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What Really Did Kill The Dinosaurs?

Lord_Flashheart said:
Doglas adams might have but terry pratchett also used this idea in his book "Strata".

IIRCC the leading character in 'Strata' had been sacked for a joke with a fossil dinosaur holding a sign saying "No More Nukes"

In the cartoon strip 'Life the Universe and (almost) everything' by Kate Charlesworth that ran in New Scientist for several years (history of science explained by a cat and a chicken), there was strip on the theories of dinosaur extinction, with...

'They all migrated to the same side of Atlantis and it capsized and sank.' ;)
 
Timble said:
'They all migrated to the same side of Atlantis and it capsized and sank.' ;)

IIRC, wasn't there an SF story along the lines that they were doing just fine until they (eventually) discovered nicotine and alcohol?...

('Scuse while I drop the fag butt into the empty beer can...:eek!!!!: ! )
 
Originally posted by DerekH
IIRC, wasn't there an SF story along the lines that they were doing just fine until they (eventually) discovered nicotine and alcohol?...

I'm pretty sure there's a Gary Larsen cartoon to that effect anyway ;)
 
I always found it somewhat humourous when ( in books and documentaries ) they make it seem as though the dinosaurs were somehow ( in terms of evolution ) failures, and were swept aside just in time for us far superior teat-bearing rodents to carpe diem. Godzilla and his kin were here for a whole helluva lot longer than us, and in light of our track record as a species...well, we might not want to slap ourselves on the back just yet...
 
Scientists challenge theory of how dinosaurs died out

13.10.2004

Birds and mammals may have displaced dinosaurs gradually in the 20 million years before the disastrous asteroid collision traditionally blamed for wiping dinosaurs out, say a New Zealand evolutionary biologist and a British colleague.

Professor David Penny, from Massey University, and Matt Phillips from Oxford University said fossil and molecular evidence did not support the theory of an asteroid-impact extinction.

Popular theory says that birds and mammals flourished on Earth only after an asteroid impact wiped out the dinosaurs and pterosaurs, or flying reptiles, at the end of the Cretaceous period 65 million years ago. But Professor Penny and Dr Phillips are not convinced.

"We agree completely with the geophysicists that an extraterrestrial impact marks the end of the Cretaceous," Professor Penny said. "But after 25 years they have still not provided a single piece of evidence that this was the primary reason for the decline of the dinosaurs and pterosaurs."

Writing in the October issue of Trends in Ecology and Evolution, the two scientists have said that instead of accepting the geophysicists' theory at face value they want scientists to take a closer look at the fossil and genetic evidence.

"Although the asteroid at the end of the period was real, we think it's natural evolutionary processes that made the difference," Professor Penny said.

They believe that mammals and birds started to out-compete dinosaurs between 80 to 90 million years ago.

Professor Penny said fossils could tell scientists when the different species of dinosaurs, birds and mammals roamed the Earth, which would indicate when the dinosaurs started to decline and when birds and mammals began to proliferate and diversify. But the fossil record was patchy.

Increasingly sophisticated technology and techniques in molecular biology that had enabled advances such as the sequencing of the human genome were powerful new tools in the scientists' arsenal.

By looking at the molecules from living animals scientists could, theoretically, reconstruct the family trees of all living animals. This could help to show whether the ancestors of the living birds and mammals arose very quickly after the asteroid hit the Earth or whether their appearance was far more gradual.

"So far, this evidence contradicts the popular theory," Professor Penny said. "The evidence from fossils and molecules appears to support an expansion of birds and mammals, and a decline of pterosaurs and dinosaurs, starting many millions of years before the end of the Cretaceous."

The impact, its effects and disruption to ecosystems, probably finished off the dinosaurs that were not bird-like.

But the two biologists said a dogmatic adherence to the popular theory had steered scientists from examining the real reasons behind the mass extinction.

"I see the discovery of the asteroid impact that marked the end of the Cretaceous as simultaneously a high point and low point of 20th-century science," said Professor Penny.

"It was a high point from the view of a brilliant new explanation of the iridium layer that correlated geological strata world-wide: outstanding.

"But it was also a low point, equivalent to the report of supposed N-rays, cold fusion, and inheritance of acquired characters on the pads of the midwife toad."

The killer comet idea was first aired by United States geologist Walter Alvarez in 1980. He argued that large amounts of the metallic element iridium found in sediments deposited at the end of the Cretaceous period showed that a huge meteorite had struck Earth with such force that it raised a suffocating dust cloud that shut out the sun for years.

In New Zealand, coal seams in a stream bank next to the Moody Creek coal mine north of Greymouth have an iridium concentration of 71 parts per billion, the highest for non-marine rocks anywhere in the world.

But the asteroid impact theory has long been controversial. A professor of palaeontology at the French Natural History Museum, Leonard Ginsburg, started arguing more than 30 years ago that a gradual drop in world sea levels led to disastrous climate changes for dinosaurs.

Professor Ginsburg has strongly criticised the American theory that the dinosaurs were wiped out after a giant meteorite smashed into the Earth with a force estimated at five billion times that of the Hiroshima nuclear bomb.

"It is obvious the dinosaurs died over the space of millions of years and not in one cataclysmic event," he told the Reuters news agency. "The trouble is Americans like wonderful disaster scenarios and my idea is not spectacular enough."

Solid evidence that at least some dinosaurs slowly dwindled into oblivion rather than being blasted off the face of the Earth has been found in fossil sites in the US state of Montana.

Digs have shown that 75 million years ago there were 30 species of giant reptiles living in the area. Five million years later there were 23, within two million years the number had fallen to 18, and so on down until the end of the so-called Cretaceous period when all the dinosaurs had died out.

Earth's history has been marked by a succession of mysterious periods of mass-extinction, when whole families of animals disappeared for good. They occurred at the end of the periods known as the Permian (245 million years ago), Devonian (360 million years ago), Ordovician (438 million years ago) and the Cambrian (510 million years ago).

Professor Ginsburg has argued that a common element probably tied all of these events together - sea movement caused either by changes to the polar icecaps or shifts in the Earth's crust.

Source
 
Multiple Impacts?

by Leslie Mullen




Rather than a single meteorite impact 65 million years ago, could Earth have been hit with a scattershot of several rocks from space?

It may have happened before. There is evidence that about 35 million years ago, at least five comets or asteroids collided with Earth. If the effects of a single large meteorite impact seem overwhelming, imagine how life on Earth would reel from a barrage of rocks from space.

One way such impact clustering happens is to have a single bolide break up as it approaches a planet. The comet Shoemaker-Levy 9 provides a recent example. Before striking the planet Jupiter in 1994, the comet was torn into 21 different pieces by Jupiter's immense gravity. These fragments struck Jupiter over 5.6 days, some creating large fireballs as they entered Jupiter's vast gaseous atmosphere.

Earth's gravity is no where near as powerful as Jupiter's, so the same scenario would not happen to Earth. Yet many asteroids are thought to be rubble piles, loosely bound by gravity, and such an asteroid could rip apart as it approached our planet.

But if an asteroid "rubble pile" broke up before it entered Earth's atmosphere, the pieces would only result in one crater, or at most two, because most of the pieces would fall into the same hole.

"Once such a rubble pile enters Earth's gravity it's too late," says Christian Koeberl, a geochemist at the University of Vienna in Austria. "It would only get into the attraction field of Earth's gravity a few hours before it hits at best, and this is not time enough to spread it out appreciably."

However, having an asteroid break up as it approaches Earth is not the only way to end up with multiple craters.

Within the asteroid belt that orbits the sun between Mars and Jupiter, collisions between asteroids sometimes occur. The resulting fragments can then rain down on Earth. Simon Kelley, a geologist at the Open University in England, says that such a collision occurred 470 million years ago, and many of those fragments traveled to Earth. In fact, some of the fragments still are impacting Earth today.

A similar shower of fragments can come from a collision within the Oort cloud, a comet-filled region in the outer-most portion of the solar system. Kelley says such a cometary shower may be responsible for the cluster of impact craters dated to be 35 million years old, including two of the largest impact craters on Earth: the 100 kilometer Popigai crater in Siberia, and the 90 kilometer Chesapeake Bay crater off the shore of Maryland. This cometary shower is thought to have lasted for 2 to 3 million years.

tunguska_event


Looking over the Planetary and Space Science Centre's Earth Impact Database, several of the crater dates overlap. Much of this overlap reflects the limitations of current dating techniques, where ages can't be narrowed down further than hundreds of thousands of years. But it is possible that some of the craters point to a multiple impact scenario.

The Boltysh crater in the Ukraine may be proof that multiple impacts occurred during the Cretaceous-Tertiary (K-T) extinction. Kelley has dated the crater to be about 65 million years old.

The Boltysh crater had previously been assigned ages ranging from 88 million to 105 million years old, a variation that arose due to the different dating methods used. Kelley used Argon-Argon dating to determine the correct age for the crater. The crater rocks melted in the heat of impact, and when they cooled they trapped argon out of the air. Different isotopes of argon decay at different rates, so by measuring the ratio of argon isotopes, Kelley was able to estimate when the rocks melted, within a margin of error of plus or minus 600,000 years.

The margin of error prevents scientists from saying Boltysh was definitely part of the same meteorite strike as Chicxulub. And Kevin Pope of Geo Eco Arc Research points out that while Chicxulub has been linked directly to the K-T boundary and extinctions by the stratigraphy of its ejecta, the same is not true for Boltysh.

"Craters the size of Boltysh form every few million years, so the fact there is a crater this size close to the boundary is no surprise," says Pope.

The Boltysh crater measures only 24 kilometers in diameter - compared to Chicxulub's nearly 200-kilometer-wide monster - so even if the smaller meteorite that made this crater hit around the same time as Chicxulub, its effects wouldn't have been as catastrophic.

But if Boltysh did occur around the same time as Chicxulub, it could aid in our understanding of the K-T extinction event. Craters can act as time capsules, preserving information about the environment at the moment of impact.

Like Chicxulub, the Boltysh crater is buried underground. But while Chicxulub filled with seawater, the Boltysh crater became a fresh water lake. Kelley is writing a paper with Dave Jolley at the University of Sheffield on the microflora and fauna they have found in the Boltysh crater fill. They hope to determine how rapidly life recovered in the vicinity of the impact, and how that recolonization occurred.

As for other K-T impact craters, Kelley notes that the record is very poorly dated. He plans to study other craters, currently dated to be from the Devonian (around 380 million years ago) to the Eocene (34 million years ago), to see if their ages are accurate. By determining the correct dates for impact craters, scientists will be able to better understand how often multiple impact events have occurred in the past.

So do multiple impacts play a role in mass extinctions? Kelley says that as far as we know, they don't. For instance, Kelley says there is no evidence that the barrage of comets 35 million years ago led to a mass extinction event.

"The effects would have been truly devastating locally, but they didn't amount to global catastrophes," says Kelley. "You can argue that minor extinctions are associated, but not a K-T-like event."

A single large meteorite impact like Chicxulub may be more harmful to life than a cluster of several smaller meteorites or comets spread out over a million years or less. Yet determining why certain species die out can often be difficult. Extinctions are a natural part of the cycle of life, and may occur due to a whole host of interrelated factors, including competition for food, climate change, and even sea level change. Perhaps tossing a few meteorites into the mix also can upset the scales, tipping some species too far off balance to recover.

------------------
Part 1, Debating the Dinosaur Extinction. Part 3 of this series will discuss a controversial impact crater off the coast of India. Part 4 will cover the debate over whether the K-T extinction was brought on by global warming.

Source
 
Ancient tsunami scrambled the fossil record - link

ONE of the few remaining objections to the idea that an asteroid killed off the dinosaurs has been put to rest. The giant tsunami caused by the impact may have scrambled fossil evidence, explaining puzzling finds that suggested the asteroid and the mass extinction were unrelated.

Many geologists have long argued that the Cretaceous period ended when a 10-kilometre-wide asteroid slammed into the Gulf of Mexico, just off the Yucat´n coast. That point in history bears the hallmarks of a global disaster - mass extinction, a giant crater, impact debris such as solidified rock droplets, and traces of iridium typical of an asteroid. However, fossils from the end of the Cretaceous have been found above layers of rock linked to the asteroid impact, and Gerta Keller of Princeton University has used this evidence to argue that the extinction may have come 300,000 years after the impact.

Now Tim Lawton of New Mexico State University in Las Cruces and his colleagues have shown how fossil records could have been shuffled by the tsunami following the impact. Earlier evidence has shown that the tsunami may have been 150 metres high, carrying water up to 300 kilometres inland.

Lawton's team studied rocks in the La Popa basin in north-east Mexico and found layered deposits that contain a jumble of debris from the asteroid impact and fossils of various organisms that lived in disparate environments. "They are organisms that did not originally live together," says Lawton.


The deposits include oysters from bays, snails from marshes and foraminifera that float in open water, along with impact debris such as solidified molten rock, says Lawton. As the tsunami retreated it dumped these together at La Popa (Geology, vol 33, p 81). The team argues that the tsunami could also have messed up fossil records elsewhere, explaining how Cretaceous fossils ended up above debris from the asteroid impact. Keller, however, remains unconvinced that the tsunami formed the deposit at La Popa or that it happened at the end of the Cretaceous.
From issue 2486 of New Scientist magazine, 12 February 2005, page 15
 
Were the dinosaurs done in by fungus?

Quest to figure out why mammals are warm-blooded led to new theory

By Carolyn Y. Johnson, Globe Correspondent | February 22, 2005

In the still unsolved mystery of how the dinosaurs died, there's a new suspect -- fungus.

After a meteor slammed into the Earth 65 million years ago, "the great dying" began, decimating life in the oceans and killing off the dinosaurs -- with mysteriously little effect on mammals. Conjecture over what did in the reptiles has long fascinated everyone from school children to paleontologists, but a new theory suggests that a less earth-shaking possibility could have played a role.

"The forests went out. The fungi proliferated, and the Earth became a giant compost pile. An enormous number of spores were released," said Dr. Arturo Casadevall, an infectious disease researcher who proposed last month that air thick with fungal spores after the meteor hit could have overwhelmed animals' immune systems, causing sickness and death. If he's right, the large numbers of warm-blooded mammals and birds that survived the mass extinction might have had a natural advantage -- body temperatures too hot for fungal infections to take hold.

"It's just a beautifully creative suggestion," said Nicholas Money, a mycologist, or mold expert, from Miami University of Ohio and author of "Carpet Monsters and Killer Spores: A Natural History of Toxic Mold."

Casadevall, of Albert Einstein College of New York, laid out his suggestion in this month's issue of Fungal Genetics and Biology when considering a much larger question: "I ask you, why are we so hot?"

He has long been troubled by the lives of warm-blooded animals, who must live a virtual food-finding mission because they burn so many calories each day just heating their bodies. Cold-blooded animals, on the other hand, need only eat once every few days. Where, he wondered, is the advantage in a life of constant scurrying, foraging, and saving up food for the winter?

That question coincided with another puzzling trend: Fungal infections rarely give mammals more than a mildly irritating case of athlete's foot or a yeast infection but are often deadly to plants, fish, and insects.

At a crucial time in natural history, the world's 1.5 million species of molds, yeasts, rusts, and mushrooms, also might have been a vehicle for natural selection.

In the aftermath of the meteor that carved out the Chicxulub crater on the Yucatan Peninsula, the Earth probably was a cool, shady place. Researchers last year discovered fossil evidence of a post-collision "fungal spike," and in a world dense with potentially pathogenic fungi, warm-blooded animals might have had a unique advantage.

In such a situation, "every warm-blooded generation has a little advantage, and when the dust settles and the sun comes out again . . . the warm-blooded find themselves in a world with a lot more space," Casadevall said.

Other evidence shows that the mass die-off didn't occur immediately after the collision, but about 300,000 years afterward -- raising the possibility that an intermediary factor, like fungi, could have played a part.

The trouble with the theory, experts said, is that no one is sure whether the dinosaurs were warm- or cold-blooded. Smaller cold-blooded animals like turtles, lizards, snakes, and frogs were able to weather the mass extinction, indicating that size, not body temperature, may have been a deciding factor.

And, while there is wide agreement that a massive meteor struck the Earth 65 million years ago, other theories suggest that increased volcanic activity could have played a role in the extinction.

Stephen McLoughlin, a geologist from Queensland University of Technology in Australia who discovered evidence of the long-ago fungal explosion, said the spores that his group studied, which were preserved in a layer of coal in New Zealand, probably did not harm animals.

He stated in an e-mail that he finds Casadevall's idea "intriguing" but, "while this may have been the case, it is virtually impossible to test."

Nonetheless, the main idea behind Casadevall's research -- that deadly fungi could have helped establish the age of the mammals -- is timely.

Fungal infections are now emerging as an important force in nature again: Fungal diseases also may be contributing to the worldwide decline of the coral reefs, and appear to play a poorly understood role in the steady decline of amphibians.

A study last year reported that a third of all amphibian species worldwide are facing extinction -- and while climate change, pollution, and habitat loss are all thought to play a role, many of the extinct and endangered frog species have been infected with the chtyrid fungus, which may interfere with their delicate, breathable skin, produce a toxin, or something else.

"Like everything in life, it wasn't just one thing" that killed the dinosaurs, Casadevall said. In the case of the amphibians, "you can imagine [the culprit] could be a weakening of their immunity caused by a fungus."

-------------------------
© Copyright 2005 Globe Newspaper Company.

Source
 
Birds and mammals may have displaced dinosaurs gradually in the 20 million years before the disastrous asteroid collision traditionally blamed for wiping dinosaurs out, say a New Zealand evolutionary biologist and a British colleague.
How times change!

When the asteroid impact theory was first proposed (1980?) it was considered sensationalist, crazy science, and most 'traditional' palaeontologists rejected it - until the evidence piled up.
 
And, to the best of my knowledge, said evidence is still piling up.

Didn't they recently decide that some of the "contrary" evidence was actually the result of the impact (and related tsunamis etc) mixing up the sediment layers?

Now, the impact might have given the fungus a chance to get a hold, but I think the impact probably had the most significant effect.

As to how mammals survived - maybe it's because they were much smaller and more ready to adapt to the conditions after the impact. (Some mammal species were extinguished by the impact as well.) Or maybe it's just luck.
 
Mass extinctions: a threat from outer space or our own plane

Mass extinctions - a threat from outer space or our own planet's detox?

University of Leicester scientists suggest extraterrestrial theories are flawed and that more down to earth factors could have accounted for past mass extinctions
Earth history has been punctuated by several mass extinctions rapidly wiping out nearly all life forms on our planet. What causes these catastrophic events? Are they really due to meteorite impacts? Current research suggests that the cause may come from within our own planet – the eruption of vast amounts of lava that brings a cocktail of gases from deep inside the Earth and vents them into the atmosphere.

University of Leicester geologists, Professor Andy Saunders and Dr Marc Reichow, are taking a fresh look at what may actually have wiped out the dinosaurs 65 million years ago and caused other similarly cataclysmic events, aware they may end up exploding a few popular myths.

The idea that meteorite impacts caused mass extinctions has been in vogue over the last 25 years, since Louis Alverez's research team in Berkeley, California published their work about an extraterrestrial iridium anomaly found in 65-million-year-old layers at the Cretaceous-Tertiary boundary. This anomaly only could be explained by an extraterrestrial source, a large meteorite, hitting the Earth and ultimately wiping the dinosaurs – and many other organisms - off the Earth's surface.

Professor Saunders commented: "Impacts are suitably apocalyptic. They are the stuff of Hollywood. It seems that every kid's dinosaur book ends with a bang. But are they the real killers and are they solely responsible for every mass extinction on earth? There is scant evidence of impacts at the time of other major extinctions e.g., at the end of the Permian, 250 million years ago, and at the end of the Triassic, 200 million years ago. The evidence that has been found does not seem large enough to have triggered an extinction at these times."

Flood basalt eruptions are – he says - an alternative kill mechanism. These do correspond with all main mass extinctions, within error of the techniques used to determine the age of the volcanism. Furthermore, they may have released enough greenhouse gases (SO2 and CO2) to dramatically change the climate. The largest flood basalts on Earth (Siberian Traps and Deccan Traps) coincide with the largest extinctions (end-Permian, and end-Cretaceous). "Pure coincidence?", ask Saunders and Reichow.

While this is unlikely to be pure chance, the Leicester researchers are interested in precisely what the kill mechanism may be. One possibility is that the gases released by volcanic activity lead to a prolonged volcanic winter induced by sulphur-rich aerosols, followed by a period of CO2-induced warming.

Professor Andy Saunders and Dr. Marc Reichow at Leicester, in collaboration with Anthony Cohen, Steve Self, and Mike Widdowson at the Open University, have recently been awarded a NERC (Natural Environment Research Council) grant to study the Siberian Traps and their environmental impact.

The Siberian Traps are the largest known continental flood basalt province. Erupted about 250 million years ago at high latitude in the northern hemisphere, they are one of many known flood basalts provinces - vast outpourings of lava that covered large areas of the Earth's surface. A major debate is underway concerning the origin of these provinces –including the Siberian Traps - and their environmental impact.

Using radiometric dating techniques, they hope to constrain the age and, combined with geochemical analysis, the extent, of the Siberian Traps. Measuring how much gas was released during these eruptions 250 million years ago is a considerable challenge. The researchers will study microscopic inclusions trapped in minerals of the Siberian Traps rocks to estimate the original gas contents. Using these data they hope to be able to assess the amount of SO2 and CO2 released into the atmosphere 250 million years ago, and whether or not this caused climatic havoc, wiping out nearly all life on earth. By studying the composition of sedimentary rocks laid down at the time of the mass extinction, they also hope to detect changes to seawater chemistry that resulted from major changes in climate.

From these data Professor Saunders and his team hope to link the volcanism to the extinction event. He explained: "If we can show, for example, that the full extent of the Siberian Traps was erupted at the same time, we can be confident that their environmental effects were powerful. Understanding the actual kill mechanism is the next stage….watch this space."

More information is available from the website: Source: http://www.le.ac.uk/gl/ads/SiberianTraps/Index.html

Source: University of Leicester


http://www.physorg.com/printnews.php?newsid=11585
 
Why are they the stuff of Hollywood? They happened - it's not fantsy. And there have been more cheesy Hollywood movies about apocolyptic volcanic eruptions than meteroite impacts.
 
I merged it with the more specific thread for the death of the dinosaurs.

------------
More Evidence Acquits Dino-Killer Meteor

By Larry O'Hanlon, Discovery News


March 27, 2006— The infamous dino-killer meteor was nothing of the sort, according to a story told by melted glass spheres ejected from the Yucatan's Chicxulub impact crater that landed as far away as Texas and Haiti.

The ages and chemical signatures of the glass spherules from northeast Mexico, Guatemala, Belize, Texas, and Haiti all indicate that the giant Chicxulub meteorite impact came 300,000 years too early to have been the culprit in one of the Earth's biggest mass extinction events 65 million years ago.

"The original spherule layer is not at the K-T boundary," said Chicxulub researcher Markus Harting of Utrecht University in the Netherlands.

By "original," he means the layer of spherules found in sediments from the various locations that mark the first fall of the spherules from the sky immediately after the impact.

Subsequent layers of "ejecta" spherules have been identified, but Harting's geochemical analysis shows they are all more worn and weathered, indicating that they are just re-worked spherules from the original layer.

Harting is scheduled to present his latest findings on April 3 at the co-meeting of the Geological Society of America and the Asociación Geológica Argentina in Mendoza, Argentina.

"He has the most comprehensive dataset on Chicxulub impact glass from the entire Central America area and now Texas," said geologist Gerta Keller of Princeton University.

She has studied some of the very same rocks to unravel the environments they represent. Her work also points to the Chicxulub impact being too early to account for the mass die-offs and the famous extraterrestrial iridium layer found worldwide in rocks at the Cretaceous-Tertiary (K-T) boundary, 65 million years ago.

The iridium layer is the original evidence used to argue for a mega-murderous impact.

"In essence, his work has confirmed that the oldest ejecta layer is the original Chicxulub fallout and that over time erosion, transport and re-deposition of the original spherule layer in shallow areas resulted in the additional spherule deposits," said Keller.

For those reluctant to abandon the killer meteor theory, Harting offers up the possibility that Chicxulub may have been in some way responsible for the extinction of ammonites, the once common nautilus-like sea creatures.

"Ammonites start to go extinct and are almost done before the K-T boundary," said Harting.

On the other hand, turtles and crocodiles weathered the K-T boundary just fine, which would seem to argue against the idea that the Chicxulub impact caused a worldwide dust storm that cooled and killed almost all life with a "nuclear winter" scenario. Turtles and crocs are warmth and sun-loving creatures, after all, he said.

And as for what, then, caused the famous iridium layer, which is undoubtedly from outer space, Harting said there are other possibilities.

One is that Earth and the entire solar system passed through a dense cloud of galactic dust 65 million years ago.

That would have caused harmless meteor showers that would have left their iridium in the atmosphere, and that would have been quickly rained out and deposited in lakes and oceans worldwide.

http://dsc.discovery.com/news/briefs/20 ... r_din.html
 
I'm betting that it was captain kirk. All evidence (unreferenced natch) points his way. Banned episode nuff said. :lol:
 
Did Million-Year-Long Eruptions Cause Mass Extinction?

Stefan Lovgren
for National Geographic News
May 2, 2006

As Earth-shattering events go, nothing comes close to the mass extinction that punctuated the Permian period some 250 million years ago.

Around 95 percent of marine species and 70 percent of land species were abruptly wiped out (related photos).

So sharp is the break in the fossil record at this geologic boundary that scientists in the 1800s believed they were dealing with two separate, unrelated starts to life on Earth.

But what caused the Permian extinction is one of science's greatest mysteries.

In his newly published book Extinction: How Life on Earth Nearly Ended 250 Million Years Ago, Douglas Erwin explores the many theories put forth to explain the phenomenon, from plate tectonics to meteor strikes.

Erwin, a paleobiologist at the Smithsonian Institution in Washington, D.C., doesn't pinpoint a definite culprit in the quarter-billion-year-old whodunit—but he has a suspect.

The extinction coincided with the million-year-long eruption of Siberian flood basalts. A flood basalt is a giant volcanic eruption that coats vast stretches of land with basalt lava. (Related feature: Build your own volcano.)

The Siberian event was one of the most massive volcanic events in the last 600 million years.

Erwin suggests the eruptions may have produced everything from acid rain to global warming, which helped kill the majority of life on Earth.

"The most likely explanation at this point is that the effects of the Siberian flood basalts were responsible," Erwin said.

No Crater

Until a decade ago scientists thought the Permian extinction was a continuous event that lasted for up to ten million years. But experts have now concluded that the die-offs occurred in two waves separated by eight million years or so.

The second wave was far more severe and may have happened over as little as 100,000 years.

The relative speed of the extinction seems to rule out gradual processes, like plate tectonics, as the cause. Instead, clues point to a sudden, catastrophic event, such as an extraterrestrial impact.

A giant asteroid that slammed into Mexico's Yucatán Peninsula caused the last mass extinction on Earth, spelling the end of the dinosaurs some 65 million years ago.

The entire global ecosystem collapsed as dust from the impact blocked out the sun and blanketed the planet with thick ash, most scientists believe. (Get the opposing side of the debate: "Yucatán Asteroid Didn't Kill Dinosaurs, Study Says.")

But no spike in iridium—a metal that is rare on Earth but common in meteorites—or other telltale minerals have been found in the geological evidence from the time of the Permian extinction.

Most important, no physical evidence of an impact, such as a gigantic crater, during that period has been found anywhere on Earth.

"Most of what we know [about the Permian extinction] is consistent with [a meteor] impact, but we don't actually have any evidence that that's what happened," Erwin said.

Lava "Floods"

Instead, Erwin believes the answer can be found in the Siberian flood basalts.

The largest flood basalt region in the United States covers most of southeastern Washington State, stretching from the Pacific Ocean and into Oregon.

The eruptions of the Siberian flood basalts, which lasted up to a million years, spilled lava across an area larger than the continental U.S.

Most notably, the eruptions happen to coincide with the Permian extinction.

The problem is that no human has ever seen this type of eruption, which is much larger than any regular volcanic eruption. Scientists don't know what the climactic effects of such an event might be.

"Correlation is not causality," Erwin said. "We would like to know how this volcanic eruption actually killed stuff."

Erwin suggests a series of possible effects, including acid rain, which may have been produced by the sulfur released by volcanoes.

Geological evidence also shows that the volcanic event destroyed a lot of coal in the area where the eruptions took place.

The heating of the coal would have released carbon dioxide and methane into the atmosphere, producing a serious bout of global warming.

"I think it's all of those things put together that explain how the Earth's [life] was wiped out," Erwin said.

New Life

Peter Ward is a biologist at the University of Washington in Seattle and author of an upcoming book, The Greenhouse Extinctions.

He says the geologic record indicates that carbon dioxide concentrations skyrocketed immediately before the start of the extinctions and then stayed high for a few million years.

"A byproduct of these major volcanic events would have been enormous volumes of carbon dioxide and methane entering the atmosphere, which would have caused a short but rapid interval of global warming," Ward said.

But other scientists are not so sure.

"Most of the explanations up until now have sort of been arm-waving without any real evidence," said Michael Rampino, an earth scientist at New York University in New York City.

"We have no smoking gun for the Permian extinction."

One thing is certain: Earth was a pretty bleak place to live at the time.

"The recovery from the mass extinction didn't even get started for four million years, and then it took another 10 to 20 million years for life to get diverse again," Erwin said.

"That's in remarkable contrast to most other mass extinctions, where the recovery is going within a few hundred thousand years."

Erwin says "recovery" may even be the wrong word to describe the process.

"Ecosystems of the Permian didn't recover, they're gone," he said. "Life had to construct new ecological relationships. This was a turning point in the history of life on Earth."

http://news.nationalgeographic.com/news ... ction.html
 
Dinosaurs 'could have been wiped out by 25 mile wide meteor'
Dinosaurs may have been wiped out by a massive 25 mile wide meteor - four times bigger than the asteroid previously though to be behind their extinction.
Published: 7:00AM BST 19 Oct 2009

Researchers believe they have discovered the world's biggest crater off the coast of India which they think may be responsible for the extinction of dinosaurs 65 million years ago.

The mysterious Shiva basin, named after the Hindu God, has a diameter of 310.7 miles along the seafloor and has a central peak of some 3 miles, as tall as Mount McKinley, the highest mountain in North America.

This dwarfs the meteor that was thought to have killed off the dinosaurs which measured between five and six miles and lies in the Yucatan Peninsula in Mexico.

That impact left a crater with a diameter of 180 kilometres.

Sankar Chatterjee of Texas Tech University, who led the research, said: "If we are right, this is the largest crater known on our planet.

"Rocks from the bottom of the crater will tell us the telltale sign of the impact event from shattered and melted target rocks. And we want to see if there are breccias, shocked quartz, and an iridium anomaly."

Asteroids are rich in iridium, and such anomalies are thought of as the fingerprints of an impact.

Mr Chatterjee believes the impact of an asteroid or comet of this size would have vaporized the Earth's crust on collision, killing most life and leaving ultra-hot mantle material to well up in its place.

The force of the impact broke the Seychelles islands off of the Indian tectonic plate and sent them drifting towards Africa. Much of the 30-mile-thick granite layer in the western coast of India was also destroyed.

Most of the crater lies submerged on India's continental shelf, but some tall cliffs rise above the sea, bringing active faults and hot springs. The area is a rich source of oil and gas reserves.

The team plans to visit India again to drill into the centre of the crater for clues to prove the basin was formed by a gigantic impact.

Mr Chaterjee will present his research this month at the Annual Meeting of the Geological Society of America.

http://www.telegraph.co.uk/science/dino ... eteor.html
 
Not one massive meteorite strike, but two, or possibly more. Did multiple strikes put paid to the dinosaurs?
http://www.bbc.co.uk/news/science-environment-11112417

Double meteorite strike 'caused dinosaur extinction'

BBC News Howard Falcon-Lang Science reporter, 27 August 2010

The dinosaurs were wiped out 65 million years ago by at least two meteorite impacts, rather than a single strike, a new study suggests.

Previously, scientists had identified a huge impact crater in the Gulf of Mexico as the event that spelled doom for the dinosaurs.

Now evidence for a second impact in the Ukraine has been uncovered.

This raises the possibility that the Earth may have been bombarded by a whole shower of meteorites.

...
makes you think...

:shock:
 
Fossil from the 'last dinosaur' proves species WAS wiped out by killer asteroid, say scientists
By Daily Mail Reporter
Last updated at 2:26 PM on 13th July 2011

A horn from one of the last surviving dinosaurs could finally prove that a massive meteor strike ended the reptiles' reign on Earth.
The 45cm-long fossilised browhorn belonged to a family of plant-eating dinosaurs that included the famous three-horned Triceratops.
It was found at a geological site known as the Hell Creek Formation in the bleak badlands of south-east Montana, U.S., where many other dinosaur fossils have been unearthed.

What made this find so remarkable was its location, just 13cm below the rock layer that marks the Cretaceous-Tertiary or 'K-T' boundary - the point in the fossil record where the dinosaurs died.
This suggests dinosaurs were around right up to the time all traces of their existence vanished.
Scientist believe they then disappeared suddenly after an abrupt global disaster rather than a slow extinction.

A huge asteroid or comet smashing into the Earth off the coast of Mexico at the end of the Cretaceous Period 65 million years ago is widely believed to have killed off the dinosaurs.
But some sceptics have repeatedly pointed to an absence of dinosaur fossils for three million years leading up to the impact as evidence that the creatures may have already gone when the meteor struck.
There has been an apparent lack of fossils buried within the 10 feet of rock below the K-T boundary. The area has become known as the 'three-meter gap.'
The 'three-meter gap' theory has helped drive controversy over what happened to the dinosaurs, some of which evolved into birds.
Some scientists have suggested that the dinosaurs slowly died off.

However, the horn fossil appears to close the gap, according to scientists writing in the Royal Society journal Biology Letters.
The discovery gives weight to the idea that a devastating asteroid obliterated the beasts.
'This discovery provides some evidence that dinosaurs didn't slowly die out before the meteor struck,' Dr Tyler Lyson, from Yale University in New Haven told physorg.
'The fact that this specimen was so close to the boundary indicates that at least some dinosaurs were doing fine right up until the impact.
'The in situ specimen demonstrates that a gap devoid of non-avian (bird) dinosaur fossils does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the K-T boundary impact event.'

The scientists pointed out that a 125cm section of rock strata laid down after the impact was completely devoid of fossils.

http://www.dailymail.co.uk/sciencetech/ ... saurs.html
 
Case closed?

Two separate extinctions brought end to dinosaur era
http://www.newscientist.com/article/dn2 ... r-era.html
17:04 03 August 2012 by Jeff Hecht

The mass extinction that wiped out the dinosaurs 65 million years ago was almost unprecedented in its size. There may be a simple reason why three-quarters of Earth's species disappeared during the event – there were actually two extinctions at the end of the Cretaceous, each devastating species in distinct environments.

Famously, the dinosaurs met their end when a massive meteorite crashed into Mexico's Yucatán Peninsula around 65 million years ago. The extinction paved the way for the rapid evolutionary diversification of mammals.

But sceptics have long questioned whether the meteorite was solely responsible for the extinction. They point out that there were massive volcanic eruptions in India more than 100,000 years earlier, which triggered global warming that might have contributed to the species fatalities. But convincing evidence for those claims has proved elusive, so the impact has taken most of the blame.

A key problem has been finding sedimentary rocks that were formed at exactly the right time to capture all of the events that might have contributed to the extinction. The rocks need to contain plenty of fossils too, to reveal exactly when the various species disappeared.

Thomas Tobin at the University of Washington in Seattle has just found rocks that fit the bill on Seymour Island, just off the Antarctic Peninsula. "It is really far south, so any climate changes are likely to be strongest there and have more biological effects," he says.

Tobin found two layers in the rocks, which formed in a shallow sea, where several species of shelled animals went extinct. One of the layers dates to the time of the impact, but the other layer is 40 metres below. Dating showed that the lower extinction occurred some 150,000 years before the meteorite hit – at the peak of the Indian eruptions. Tobin's team looked at isotopic ratios in the rock to work out the temperatures at the time: the first extinction followed a 7 °C rise in polar ocean temperatures – probably a result of global warming triggered by the Indian volcanism.

Comparable numbers of species in the region went extinct in each event. Surprisingly, though, the types of animals affected differed strikingly.

"The stuff living at the [ocean] bottom died out during the [volcanic extinction event]," says Peter Ward, Tobin's thesis advisor and collaborator. That might be because the global warming triggered by the volcanic eruptions initially increased levels of biological activity in the oceans, but ultimately used up the oxygen dissolved in the water to create lethal anoxic conditions in deep water.

The later extinction, which is linked to the meteorite impact, wiped out creatures that lived in the surface waters.

The new data suggesting two distinct extinctions ties in with results of another new study. Gerta Keller of Princeton University and her team studied microfossils from the Bay of Bengal that lived during the end of the Cretaceous. The sea floor sediments in which they are preserved is interleaved with basalt from the massive Indian lava flows. Around half of the species went extinct during the initial volcanic eruptions, long before the meteorite impact. Here, however, it was the surface-dwelling organisms that were affected by the volcanism.

The case for multiple factors contributing to the extinction is adding up, says David Archibald, a vertebrate palaeontologist recently retired from San Diego State University, California, who was not involved in either study. "I'm not suggesting the [meteorite] impact didn't have tremendous effects, and it probably was necessary for the extinctions, but there were other things leading up to it," he says.

Journal reference: Tobin study: Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2012.06.029; Keller study: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2012.06.021
 
First the volcano's, then the asteroid strike.

Dinosaur Die out Might Have Been Second of Two Closely Timed Extinctions
http://www.sciencedaily.com/releases/20 ... 154314.htm

Thomas Tobin clears sand from around the fossil of a giant ammonite he found in 2009 on James Ross Island in Antarctica. (Credit: Image courtesy of University of Washington)

ScienceDaily (Sep. 5, 2012) — The most-studied mass extinction in Earth history happened 65 million years ago and is widely thought to have wiped out the dinosaurs. New University of Washington research indicates that a separate extinction came shortly before that, triggered by volcanic eruptions that warmed the planet and killed life on the ocean floor.

The well-known second event is believed to have been triggered by an asteroid at least 6 miles in diameter slamming into Mexico's Yucatán Peninsula. But new evidence shows that by the time of the asteroid impact, life on the seafloor -- mostly species of clams and snails -- was already perishing because of the effects of huge volcanic eruptions on the Deccan Plateau in what is now India.

"The eruptions started 300,000 to 200,000 years before the impact, and they may have lasted 100,000 years," said Thomas Tobin, a UW doctoral student in Earth and space sciences.

The eruptions would have filled the atmosphere with fine particles, called aerosols, that initially cooled the planet but, more importantly, they also would have spewed carbon dioxide and other greenhouse gases to produce long-term warming that led to the first of the two mass extinctions.

"The aerosols are active on a year to 10-year time scale, while the carbon dioxide has effects on a scale of hundreds to tens of thousands of years," Tobin said.

During the earlier extinction it was primarily life on the ocean floor that died, in contrast to the later extinction triggered by the asteroid impact, which appeared to kill many more free-swimming species.

"The species in the first event are extinct but the groups are all recognizable things you could find around on a beach today," he said.

Tobin is the lead author of a paper in the journal Palaeogeography, Palaeoclimatology, Palaeoecology that documents results of research conducted in a fossil-rich area on Seymour Island, off the Antarctic Peninsula.

That particular area has very thick sediment deposits and, for a given interval of time, might contain 10 times more sediment as the well-known Hell Creek Formation in Montana. That means scientists have much greater detail as they try to determine what was happening at the time, Tobin said.

The researchers took small surface core samples from rocks and fossils in the Antarctic sediment and used a method called magnetostratigraphy, employing known changes over time in Earth's magnetic field to determine when the fossils were deposited. The thicker sediment allowed dating to be done more precisely.

"I think the evidence we have from this location is indicative of two separate events, and also indicates that warming took place," Tobin said.

There is no direct evidence yet that the first extinction event had any effect on the second, but Tobin believes it is possible that surviving species from the first event were compromised enough that they were unable to survive the long-term environmental effects of the asteroid impact.

"It seems improbable to me that they are completely independent events," he said.

The paper's coauthors are Peter Ward, Tobin's doctoral adviser, and Eric Steig, both UW professors of Earth and space sciences; Eduardo Olivero of the Southern Center for Scientific Research in Argentina; Isaac Hilburn, Matthew Diamond and Joseph Kirschvink of the California Institute of Technology; Ross Mitchell of Yale University; and Timothy Raub of the University of St. Andrews in Scotland.

The work was funded by the National Science Foundation Office of Polar Programs and the National Scientific and Technological Promotion Agency in Argentina.

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Story Source:

The above story is reprinted from materials provided by University of Washington. The original article was written by Vince Stricherz.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Thomas S. Tobin, Peter D. Ward, Eric J. Steig, Eduardo B. Olivero, Isaac A. Hilburn, Ross N. Mitchell, Matthew R. Diamond, Timothy D. Raub, Joseph L. Kirschvink. Extinction patterns, ?18 O trends, and magnetostratigraphy from a southern high-latitude Cretaceous–Paleogene section: Links with Deccan volcanism. Palaeogeography, Palaeoclimatology, Palaeoecology, 2012; 350-352: 180 DOI: 10.1016/j.palaeo.2012.06.029
 
Dinosaur-killing asteroid was a twin terror
http://www.newscientist.com/article/dn2 ... error.html
13:02 01 February 2013 by Colin Barras

Asteroids 2, dinosaurs 0. The infamous space rock that slammed into Earth and helped wipe it clean of large dinosaurs may have been a binary – two asteroids orbiting each other.

The dino-killing asteroid is usually thought of as a single rock with a diameter of 7 to 10 kilometres, but it may really have been two widely separated rocks with that combined diameter.

The surprise conclusion comes from a re-evaluation of the proportion of asteroid craters on Earth that were formed from binary impacts. It could also spell bad news for those hoping to protect our world from catastrophic collisions in future.

Earth bears the scars of twin-asteroid impacts: the Clearwater Lakes near Hudson Bay in Canada, for instance, are really twin craters that formed about 290 million years ago. Examples like Clearwater are rare, though. Just 1 in 50 of craters on Earth come in such pairs.

Binary mismatch

That is a puzzle because counts of the rocks zooming around in the vicinity of Earth suggest binaries are far more common. "It's been known for 15 years that about 15 per cent of near-Earth asteroids are binary," says Katarina Miljkovi? at the Institute of Earth Physics in Paris, France. All else being equal, 15 per cent of Earth's impact craters should be the result of twin impacts. Why does the real figure appear so much lower?

Miljkovi? and her colleagues have found an explanation. They ran computer simulations of binary asteroids hitting Earth and found that they often form a single crater.

This makes sense, given that a crater can be 10 times the diameter of the asteroid that made it. The team found that only unusual cases involving two small, widely separated asteroids are guaranteed to form a pair of distinct craters. The researchers' simulations confirmed that such binary asteroids are rare enough to explain why paired craters account for only 2 per cent of all Earth's craters.

An obvious implication is that binary asteroids hit Earth more often than the crater record appears to suggest – with ramifications for efforts to prevent future impacts (see "Do twin asteroids pose twice the risk?", below).

Not quite symmetrical

The simulations also suggest that it is possible to identify which of Earth's single craters had binary origins. These craters should be subtly asymmetrical, and that makes the crater near Chicxulub in Mexico – thought to be the result of an asteroid impact 65.5 million years ago that wiped out the dinosaurs – a strong candidate.

"The Chicxulub crater shows some important asymmetries," says Miljkovi?. "It is worth considering that it was formed by a binary asteroid."

Petr Pravec at the Academy of Sciences of the Czech Republic in Ondrejov agrees with Miljkovi? that the crater's features make it a particularly likely contender.

Studying the gravity anomalies created by an impact is a powerful way to find out more about a crater – particularly one that is now buried, like the Chicxulub crater. Pravec says recent gravity surveys taken at the Chicxulub impact area support Miljkovi?'s conjecture. "The signatures also suggested that the Chicxulub crater might have been formed by a binary asteroid impact," he says.

What might the binary Chicxulub asteroid have looked like? Miljkovi?'s simulations, coupled with the Chicxulub crater's diameter of about 180 km, and its shape, suggests it may have been two rocks with a combined diameter of 7 to 10 km – the same diameter as the single rock previously imagined to be the culprit. The twin impactors could have been up to 80 km apart, she says, "but these numbers are just guidelines".

It has long been suspected that binary asteroids can generate single craters, says Jean-Luc Margot at the University of California, Los Angeles. "The new study puts this conjecture on solid analytical footing."

Journal reference: Earth and Planetary Science Letters, doi.org/kcx

Do twin asteroids pose twice the risk?
If binary asteroids can form single craters, then Earth is more likely to hit by a binary impact in future than our planet's crater record would suggest. Could these double whammies be harder to spot or deflect than single asteroid hits?

The existential threat posed by asteroids has gained attention in recent years – underlining the risk, one is due to skim Earth in just two weeks. There are several new efforts to scan the skies for asteroids, and a plethora of suggestions for how they might be deflected.

"I am not sure if any of the proposed asteroid deflection techniques could deflect both binary components with a single weapon," says Katarina Miljkovi? at the Institute of Earth Physics in Paris, France, who led the new study.

Alan Harris, a retired asteroid researcher formerly at NASA's Jet Propulsion Lab in Pasadena, California, has one idea: "A nuclear explosion might be directed at the smaller body, and by blowing it away, the recoil on the main asteroid might effectively deflect it from a collision course."

Being able to deploy the appropriate defence would depend on our ability to spot whether or not an object heading our way is a binary. Don Yeomans of NASA's Near Earth Object Program thinks that won't be a problem for a future asteroid-deflecting spacecraft. "There is a slim chance that the autonomous navigation camera might be confused with two images in its field of view, but I should think these issues would be easily overcome," he says.
 
Dinosaur-killing space rock 'was a comet'
http://www.bbc.co.uk/news/science-environment-21709229
By Paul Rincon
Science editor, BBC News website, The Woodlands, Texas

The impact 65 million years ago killed off 70% of species on Earth - including the dinosaurs
'
The space rock that hit Earth 65 million years ago and is widely implicated in the end of the dinosaurs was likely a speeding comet.

That is the conclusion of research which suggests the 180km-wide Chicxulub crater in Mexico was carved out by a smaller object than previously thought.

Many scientists consider a large and relatively slow moving asteroid to have been the likely culprit.

Details were outlined at the 44th Lunar and Planetary Science Conference.

But other researchers were more cautious about the results.

"The overall aim of our project is to better characterise the impactor that produced the crater in the Yucatan peninsula [in Mexico]," Jason Moore, from Dartmouth College in New Hampshire, told BBC News.

The space rock gave rise to a global layer of sediments enriched in the chemical element iridium, in concentrations much higher than naturally occurs; it must have come from outer space.

Extra-terrestrial chemistry
However, in the first part of their work, the team suggests that frequently quoted iridium values are incorrect. Using a comparison with another extraterrestrial element deposited in the impact - osmium - they were able to deduce that the collision deposited less debris than has previously been supposed.

The recalculated iridium value suggests a smaller body hit the Earth. So for the second part of their work, the researchers took the new figure and attempted to reconcile it with the known physical properties of the Chicxulub impact.

Continue reading the main story
Death of the dinosaurs

Comets and volcanoes: Find out how entire species were wiped out in mass extinctions
Why do comets get attracted to our planet so often?
For this smaller space rock to have produced a 180km-wide crater, it must have been travelling relatively quickly. The team found that a long-period comet fitted the bill much better than other possible candidates.

"You'd need an asteroid of about 5km diameter to contribute that much iridium and osmium. But an asteroid that size would not make a 200km-diameter crater," said Dr Moore.

"So we said: how do we get something that has enough energy to generate that size of crater, but has much less rocky material? That brings us to comets."

Dr Moore's colleague Prof Mukul Sharma, also from Dartmouth College, told BBC News: "You would need some special pleading for an asteroid moving very rapidly - although it is possible. But of the comets and asteroids we have looked at in the skies, the comets are the ones that are moving very rapidly."

Long-period comets are balls of dust, rock and ice that are on highly eccentric trajectories around the Sun. They may take hundreds, thousands or in some cases even millions of years to complete one orbit.

The extinction event 65 million years ago is now widely associated with the space impact at Chicxulub. It killed off about 70% of all species on Earth in just a short period of time, most notably the non-avian dinosaurs.

The enormous collision would have triggered fires, earthquakes and huge tsunamis. The dust and gas thrown up into the atmosphere would have depressed global temperatures for several years.

Lost in space
Dr Gareth Collins, who researches impact cratering at Imperial College London, described the research by the Dartmouth team as "nice work" and "thought-provoking".

But he told BBC News: "I don't think it is possible to accurately determine the impactor size from geochemistry.


This gravity map reveals the structure of the Chicxulub crater in Yucatan
"Geochemistry tells you - quite accurately - only the mass of meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater."

He added: "The authors suggest that 75% of the impactor mass is distributed globally, and hence arrive at quite a small-sized impactor, but in reality this fraction could be lower than 20%."

That could keep the door open for a bigger, more slowly moving asteroid.

The authors accept this point, but cite recent studies suggesting mass loss for the Chicxulub impact was between 11% and 25%.

In recent years, several space objects have taken astronomers by surprise, serving as a reminder that our cosmic neighbourhood remains a busy place.

On 15 February this year, 2012 DA14 - an asteroid as large as an Olympic swimming pool - raced past the Earth at a distance of just 27,700km (17,200mi). It had only been discovered the previous year.

And on the same day, a 17m space rock exploded over Russia's Ural mountains with an energy of about 440 kilotonnes of TNT. About 1,000 people were injured as the shockwave blew out windows and rocked buildings.

Some 95% of the near-Earth objects larger than 1km have been discovered. However, only about 10% of the 13,000 - 20,000 asteroids above the size of 140m are being tracked.

There are probably many more comets than near-Earth asteroids, but Nasa points out they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 10% to the census of larger objects that have struck the Earth.

[email protected]. and follow Paul on Twitter
 
Never mind yer asteroids, apparently it was oil. Well, all right, asteroids and oil: a new take on the Alvarez theory suggests the impact ignited subterranean oil supplies, sending plumes of soot high into the atmosphere and helping to suppress temperatures. What strikes me as most interesting about the article, though, is the way that the theory changes to take account of contemporary environmental concerns.

The failure of any of the proposed kill mechanisms to account for the details of the fossil record has led some paleontologists to propose that the asteroid was not a lone assassin but struck a global ecosystem already weakened by other injuries. The most frequently cited accomplice is volcanic activity, in particular the eruptions that produced the Deccan Traps, a mile-thick stack of basalt flows in present-day India. For tens of thousands of years leading up to the extinction, the oozing lavas released enormous quantities of carbon dioxide. In recent reconstructions of the Cretaceous finale, the murderous asteroid has been forced to share the stage with unglamorous greenhouse gases.

Now, a paper just published in Scientific Reports has named another possible conspirator: crude oil. According to Kunio Kaiho and his colleagues at Tohoku University, in Sendai, Japan, the sudden ignition of underground oil at the Yucatán impact site could have jetted into the upper atmosphere a mass of fine black carbon, also known as soot. Human-made black carbon, the bane of Beijing, remains in the lower atmosphere for only a matter of days before falling back to the surface, where it warms the planet by absorbing heat. But black carbon injected into the stratosphere would have the opposite effect, acting as a long-lived sunshade that could abruptly cool Earth and inhibit photosynthesis over a period of years.
 
I've just come across this recent iPlayer investigation (first shown 15th May 2017) which claims to dot the i's and cross the t's:

The Day the Dinosaurs Died investigates the greatest vanishing act in the history of our planet - the sudden disappearance of the dinosaurs 66 million years ago.

Experts suspect that the dinosaurs were wiped out after a city-sized asteroid smashed into the Gulf of Mexico causing a huge crater. But until now, they haven't had any proof. In a world first, evolutionary biologist Ben Garrod joins a multimillion-pound drilling expedition into the exact spot the asteroid hit to get hard evidence of the link. The team overcomes huge obstacles as it attempts to drill 1,500 metres beneath sea level to pull up rock from the Chicxulub crater.

Meanwhile, paleopathologist Professor Alice Roberts travels the globe meeting top scientists and gaining exclusive access to a mass fossil graveyard in New Jersey - believed to date from the same time the asteroid hit. Alice also treks by horseback across the remote plains of Patagonia, to see if the effects of the asteroid impact could have wiped out dinosaurs across the world - almost immediately.

Alice and Ben's investigations reveal startling new evidence of a link between the asteroid and the death of the dinosaurs, presenting a vivid picture of the most dramatic 24 hours in our planet's history. They illustrate what happened in the seconds and hours after the impact, revealing that had the huge asteroid struck the Earth a moment earlier, or later, the destruction might not have been total for the dinosaurs. And if they still roamed the world, we humans may never have come to rule the planet.

http://www.bbc.co.uk/iplayer/episode/b08r3xhf/the-day-the-dinosaurs-died

59 min.

Edit: Quite interesting, but it never mentioned the world wide iridium layer that first suggested the asteroid impact theory. Not that the iridium killed the dinos - it was just a marker in time that synchronised with their extinction.


 
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Global cooling.
That's what killed the dinosaurs that were left after the asteroid hit.
Perpetual darkness and cold killed their food supply and the largest ones died off first.
 
Still more than twice the number of dinosaur species on the planet than mammals. The question of what killed the dinosaurs is pretty outdated. It's a shame the BBC isn't more up to speed.
 
Still more than twice the number of dinosaur species on the planet than mammals. The question of what killed the dinosaurs is pretty outdated. It's a shame the BBC isn't more up to speed.
Not sure what you mean by this. The changed environmental conditions meant the bigger animals (dinos) died out before the smaller ones (mammals), which could still scrabble some sort of small-scale living.

Can you explain why the BBC (and its army of experts, universities, and researchers) should be 'more up to speed'?
 
"The changed environmental conditions meant the bigger animals (dinos) died out before the smaller ones (mammals)"

But even the tiniest (chicken sized) dinosaurs disappeared, whereas the larger (badger sized) mammals survived.

Still a lot of mystery surrounding the extinction event.
 
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