe a sharp cooling after the K/T boundary (for example, “impact winter”) or a sudden warming. Additionally, they did not find evidence of long-term warming caused by volcanic activity.

The range of observed temperature changes is significantly wider than that obtained based on marine proxy records for the same period. Thus, these results more accurately determine the scale and duration of temperature change on land during this critical period (O’Connor et al., 2023).¹⁴

Greenhouse Effect Instead of Cooling?

And, as usual, if the direct assumption of an asteroid winter does not work, the exact opposite should be used. The emission of carbon dioxide, methane, and water vapor into the atmosphere could have caused greenhouse warming by several degrees (Beerling et al., 2002).

1. Julio Sepúlveda et al., 2009. Rapid Resurgence of Marine Productivity After the Cretaceous-Paleogene Mass Extinction. ↩︎
2. Alvarez L. W., et al. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 1980, v. 208, p. 1095–1108. ↩︎
3. Schultz, P. H., and S. D’Hondt (1996), Cretaceous-Tertiary (Chicxulub) impact angle and its consequences, Geology, 24, 963 – 967. ↩︎
4. C.B. Senel et al., 2023. Chicxulub impact winter sustained by fine silicate dust. ↩︎
5. Eric Hand, 2016. Drilling of dinosaur-killing impact crater explains buried circular hills. ↩︎
6. Ronald G. Prinn and Bruce Fegley, Jr., 1987, Bolide impacts, acid rain, and biospheric traumas at the Cretaceous-Tertiary boundary ↩︎
7. Alexander A. Cox & C. Brenhin Keller, 2023. A Bayesian inversion for emissions and export productivity across the end-Cretaceous boundary. ↩︎
8. Lyons et al., 2020. Organic matter from the Chicxulub crater exacerbated the K/Pg impact winter. ↩︎
9. Molly M. Range et al., 2022. The Chicxulub Impact Produced a Powerful Global Tsunami ↩︎
10. Hermann Bermúdez, 2022. The Chicxulub Mega-Earthquake: Evidence from Colombia, Mexico, and the United States ↩︎
11. Deposition and age of Chicxulub impact spherules on Gorgonilla Island, Colombia. Paula Mateo; Gerta Keller et al., 2019 ↩︎
12. J. D. Archibald, 1996 «Dinosaur Extinction and the End of an Era» ↩︎
13. Bardeen et al., 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections ↩︎
14. O’Connor et al., 2023. Steady decline in mean annual air temperatures in the first 30 k.y. after the Cretaceous-Paleogene boundary ↩︎