So what did-in the dinosaurs? A murder mystery…

Scientists have assembled a slew of new forensic evidence – from high-resolution dates to microscopic fossils – to prosecute the dino-killer. Their indictment has worrying implications for us.

Everyone knows that the dinosaurs were wiped out - along with about 70% of all species - by a massive asteroid slamming into Mexico, right? Well, not so fast. Like a good murder-mystery, a steady drip of evidence and some major new revelations have implicated another suspect – were they in it together or is one innocent?

Dead dino

Suspect A – the impact

Our first suspect is the asteroid impact at a place called Chicxulub, in Mexico.

This one is not a serial killer. In all the episodes of mass extinction through deep time, it has only credibly been implicated in the end-Cretaceous catastrophe. It’s weapons of death include a violent blast that destroys everything for thousands of miles around the impact, a heat flash from the blast that incinerates everything in a similar radius, followed by a near-global rain of red-hot ejecta that turns the sky into a broiler (“grill” if you are outside the US) inflicting fatal burns and igniting a global conflagration. The blast, centered in shallow ocean, generates a colossal tsunami across the juvenile Atlantic and the shock wave triggers earthquakes and tsunamis around the world far more violent than the 2011 magnitude 9 T?huku earthquake in Japan. Finally, the great quantity of dust and incinerated debris flung into the upper atmosphere blocks out the sun, turning the world dark and the climate frigid for years.

That should do it.

Except lately the idea that the impact’s heat flash could ignite everything has been challenged by experiments, which show: “any fires ignited by impact-induced thermal radiation cannot be directly responsible for plant extinctions, implying that heat stress is only part of the end-Cretaceous story.” Plant fossils around the world also show no sign that fire was above normal levels at the time. So that’s one weapon that probably didn’t cause global extinction. Neither the blast, nor the extreme earthquakes would have been enough to cause a global extinction by themselves. What about the tsunami? Tsunami deposits have been identified around the Gulf of Mexico and into the Atlantic, but further afield the deposits are elusive. Even those in Mexico and in Texas can be interpreted, with convincing detail, as just normal sediments rather than tsunami deposits. So the tsunami was of doubtful reach, its rock evidence questionable, and it can’t have caused global extinction alone.

How about blanketing the planet in hot fallout? While it is true that tell-tale traces of apparent impact fallout (high concentrations of iridium, “shocked” quartz crystals, and tiny glass droplets called  “spherules” or “tektites”) have been found in many parts of the world, the physical layer of impact deposits dwindles from  a thickness of 2 meters (6 feet) around the Gulf of Mexico, to 3 to 5cm (1 to 2 inches) in Europe, and is apparently absent in China, Alaska, Japan and New Zealand. An iridium-rich layer has been detected across North America, Europe and North Africa, and as far afield as the India-Bangladesh border, but has not been reported from China, Alaska and Japan. So the fallout by itself does not seem convincing as a worldwide killer, and there has even been a suggestion that some of those fallout traces may be from volcanic eruptions rather than an impact.

Was the impact capable of causing the ocean acidification that bumped-off so many marine species?

Professor Toby Tyrrell of the University of Southampton, England, and his coauthors tested this. At the American Geophysical Union conference in December they revealed their dramatic answer: “no.”

The impact cannot feasibly have generated the ocean acidification that killed off calcifying marine species in the mass extinction.

They looked at a number of possible ways the impact might have acidified the ocean. Did the impact’s pressure wave turn nitrogen to nitric acid? If it did, it didn’t generate enough to acidify the ocean. Vaporization of carbon from limestones at the impact site? Not enough. Liberation of carbon from terrestrial decay, soil respiration, wild fires, hydrocarbons? Nope. Tsunami stirring the oceans? No. All of the above together. Still no. Sulfur vaporized from the sulfur-rich rocks at Chicxulub and sucked into the atmosphere? It would take 800 billion tonnes of sulfate to achieve the acidification observed, but to get that number you have to max-out every assumption in the calculation to a ridiculous extent. So no, not even close. The impact cannot feasibly have generated the ocean acidification that killed off calcifying marine species in the mass extinction.

That just leaves the “impact winter” as our suspect’s last remaining, potentially globally-fatal weapon. But plant fossils around the world show that any impact winter severe enough to prevent plant growth can’t have lasted more than a couple of years. Maybe a couple of years was enough? Sediments in Texas and New Jersey do show a strong cooling spike thought to represent the transfer of cold impact winter atmospheric temperatures into the ocean, within months to decades after the impact. But it takes many decades for the ocean to cool or warm significantly, due to its great volume and thermal inertia. Such a brief climate blip is unlikely to have cooled oceans globally fast enough to produce the rapid, sharp cooling spike observed in the sediments. In any case, large eruptions are known to cause temporary climate cooling too. In the absence of any other corroborating evidence, the impact winter seems busted as an effective cause of global extinction by itself.

So, despite its murderous image, the Chicxulub impact seems to lack WMEs (Weapons of Mass Extinction). What about our other suspect?

Suspect B – climate change caused by massive volcanic eruptions

This one has a rap sheet as long as your arm. Unlike asteroid impacts, this suspect is a known serial killer, linked to four of the “big five” mass extinctions, as well as many other global extinctions and global warming events. Still, we should presume innocence until guilt is proven.

The Deccan eruptions in India were in a rare and exotic league of hyperactive eruptions known as a “Large Igneous Provinces” not seen on the planet in the last 16 million years.  They inundated an area of India 3 times the size of Texas (or France) in superhot rivers and lakes of lava, including the longest lava flow ever measured (over 1500 km/930 miles) that only stopped when it reached well out into the ocean. Around 3 kilometers (2 miles) thickness of lava built up episodically in around 750,000 years, but at the time of the mass extinction there were four especially massive mega-eruptions packed into just a few millennia.

But it’s not the lava that kills on a global scale, it’s the gasses.

Artist's impression of a Large Igneous Province

Artist’s impression of a Large Igneous Province mega-eruption such as the Deccan eruptions. Horizontal scale is approximately 1,500 km (930 miles). Vertical scale is exaggerated - the top of the box is the stratosphere (about 15 km above the ground). The modern world’s largest volcano – Hawai’i’s Big Island - is shown for approximate scale.

At the cataclysmic onset of each mega-eruption, towering columns of ash and gasses (including steam, CO2, sulfur dioxide (SO2), chlorine and fluorine) rose to stratospheric levels, where they spread around the planet. The SO2 became sulfate aerosols, just as it does in large eruptions today, which acted as sunscreen to cool the planet for a few years (much like the impact winter), while the chlorine and fluorine may have decimated the ozone layer causing a dramatic increase in harmful UV radiation reaching the ground.

If any of this sounds familiar, it’s because we’re projected to do something very similar to ourselves if we continue CO2 emissions at the same rate as today

The Deccan gas emissions were so massive and rapid they outstripped the ability of the ocean and other feedbacks to absorb them – causing CO2 to build sharply in the atmosphere over a few millennia. As the short-term volcanic winter diminished, it unmasked the really lethal weapons of abrupt global warming and ocean acidification. The planet warmed by 8°C (14°F) on land and 4°C (7°F) in the oceans, while the excess CO2 dissolved in ocean water, turning it increasingly acidic. The sulfur gradually rained down as sulfuric acid, which pickled land and sea alike until oceans were acid enough to dissolve shelly sea life alive.

If any of this sounds familiar, it’s because we’re projected to do something very similar to ourselves if we continue CO2 emissions at the same rate as today (our CO2 emission rate is comparable to that from the Deccan eruptions).

So yes, the eruptions had effective WMEs (Weapons of Mass Extinction), and Large Igneous Provinces have a record of doing all of this before, such as in the end-Triassic and end-Permian mass extinctions.

Placing the suspects at the scene of the crime.

Since we are talking global mass extinction this isn’t so much a problem of where, as when. At the scale of many millions of years that geologists are used to working with, the dates calculated from radioactive element decay (“radiometric dates”) for the mass extinction, the eruptions, and the impact are near identical. But not quite near enough to rule our defendants either in or out.

To tease out which of our suspects caused the mass extinction, we need to refine the dates to within a few tens of millennia. Zooming up to that level of detail puts us in the blur of radiometric dating uncertainty, so scientists have to resort to forensic sleuthing using fossils, traces of orbital wobbles, and magnetic field reversals to narrow the timeframes. The inevitable discrepancies between different studies, locations, and between marine and non-marine rocks, make establishing definitive timelines like trying to find a level line across small boats bobbing up and down on high seas.

So it helps to have one fixed point to refer to. The official end of the Cretaceous is defined in northern Tunisia at the level of the mass extinction recorded by marine rocks. A kind of tiny shelly sea life called foraminifera (“forams” for short) frequently evolved new species with different shell shapes, so they can be used to establish “level” time zones. The Tunisian rocks show a changeover from the “CF1” to the “P0” fossil time zones at the end-Cretaceous as a marker of the mass extinction, which can be traced in marine rocks around the world, along with a distinct layer of clay with a red layer at its base and a spike in iridium levels, and a kick in carbon isotopes that shows a big upset in the global carbon cycle at that time.

But the best date of the end-Cretaceous so far is defined in non-marine rocks from Montana, sandwiched between the last appearance of Cretaceous pollen and the first post-Cretaceous pollen fossils, dated to 66.043 million years ago, with an uncertainty of 43,000 years in either direction. That date is within 5,000 years (effectively identical) to a date for tektites – traces of the impact – found in Haiti. That would seem to put the impact conclusively at the time of the mass extinction, but the marine fossils associated with those Haitian tektites are from a time zone at least 100,000 years younger, showing that those tektites must have been recycled by later sedimentary processes, and cannot give the true date of the impact.

The plot thickens.

Chicxulub wrongly convicted?

You might expect that the impact fallout would generate a clear global signal, a time “level” against which all other events can be compared, but sadly that is not the case. It turns out that the spherules, shocked quartz, and the iridium spike can all be moved by sedimentary processes and groundwater. Just as we saw in Haiti, a number of locations from New Jersey to the Caribbean, where the signs of impact were considered proof that the impact coincided with the mass extinction, have large gaps lasting several hundred thousand years up to 3 million years, spanning the crucial time period at the end Cretaceous! They can’t be used to narrow down the date of the impact either.

And what if there was a doppelganger – another impact that occurred around the same time generating some of the same signals, potentially throwing us of the scent? It turns out there was.

A smaller asteroid blasted a crater at Boltysh in the Ukraine, dated at 65.59 million years ago, with an uncertainty of more than half a million years in either direction, amply overlapping the events we’re investigating. It appears to have inflicted negligible ecological trauma beyond its local neighborhood, and fossils inside the crater show the Boltysh impact happened a few thousand years before the end of the Cretaceous. The Earth Impact Database shows that there are an additional 3 known (small) impacts that might possibly have occurred in this timeframe, but which are very imprecisely dated. In other words Chicxulub may have been the largest by far, but it wasn’t the only impact broadly at that time capable of generating similar tell-tale impact traces.

Sediments drilled from within the Chicxulub crater itself tell a remarkably similar story to that at Boltysh. Once thought to be the settlings from the immediate aftermath of the impact and tsunami, they have since been shown to include a regular marine limestone containing the distinct late-Cretaceous CF1 fossils - so the crater must have been formed before the end-Cretaceous mass extinction! Corroborating that, rocks from Texas and Mexico show that the impact fallout (in the form of the oldest layer of impact spherules) predates the mass extinction by more than 100,000 years!

So – amazingly - it looks like the Chicxulub impact has an alibi. It wasn’t at the scene of the crime during the mass killing, but what about our other suspect?

Chicxulub's alibi: it was before the main extinction event

The impact’s alibi – it was about 100,000 years before the end-Cretaceous mass extinction. Left: oldest spherule layer at El Peñon in Mexico (formed in the early part of the CF1 fossil time zone). Center: close-up of the spherule layer. Right: microscopic view of spherules bent round each other showing they were still hot and soft when they settled. Photo credit: Gerta Keller, Princeton University.

Incriminating volcanic-induced climate change.

At the end of last year some new radiometric dates were published for the Deccan eruptions that were 10 to 100 times more precise than previously-published dates, placing the start of the main phase of Deccan eruptions within 250,000 years of the mass extinction and showing that the eruptions continued through the extinction event. For a finer-grained link to the mass extinction we need those marine fossils – but the Deccan lavas were erupted on land. Fortunately at the fringes of the lava flows near the Bay of Bengal, sediments between and below the longest mega-flows are characteristic of the latest Cretaceous, and sediments immediately above the lavas have the distinct fossils of the very first post-Cretaceous time zones, showing that the marine mass extinction occurred during the mega-eruptions.

The isotopic makeup of marine fossils varies with temperature and changes in the ocean carbon cycle, giving scientists a picture of the fluctuating climate leading up to the extinction. There were 4 distinct global warming phases punctuated by cooler episodes. Sea levels rose and fell, while some land areas suffered severe drought. This “global weirding” tortured life through wild climate instability, culling biodiversity in mega-eruption steps, culminating in the most abrupt climate change and ocean acidification during the 4th mega-eruption. By its end most Cretaceous life was rubbed-out.

Most shelly creatures make their shells from calcium carbonate – chemistry that only works in alkali water with sufficient carbonate (a throwback to the Cambrian Explosion when seawater first turned alkali and animals had to evolve ways to deal with this new ocean chemistry). Ocean acidification was deadly for these creatures, to the extent that more than 90% of calcareous nanoplankton were wiped out, along with many more well-known, photogenic species like belemnites and ammonites.

On land, forests died across most of the world, moldered, and gave way to open land covered in ferns, although eastern Russian and Antarctic vegetation doesn’t seem to have been so severely affected. Fluctuating climate and drought prevented the return of forests for many thousands of years.


new dates... do not support an impact as the cause of the environmental changes

An incredibly detailed set of dates has just been published for end-Cretaceous sediments, which contain dinosaur fossils, in Montana. What they reveal is a sequence that is “not obviously consistent with an instantaneous forcing mechanism.” In other words, they do not support an impact as the cause of the environmental changes recorded by the sediments! In fact the new dates fit very well with the rest of the evidence incriminating the Deccan eruptions.

In North America and Europe the dinosaurs were healthy and diverse right up to within about 200,000 years of the end-Cretaceous, at which point they disappeared. Mammals and amphibians continued but declined markedly through the final 200,000 years of the Cretaceous, coinciding with the Deccan eruptions. A very similar story is told by the fossils of India, where you see fossils of flourishing vegetation and abundant animals, including nesting dinosaurs, right up to the Deccan eruptions. Once the eruptions start, sediments between the lava flows capture life dwindling away like a tragic stop-motion film. Dinosaurs and forests are decimated by the onset of the eruptions about 250,000 years before the end of the Cretaceous. The few that survive don’t make it past the next eruption, disappearing from the Indian fossil record well before other reptiles like turtles and snakes. During the final 18,000 years or so of the Cretaceous, terrestrial plant life in North America declines up to the end-Cretaceous boundary, matching the timing of the marine extinction and the Deccan mega-eruptions.

So it seems that the eruptions, not the Chicxulub impact, did-in the dinos, just as they dispatched so much other life on land and in the seas.


So we have reached a verdict. All rise.

Deccan eruptions - for the Cretaceous global warming, ocean acidification, and extinction in the marine realm: guilty! For the terrestrial extinction including the dinosaurs: also guilty – but some may still claim reasonable doubt.

Chicxulub impact – for the Cretaceous global warming, ocean acidification, and extinction in the marine realm: not guilty! For the terrestrial extinction and doing-in the dinos: not guilty - It has an alibi, and there’s insufficient evidence of its ability to kill on a global scale to prosecute. After 30 years it’s time to let this one go. It has done its time.

Case closed? Probably not. There’s room for appeals and fresh evidence in the years ahead – perhaps even a “Serial” podcast. But the many strong strands of scientific evidence that global warming and ocean acidification was behind the demise of so much life, including the dinosaurs, should give us pause.


Hat tip to the Geological Society of America Special Paper 505, the Geological Society of America October 2014 meeting in Vancouver, the American Geophysical Union December 2014 Conference, to NPR’s “Serial” podcast, and to Professors Toby Tyrrell and Gerta Keller for several clarifications, corrections, and explanations.

Howard Lee’s latest book is “Your Life as Planet Earth,” an account of past climate changes, how they affected life, and how Earth and life affected climate.

The theory that the end-Cretaceous mass extinction was due to global warming and ocean acidification goes all the way back to 1978 with the publication of a paper by Dewey McLean in Science. Even back then Dewey saw the parallels to modern climate change. When Alvarez et al published their theory that an impact was the cause, this captured the public’s imagination, but debate continued and became rancorous enough to make the newspapers. When the previously identified Chicxulub crater was linked to the extinction in 1994 the impact theory became mainstream, yet it didn't completely match the observations.  So a new theory that combined the Deccan eruptions and the Chicxulub impact was developed, which has been generally accepted since 2008 (the Press-Pulse theory of mass extinction, where the eruptions pressed ecosystems to the brink before the impact pulse finished the job). In 2010, responding to papers by Keller, Schulte et al concluded that the impact was indeed the ultimate cause of the extinction, but the debate continued. The environmental data combined with the slew of high-precision dates since 2013 linking LIPs to mass extinctions in general, and the Deccan LIP to the end-Cretaceous specifically, has now shown the dominant role of the eruptions. But it’s fair to say that the timing and ecological trauma inflicted by the Chicxulub impact will continue to be debated and refined alongside the effects of the eruptions.


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Posted by howardlee on Thursday, 12 March, 2015

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