Dinosaurs ruled the Earth for 160 million years, beginning about 225 MY ago and ending 65 MY ago.  In the geological strata, the layer of clay  known as the Cretaceous-Tertiary (K/T) boundary layer marks the transition from the era of the dinosaurs (the Cretaceous) to the following post-dinosaur era (the Tertiary).  This boundary layer is well marked and recognized world-wide and has been long known to mark one of the largest mass extinctions in the fossil record.

What has always clearly marked this boundary layer is the fossils above and below. Below, in the older Cretaceous sediments, the tiny foraminifera  - single-celled organisms that have inhabited the oceans for more than 500 million years - come in a variety of shapes and sizes.
In the younger, Tertiary sediments, there are only tiny, less ornate foraminifera.

Other creatures, prominently the ammonites, the fish of the oceans (except they are cephalopods like the octopus and the chambered nautilus) in the Paleozoic and Mesozoic Eras, some 400 to 65 million years ago, abruptly disappeared.

And of course, the terrible reptiles, the dinosaurs, disappeared from the face of the Earth. Clearly, something happened 65 million years ago to cause a mass extinction.

The Gubbio Clay

In the late 1970s, a geologist at UC Berkeley, Walter Alvarez was conducting research on the geological strata surrounding the so-called K/T  boundary layer, in Gubbio, Italy.  A core sample of rock, taken across the boundary layer (but not from Gubbio) is shown here:

 core sample across K/T boundary layer

Alvarez brought samples of the boundary layer back to his lab at Berkeley and ran some standard tests.  One of those tests revealed that the boundary clay was heavily enriched (up to 5 ng/gm) in the element iridium, i.e., the amount of iridium (per gram of material) was tens to hundreds of times greater in the KT boundary layer clay than in normal, terrestrial rocks (~0.05 ng/gm) though less than chondritic abundance (500 ng/gm).  Iridium is an element that has very few uses: one of these is as a hardening agent for gold.  So the initial tests were thought contaminated by material sloughing off the gold wedding bands worn by the scientists working with the clay.  But more tests revealed no such contamination.  So, what does the enhanced abundance of iridium mean?

Iridium is known as a siderophile - or iron-loving - element.  Iridium attaches itself to iron and goes where the iron goes.  If you want to find or mine iridium, the best place is an iron deposit.  And this means that most of the iridium on Earth went to the core when the Earth differentiated.  Consequently, the abundance of iridium in almost all terrestrial rocks is very small.  Where could the iridium have come from?

Walter Alvarez obtained the answer after consulting with his father, Luis Alvarez, a nobel prize recipient for his work in physics: the iridium came from space,  from an asteroid.  Meteorites have lots of iridium, especially the undifferentiated and the iron meteorites. So, if an asteroid hit the Earth, and if the debris from the collision was distributed into a cloud surrounding the planet, and then if that debris cloud gradually rained down to Earth, we would find a small sedimentary layer with an enhanced level of iridium.

The Alvarezes estimated that the likely culprit would have been an asteroid perhaps 10 km in diameter, in order to produce the necessary amount of iridium.

Later tests confirmed that the iridium level is enhanced in the K/T boundary clay throughout the world, not just in Italy, by a factor of 10-50 compared to normal rocks.

So what happened?  Presumably an asteroid hit the earth.  The collision through an enormous amount of ejecta, including the vaporized asteroid, into the atmosphere.  Somehow (darkness? shock waves? global fires?), this event precipitated global mass extinctions.

Many scientists, including the Alvarezes and Eugene Shoemaker - famous for identifying meteor crater in Arizona as an impact crater, for training astronauts how to collect rocks on the moon, for Comet Shoemaker/Levy IX - went so far as to claim that all mass extinctions on Earth were likely caused by asteroid impacts and that extraterrestrial events such as this have been the single most important contributor to biological change in the history of Earth.  Darwin's fittest were the few creatures that could survive the impact of an asteroid, they claimed.  But this kind of evolution is profoundly non-Darwinian in that it posits extraterrestrial catastrophism as the primary mechanism for evolution.

Other evidence for the asteroid impact hypothesis

Soon, new evidence was being found in support of the impact hypothesis.  These include:

• iridium: see above.  In addition, note however that the iridium rich layer varies in thickness around the globe, and thus corresponds to a depositional time interval that varies from 10,000 to 100,000 years.
• amino acids: two amino acids, isobutyric acid and isovaline, have been found in the K/T sediments.  On earth, these two never occur together, are extremely rare, and are not found in life.  They have been found in meteorites.
• soot and ash: a thin layer of soot, presumably the result of global firestorms triggered by the impact, are found at the very top of the K/T boundary layer.
• shocked quartz grains (stishovite):  tiny grains of quartz, found in the boundary layer, show a mineral structure that is generated only when normal quartz  grains are hit so hard that a shock wave propagates through the quartz or are subjected to enormous pressures.  In normal quartz, each silicon atom is surrounded by four oxygen atoms.  In stishovite, each silicon atom is surrounded by six oxygen atoms.. Shocked quartz grains are found at only two places on Earth: in the ejecta from asteroidal impacts and in the debris from a nuclear test site explosion.  They are not associated with volcanic debris. Since we can safely assume that the dinosaurs did not possess nuclear weapons, and since the stishovite is found globally, it is most likely that its existence testifies to the impact of an asteroid.
• tektites: tektites are are small, glassy beads that are formed from rocks that are melted and ejected during a very high velocity impact.  As pieces of the molten rock fall back to Earth, they cool rapidly, yielding aerodynamic shapes to these pieces of glass.  Tektites are found associated with a few "strewn fields," i.e. the ejected material from known impact sites.  And tektites are found in the K/T boundary layer, but not globally; they are found exclusively in Caribbean and Gulf of Mexico region samples, from Alabama to Haiti.
• tsunami deposits: in the Gulf of Mexico, or more properly at the K/T boundary surrounding the Gulf of Mexico as it was 65 million years ago, a sedimentary layer of sand, 50 m - 100 m thick, deposited violently (i.e., with big and small rocks mixed together) suggests a tremendous wave that ripped through the Gulf.  At the top of this tsunami-like deposit is a thin layer of iridium-rich silt.
• buckyballs (fullerenes): These are hollow, cage-like molecules made of 60 carbon atoms (C₆₀). They can trap gases within.  In buckyballs found in the K/T boundary layer, gases enriched in ³He relative to  ⁴He (as compared to the ratio of these helium isotopes in terrestrial reservoirs) has been found.  Cosmic sources are richer in  ³He  than are terrestrial sources because terrestrial sources of helium are enriched by the ⁴He that is produced through radioactive decay of uranium and thorium.
• a crater: a crater of the right age and approximately the right size has been found in Mexico, on the tip of the Yucatan peninsula, near the town of Chicxulub.  This crater is buried beneath about 1 km of sediments, is at least 180 km and perhaps as large as 300 km in diameter, is roughly circular, and has a radiometric age of 64.98  +/- 0.05 million years.  The size of the crater is quite important to consider carefully.  The crater is inaccessible except through core drilling and in examinations of the magnetic and gravitational signature of the rocks.  The initial discovery of the crater was based on the circular patterns in the gravity and magnetic field data, with a diameter of 105 +/- 10 km.  Later, the gravity data revealed two concentric rings outside of the initially identified pattern; this extended the crater size to 170 +/- 25 km.  At this size, impact scaling arguments indicate that the impact event would have excavated a crater to a depth of 45-60 km, initially, filling back in quickly to a depth of 17-20 km.  Then, another ring was found with a minimum diameter of ~280 km.  Thus, Chicxulub is well established as a multi-ring basin with D = 260-300 km.  Astoundingly, since the heavy bombardment ended ~4 billion years ago, the only post-heavy bombardment crater of comparable size to Chicxulub that is known is Mead crater, a 280-km crater (the largest) on Venus. No other craters of this size that formed in the last 4 BY are known on Mercury, Venus, Mars, Earth, or the Moon.  Thus, it is likely that this impact event was the largest such event since the development of multi-cellular life 1 BY ago and possibly since the development of life itself.

The K/T Extinction: the devil is in the details

Extinctions are identified on the basis of changes in the fossil record.  So, in order to associate the K/T extinctions with the impact, one must demonstrate that:

1. a global, mass extinction occurred, and
2. this extinction was temporally associated with the impact event, and
3. some causal mechanism links the two events.

How do fossils form?

Sedimentary rocks from from the (usually gradual) deposition of eroded material on stream beds or lake bottoms.  So only certain environments will permit the formation of sedimentary rocks, at any given point in time.

Typically, a sedimentary rock from the Cretaceous-Tertiary eras can be dated to within the closest million year interval, sometimes to within +/- 250,000 years.  Even if we can obtain an accuracy of 50,000 years, is that sufficient to demonstrate than an extinction event was sudden, almost instantaneous (a year, a few years, a few decades)?
 

What is known from the fossil record?

25% of all terrestrial families went extinct.  Only small species (below ~25 kg) survived.

Higher planets, birds, mammals, and dinoflagellates (single celled photosynthetic creatures) were generally unaffected.

Dinosaurs: The dinosaurs arose 225 million years ago and flourished until the end of the Cretaceous, 65 million years ago.  But the dinosaurs that lived 180 MY ago were not the same species as those that died out 65 MY ago.  And, whereas there were 60 species of dinosaurs on Earth 75 MY ago, only 18 of those species are found in strata from 65 MY ago.  So, clearly dinosaurs were suffering, probably from some long term climatic changes. No dinosaur fossils are  found above the K/T boundary; however, no dinosaur fossils are found within a meter below the K/T boundary.  The last appearance of dinosaur eggshells and footprints occur 2 million years prior to the K/T boundary.  Did the dinosaurs die out 100s of thousands of years before the impact? Or, are conclusions in this regard  too hard to draw because there are so few large animals and even fewer fossils preserved?

Icthyosaurs: These fish-lizards (dophin shaped; up to 9-m in length) died out 30 million years before the K/T extinction.

Plesiosaurs: These creatures had decreased from 6 families to only 2, at the approximate time of the K/T extinction.

Vertebrate species: More than 50% of vertebrate species survived across the K/T boundary, in the fossil record of what is now the western United States.   Not including very rare fossil species, for which often only a single fossil is known, 70% survived.  The extinctions were concentrated in only five of 12 taxa of vertebrates.  Hit hard were  sharks & relatives (less than 20% survival), bird-hipped dinosaurs (0% survival), reptile-hipped dinosaurs (0% survival, except birds), lizards (less than 30% survival) and marsupials (less than 10% survival).  Completely unaffected were frogs & salamanders, champsosaurs, and placentals; barely affected were turtles (85% survival), crocodilians (80% survival), bony fishes (70% survival) and multituberculates (50% survival).  One must ask what kind of event would cause this kind of survival and extinction pattern.  Clearly, the K/T extinction did not affect all types of vertebrates equally.  What kind of changes would cause the noted extinctions?  A quick, sustained temperature drop is consistent with the survival and extinction patterns for only 4 groups; acid rain is consistent with only 3 groups; global wildfire is consistent with 5 groups; local wildfires are consistent with 9 groups; global marine regression and habitat fragmentation (the shallow epicontintental seas that formed in the late Cretaceous receded quickly, producing the largest single increase in non-marine area  during the past 250 million years) is consistent with 11 of 12.  Thus, the vertebrate fossil record is not consistent with an impact caused extinction.

Mosasaurs: These giant marine reptiles (up to 17-m in length) lived from about 90-65 MY ago.  They appear to have been diversifying and undergoing a mass radiation shortly before the K/T event, with most of this occurring from 75-65 MY ago.  They apparently went extinct abruptly at the K/T boundary.

Rudistid bivalves: These were marine creatures that live on reefs.  Their fossil remains are considered a proxy for tropical, shallow-marine ecosystems.  Rudistids survived the end of Jurassic, late Aptian and Cenomanian-Turonian extinctions.  At the end of the Cretaceous, they became extinct suddenly, but the demise of the reef systems occurred 1.5 - 3.0 million years before the K/T boundary.

Ammonites:  The abundance of ammonites (from K/T strata in Chile) declined gradually and disappear 5 meters below the K/T boundary.  This suggests their disappearance was due to environmental changes in the late Cretaceous (the Maastrichtian).

Radiolarians: These are protozoans with silica shells; they are closely related to foraminifera, but are exclusively marine and planktonic.    Radiolarians are absent from the vast majority of K/T boundary sections.  In modern seas, siliceous plankton are only abundant in areas of high surface productivity, where wind and current systems cause continuous upwelling of nutrient-rich deep waters.  In New Zealand strata, where radiolarians are abundant in the fossil record of the late Cretaceous and early Tertiary, 100% of radiolarian species survived and crossed the K/T boundary.  They show no indications of a mass extinction.

Benthic (deep water) Foraminifera:  There is general agreement that foraminifera underwent a major mass extinction, at least at low to middle latitudes, at or near the K/T boundary.  12% of these species went extinct 300,000 years prior to the K/T boundary, 57% appear to go extinct at the boundary, 31% of the species (i74% of all fossil individuals) survived. Thus, these data strongly support the existence of a sudden and major biotic crisis, but one that appears to have acted on an already highly stressed ecosystem.

Caribbean foraminifera: These creatures appear to have lived right up to the K/T boundary and then disappeared.

Sea Urchins (echinoids): 36% of these species went extinct in the late Maastrichtian, but it is unclear how sudden or how close to the K/T boundary this occurred.  Another 15% went extinct during the early Paleocene, i.e., after the K/T boundary. Post-Cretaceous survivors were dramatically smaller than their predecessors.  The extinctions seemed to have hit adults at depth harder than planktonic larvae, feeding at the surface.  The extinction pattern could be due to a decrease in phytoplankton abundance at the end of the Cretaceous; this change would have had to be small enough to not seriously affect surface larvae but large enough to affect deep water feeders.  The conclusion drawn from this group is that an asteroid may have been the final blow, but that the climate was already stressed for this type of creature.  Furthermore, the continued extinctions after the K/T boundary suggest a very long term stress exacerbated but not caused by an impact.

The last 50,000 years on Earth

In the last 500 years, the dodo bird as well as many other island avian species perished, when they could not withstand the predation and diseases brought by new species to previously isolated islands.

In the last 150 years, many species (buffalo, alligator, grizzly bear, rhinos, elephants,whales, carrier pigeon) have been driven (nearly) to extinction by hunting.

In the last  50 years, many species, such as wild cats (lions, tigers, leopards), the spotted owl, snail darter, many birds and butterflies, etc., are being driven to extinction through loss of habitat in competition with humans.

In the last 3000 years, we have seen the explosive abundance of cows, horses, dogs, cats, rats, mice, sheep, people, and cockroaches, all due to the increasing human control over the environment.

Now, we imagine that an asteroid hits the Earth in the year 2080 and wipes out humanity (and many other species).  Or we imagine that a nuclear war does the same thing.  What if the most successful surviving species (rats) reach modern human intelligence in 100 million years and investigate the geologic record of this epoch?  With the accuracy we currently have, they would conclude that ALL the above creatures died out suddenly and catastrophically, that they all lived right up to the asteroid impact (or nuclear war) and then were dramatically wiped out.  But, in fact, we know that there are numerous reasons for these extinctions, most of which have nothing to do with the impact (or war), and that in fact some species were proliferating at the expense of others, yet the fossil record would merely show a catastrophic wipe out!
 
 

Was the Asteroid the Cause of the Extinctions, or the Straw that broke the Camel's back?

In addition, there were enormous volcanic eruptions at about the same time, forming the Deccan traps in India.  Large scale emissions of CO₂ are associated with volcanic eruptions; thus, it is possible that the Deccan flats eruptions could have influenced climate change.
 

What about other Extinctions?

In 2001, buckyballs enriched relative to terrestrial abundances in Helium and Argon isotopes were found in the layer of strata that marks the Permian-Triassic extinction, 250 million years ago.  This extinction was much more profound, in terms of the level of extinctions that occurred, than the K/T event.  However, despite searches, no iridium enrichments have been found associated with this extinction. Did a comet (rich in helium and argon, not rich in iridium) wipe out the trilobites?
 

              Asteroid killed 90 percent of life on earth millions  of years ago

                 February 22, 2001
                 From CNN Science and Technology Correspondent Ann Kellan

                 ATLANTA (CNN) -- Before the dinosaurs were wiped out 65 million years ago,
                 there was an even more devastating event millions of years before which wiped out
                 90 percent of life on earth, scientists said.

                 In a report in the journal "Science," researchers said 250 million years ago, before the
                 dinosaurs existed, an asteroid four to eight miles across slammed into earth with the
                 force of more than a million earthquakes. That impact caused volcanic eruptions
                 that buried much of the earth in lava and kicked up so much dust and ash it blocked
                 the sun's rays. The result was a dark and cold earth, which could no longer support
                 most of the life on it, including cockroach-like creatures called trilobites.

                 "This is probably the most famous type of animal that existed in the Paleozoic era"
                 said Richard Bambach of Virginia Tech, who added that the trilobites went
                 "completely extinct."

                 Ninety percent of the 15,000 species on earth became extinct  in the aftermath,
                 including shellfish and coral. Fish fared better than most, Bambach said.

                 "These very active animals actually made it through with only 40 percent rather
                 than 90 percent extinction," he said.

                 No one knows where this asteroid hit. The one that scientists say caused the
                 extinction of the dinosaurs hit in the Yucatan Peninsula in Mexico. Another smaller
                 asteroid left a crater in Arizona.

                 Does this mean it could happen again? If so, when? Not to worry, scientists say. They
                 calculate an asteroid hits earth every million years or so. We know where the big ones
                 are and even if a smaller one was coming at us we'd get plenty of notice, said Chris
                 Chyba of Stanford University.

                 "We would almost have decades, if not centuries, to go before that impact would
                 happen so we would have a long time to think about what to do about it," he said.

                 Scientists discovered the earlier, devastating extinction event by digging deep into the
                 earth's core. They found gases which are normally only found in outer space, trapped
                 in carbon molecules called buckminsterfullerenes, or buckyballs.

                 Scientists say those alien gases rode in on a speeding asteroid and even though the
                 devastating impact destroyed life, it paved the way for the dinosaur era 25 million
                 years later.

If any life existed 4 billion years ago, would it have survived the a supposed "cataclysmic
bombardment" near the end of the great bombardment?

                Asteroid storm pounded Earth, moon as life formed

                  December 7, 2000

                  (CNN) -- A new study of lunar meteorites strongly suggests that the moon and
                  Earth suffered a brief but cataclysmic bombardment of asteroids almost 4
                  billion years ago at roughly the same time that life formed on our planet.

                  The flurry of killer space rocks could have had major consequences on early
                  Earth evolution, whether smashing the building blocks of primordial life or
                  delivering them to the planet in the first place, researchers said.

                  The burst of impacts lasted from 20 million and 200
                  million years, flanked in time by periods of relative calm. The rain of meteorites or
                  comets melted rocks, blasted out craters and reshaped the surface across the entire
                  moon, according to a new report in the journal Science.

                  The bombardment would have produced the great basins that now shape the
                  man in the moon. On Earth, the same cataclysm would have affected the
                  evolution of life, possibly forcing it to begin anew, according to scientists.

                  "Earth would have been bombarded by at least 10 times as
                  many impact events as the moon, and these impact cratering
                  processes are immense," lunar geologist David Kring said in a statement.

                  The findings are based studies of four lunar meteorites -- rocks that were ejected at
                  random from the moon's surface and landed on Earth after a million or so years in
                  space.

                  The scientists applied a sophisticated dating technique to the meteorites to determine
                  when the lunar bombardment occurred.

                  The Chixculub crater in Mexico, which is linked to the mass extinction of dinosaurs
                  65 million years ago, "is puny by the scheme of we are talking about. Here we are
                  talking about impacts that are 10 times larger, impacts that blasted craters rim to rim
                  the size of continents on Earth today," he said.

                  The blasts would have vaporized the oceans and filled the atmosphere with a
                  life-choking fog, all but eliminating terrestrial life if it existed.

                  "If there was life on Earth before the bombardment, the question is what, if anything
                  survived," said lunar researcher Timothy Swindle in a statement.

                  Unlike the lifeless moon, where the ancient scarring remained easily visible, the
                  Earth has blotted out signs of the cataclysm through erosion and natural geologic
                  changes over the eons.

                  Rather than death, ancient meteorites or comets could have brought life, some
                  scientists theorize. Proponents of a theory called "panspermia" suggest that complex
                  organic molecules or even primitive life forms seeded the Earth after riding aboard
                  rocky or icy cosmic debris.

                  Previous analysis on lunar rocks returned by the Apollo and Luna missions in the
                  1970s suggested that the moon took a pounding near the equator where the samples
                  originated.

                  But a sophisticated examination of lunar meteorites now offers strong evidence that
                  the maelstrom hammered the entire lunar surface, according to a December 1 report
                  in the journal Science.

                  Barbara Cohen of the University of Tennessee-Knoxville was the lead author of the
                  report, co-authored by Kring and Swindle, both of the University of Arizona's Lunar
                  and Planetary Lab.
 

Note: the origin of the dinosaurs?

http://www.cnn.com/2002/TECH/space/05/16/asteroid.dinosaurs/index.html